Biotite

June 13, 2025September 29, 2015 by Siim

Biotite is a very common and widespread mineral group. The minerals of the group occur often in metamorphic and igneous rocks. It is much less common in sediments and sedimentary rocks because it yields to clay minerals in the weathering environment. Biotite is one of the two most common members of the mica group. The other one is muscovite. Biotite with other micas are sheet silicates – their structure is composed of many sheetlike layers which are connected to each-other by weak chemical bonds. That gives rise to the most characteristic feature of micas: perfect basal cleavage. As a result of that, biotite readily cleaves into parallel and flexible sheets. This property together with the softness (2.5-3 on the Mohs scale) and good reflectance makes it one of the most easily identifiable common minerals.

Large biotite and other mica samples come from pegmatitic rocks. Luumäki, Finland. Width of sample 13 cm.

Composition

A sample of the mineral with usual composition may be easily identifiable, but it is actually a mineral group because there are numerous possible replacements in the crystal lattice which gives rise to several end-members with different compositions. This is common for silicate minerals and especially widespread and complicated are the substitutions for sheet silicates. End-members such as phlogopite, annite and siderophyllite all belong to the group.

It is a common practice to distinguish between biotite and phlogopite like these two are mutually exclusive minerals with different compositions. This is no longer true. The term “biotite” has been discredited as a mineral species name by the International Mineralogical Association since 1999. It is now recommended that the term should be used for the entire series which includes phlogopite. However, phlogopite is often recognizably colored and has somewhat different occurrence which makes it logical to treat it separately from the other more iron-rich varieties.

Another interesting aspect that makes the use of the term necessary is that although there are several minerals in the mineral group, there are no clearly defined demarcation lines between them¹. Therefore the naming may be confusing task even if the exact chemical composition is known.

There are four principal compositional end-members of the group:
K₂Fe₄Al₂[Si₄Al₄O₂₀](OH)₄ – siderophyllite
K₂Mg₄Al₂[Si₄Al₄O₂₀](OH)₄ – (eastonite)
K₂Fe₆[Si₆Al₂O₂₀](OH)₄ – annite
K₂Mg₆[Si₆Al₂O₂₀](OH)₄ – phlogopite

‘Eastonite’ is in parentheses because the term has been abandoned. Biotite close to eastonite end-member is rare in nature. Siderophyllite and annite are also more like a theoretical end-members because some of the iron in their crystal lattice is always substituted by magnesium. And in most cases some of the Fe/Mg of annite-phlogopite series is substituted by Al which moves them closer to the siderophyllite-(eastonite) series. Charge balance is maintained by replacing Al for Si in other site in the lattice (this substitution of multiple elements is known as the Tschermak’s substitution).

Biotite which is not phlogopite is therefore closer to annite or siderophyllite end-member. These so-called normal biotites tend to be black in color and they are sometimes referred to as lepidomelane (dark iron-rich biotite). Other possible replacements include F and Cl for OH and Na, Ca, Ba, Rb, Cs for K.

Dark iron-rich variety from granite pegmatite. Evje, Norway. Width of sample 11 cm.

Mg-rich variety phlogopite. Phlogopite has typically more brownish or fiery look than black iron-rich biotites. This rock (pure biotite-rock is known as glimmerite) formed similarly as part of pegmatite, but the source of fluids was mafic (gabbroic) magma which contained significantly more magnesium than felsic magma and favored the formation of phlogopite. Ødegården Verk, Norway. Width of sample 14 cm.

http://picasaweb.google.com/107509377372007544953/Rocks#5805071107438597810
The three end-members already mentioned do not cover all the possible variations in composition. Zinnwaldite, for example, is compositionally close to iron-rich variety siderophyllite with added lithium. This mineral name, however, is also no longer in official use. The sample is from the type locality (Zinnwald) at the German/Czech border in The Ore Mountains. Width of sample 7 cm.

Properties and identification

The most characteristic property of these minerals is the perfect basal cleavage:

Mica minerals have perfect basal cleavage which makes it easy to separate them into many thin folia. Biotite (on the left) and muscovite are the most widespread micas.

Most biotite group minerals are black in color, although the color gets lighter (brownish) if the sheet is thin enough for the light to penetrate it. Mg-rich phlogopite is brown or greenish. The density is highly variable: 2.7-3.3 g/cm³. Iron-rich member annite is the heaviest.

Biotite is in most cases not difficult to identify. It is micaceous (composed of cleavable sheets). Muscovite has the same property, but it is usually much lighter in color. The micaceous character, however, may not be immediately obvious if the rock is fine-grained. Hornblende is a common amphibole mineral that may occur in very similar rocks (both are common in granitoids) and may look alike. In this case a needle is a handy tool that will quickly solve the question. These minerals are notably soft. Their hardness on the Mohs scale is only about 2.5-3 which is low for a silicate mineral (although within normal range for a sheet silicate). Biotite is easily scratched with a needle and you can also force it into the “book-like” aggregates of many layers. Nothing even remotely similar can be done to hornblende which is a much harder material. Phlogopite is usually more brownish or even fiery looking. That is why phlogopite is named so – phlogopos means ‘fiery looking’ in Greek.

Occurrence

It is among the most common and widespread minerals. It occurs in a wide variety of igneous rocks which includes both felsic and mafic rocks. It has different roles in these rocks, though. It acts as the primary iron-bearing phase in many granitic rocks and as a hydrated potassium-bearing mineral in mafic rocks. Biotite is present in both silica under- and oversaturated rocks. Biotite, because it is the host of relatively uncommon chemical elements in a given magma, is a very common ingredient of pegmatites which crystallize from late-magmatic fluids. Large “books” are a common sight in pegmatitic rocks. Biotite in felsic rocks tends to be Fe-rich.

Phlogopite in igneous rocks occurs in ultramafic rocks, especially diamond-bearing kimberlites. It is least common in mafic rocks, but these too host it sometimes. Especially if the magma is contaminated by the material from pelitic crustal rocks. It is much less frequent in volcanic rocks and generally absent in basalts (except some K-rich varieties). If present, biotite may be partially altered to other minerals. The reason why biotite is abundant in intrusive but usually resorbed in volcanic rocks is that it is not a stable phase in magma at lower pressures.

In metamorphic rocks too, biotite is present in a wide variety of rocks which formed under various temperature and pressure conditions. Biotite in the majority of cases forms when clay-rich sedimentary rocks are buried deep enough for the clay minerals to metamorphose to it. Biotite also forms in impure metamorphosed carbonate rocks and in metabasic rocks.

There are many different geochemical pathways that can lead to the formation of biotite which explains its ubiquitousness in metamorphic systems. It remains the companion of several metamorphic minerals (muscovite, garnet, staurolite, Al-silicates, cordierite) that inhabit pelitic rocks at various depths in the crust. Biotite finally disappears at the granulite facies conditions where hydrous minerals (which it is) are not stable. Metapelitic rocks that contain lots of biotite are various schists (biotite schist, biotite-chlorite schist, albite-biotite schist, garnet-mica schist).

In metamorphosed mafic rocks biotite forms as a replacement of low-grade greenschist facies metamorphic rocks containing amphiboles and muscovite. They react to form biotite + quartz + water. It finally breaks down to form granulitic rock composed of pyroxene + K-feldspars.

There are many reactions in impure carbonate rocks in which biotite (including phlogopite) can be among the reactants or products:
phlogopite + calcite + quartz → tremolite + K-feldspar + CO₂ + H₂O
dolomite + muscovite → biotite + chlorite + calcite + CO₂¹

Biotite also occurs in skarns which are contact-metasomatized rocks which form when silicate magma reacts with carbonate rocks.

Biotite in clastic sediments is a common mineral when biotite-bearing rocks are exposed nearby. It is not as resistant in the weathering environment as muscovite and gets less frequent with extended transport. Biotite is usually altered to clay minerals montmorillonite and vermiculite.

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Biotite and muscovite are the two most common mica minerals that often occur together in Al-rich igneous rock. Peraluminous two-mica granitoid. A fragment from an erratic on Alnö Island, Sweden. Width of sample 9 cm.

Monomineralic rock that is composed of (almost) pure biotite is known as glimmerite. It is usually part of a larger pegmatitic assemblage. Trælen, Senja, Norway. Width of sample 25 cm.

Tonalite pegmatite (white-gray on the right) and garnet amphibolite (on the left). Black in pegmatite is biotite, white is plagioclase, gray is quartz. Trælen, Senja, Norway. Width of sample 10 cm.

Biotite gneiss rock sample — Biotite gneiss. Black mineral forming bands in the rock is biotite. White feldspar is plagioclase. Evje, Norway. Width of sample 14 cm.

http://picasaweb.google.com/107509377372007544953/2015#6190953010689058114
Trondhjemite is a leucocratic igneous rock (tonalite) in which the sole mafic mineral is biotite (hornblende is rare). Width of sample from Norway is 10 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196126438168098306
Phlogopite with apatite (greenish yellow) and enstatite (dark green) in pegmatite. Ødegården Verk, Norway. Width of sample 22 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196126412621263426
Nepheline syenite. Dark minerals are iron-rich biotite and pyroxene augite. Larvik, Norway. Width of sample 9 cm.

Phlogopite in glimmerite-carbonatite. Carbonatite is igneous rock with highly unusual carbonate composition (white mineral is calcite). It is usually phlogopite when biotite group mica is associated with carbonate minerals in igneous or metamorphic rocks. Siilinjärvi, Finland. Width of sample 9 cm.

http://picasaweb.google.com/107509377372007544953/2015#6196127385810409266
Granite having slightly lineated texture. The mineral that gives it such an appearance is biotite. Note how the elongated patches are oriented sub-parallel to each-other. Lødingen, Norway. Width of sample 8 cm.

Caption — Biotite is a very common mineral in medium-grade metamorphic rock schist. Narvik, Norway. Width of sample 10 cm.

http://picasaweb.google.com/107509377372007544953/2015#6196127882994597890
Biotite also occurs in metasomatic rocks. This skarn sample is composed of biotite with light green actinolite (Ca-Mg-amphibole). Hannukainen, Finland. Width of sample 9 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196128031282680498
Tonalite. Black biotite, white plagioclase, gray quartz. Kaatiala, Finland. Width of sample 10 cm.

No biotite here. This rock is composed of alkali feldspar and amphibole of the tremolite-actinolite series. However, it is likely that phlogopite was here before the rock was altered to its current composition. Impure metamorphosed carbonate rocks that contain phlogopite, calcite and quartz can be metamorphosed to feldspar-tremolite rock: phlogopite + calcite + quartz → tremolite + K-feldspar + CO₂ + H₂O. Sample from Southern Norway. Width of sample 9 cm.

Biotite with garnet. Unusual pegmatite assemblage. Width of sample 13 cm. Senja, Norway.

Biotite and muscovite in sand. Micas are especially common in river sand because there they are closer to the source rocks. Biotite is not stable in the weathering environment. Width of view 20 mm.

Etymology

The mineral was named in honor of Jean Baptiste-Biot. Biot was a French scientist and as usual in the early 19th century contributed successfully in several fields of study. Although he is not considered to be a geologist, he is the one who claimed that meteorites are extraterrestrial in origin. To be correct, he was not the first one to say so, but the idea got much wider acceptance after that. He also studied materials in polarized light, including minerals. This is the reason why biotite was named in his honor by a German mineralogist Johann Friedrich Ludwig Hausmann in 1847. There are some claims that Biot discovered the mineral biotite. It can hardly be true. Biotite is a very common natural material which I am sure was well-known for our Stone Age ancestors. But he clearly contributed to a better understanding of the micas and deservedly has his family name (usually unintentionally) mentioned in every mineralogy and geology textbook.

Uses

Biotite itself is not very useful material. Mg-rich variety phlogopite is sometimes used as an insulator in electrical applications. But clay mineral vermiculite which forms as an alteration product (either weathering or hydrothermal) of iron-bearing varieties has several uses. Vermiculite is an expanding clay like smectite. It has water between the layers which is the reason why it expands greatly when heated. Heated vermiculite is a light-weight fluffed up material. It is used as a filler and insulating material in the construction industry. Vermiculite is also added to potting soil to improve its quality. Vermiculite enhances drainage and aeration because of low density and absorbs water to later slowly release it when the soil is drying up.

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Vermiculite is an alteration product of biotite which expands greatly when heated (shown on the picture). Expanded vermiculite is a light-weight material well-suited for insulation and also used as a soil conditioner. Width of view 40 mm.

References

1. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.

Banded iron formation

June 12, 2025September 24, 2015 by Siim

Banded iron formation (BIF) is the principal source of iron. BIF is a rock type composed of alternating silica- and iron-rich bands. Banded iron formation is economically among the most important rock types as our society is heavily reliant on iron, which is mostly extracted from this rock.

Banded iron formation consists of layers of iron oxides (typically either magnetite or hematite) separated by layers of chert (silica-rich sedimentary rock). Each layer is usually narrow (millimeters to few centimeters). The rock has a distinctively banded appearance because of differently colored lighter silica- and darker iron-rich layers. In some cases BIFs may contain siderite (carbonate iron-bearing mineral) or pyrite (sulfide) in place of iron oxides and instead of chert the rock may contain carbonaceous (rich in organic matter) shale.

Banded iron formation is a chemogenic sedimentary rock (material is believed to be chemically precipitated on the seafloor). Because of old age BIFs generally have been metamorphosed to a various degrees (especially older types), but the rock has largely retained its original appearance because its constituent minerals are fairly stable at higher temperatures and pressures. These rocks can be described as metasedimentary chemogenic rocks.

Banded iron formations, although extensively mined, remain enigmatic in several ways. Our understanding of their genesis is greatly hampered by the fact that there are no modern analogues. BIFs formed in three episodes 3500-3000 Ma (millions of years ago), 2500-2000 Ma, and 1000-500 Ma. The BIFs from these three episodes are referred to as Algoma-, Superior- and Rapitan-types, respectively. In each case there were different triggers that led to their formation.

http://picasaweb.google.com/107509377372007544953/2015#6196126915729581058
The width of layers is not nearly uniform which somewhat speaks against the hypothesis that the layers are seasonal. Bjørnevatn, Norway. Width of sample 17 cm.
http://picasaweb.google.com/107509377372007544953/2015#6198027961977892626
Average iron content of Bjørnevatn BIF is 31%. See the hand magnet strongly attached to the rock, demonstrating its high magnetite content. Width of sample 36 cm.

Algoma-type is the oldest (from the Archaean) and seems to be associated with volcanic arcs. They are found in old greenstone (metamorphosed mafic volcanics) belts. Iron-rich mineral is almost always magnetite. Algoma-type iron ore bodies are relatively small, usually less than 100 meters in thickness and a few kilometers in lateral extent. Algoma-type deposits are mined in the Abitibi greenstone belt (Ontario, Canada), Bjørnevatn (Norway), Kostomuksha (Russian Karelia), etc.

By far the most important type of banded iron formations formed during the Paleoproterozoic (Superior-type, named after lake Superior). They formed on stable continental shelves. Superior-type deposits are large in dimensions (more than 100 meters in thickness and over 100 km in lateral extent). The main iron-bearing phase is hematite, but magnetite occurs also. Iron mines where BIFs belong to Superior-type include Hamersley Basin (Australia), Kryvyi Rih (Ukraine), Transvaal Basin (South Africa), Labrador (Canada), Lake Superior (Canada, USA), Quadrilatero Ferrifero (Brazil), Singhbhum (India)². Rapitan-type is the least important in terms of the volume of ore mined. Their genesis seems to be related to glaciations and associated environmental changes. Iron-bearing mineral in Rapitan-type deposits is hematite¹. All these terms (Algoma, Superior, Rapitan) refer to localities in Canada, but they are used to classify BIFs worldwide.

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The original banding may be severely disturbed by metamorphic processes. This rock is composed of quartz and magnetite and it comes from the Bjørnevatn mine in Norway (Algoma-type BIF). Width of sample 11 cm.

The material BIFs are composed of comes from the ocean. Iron seems to be mostly provided by the black smokers on the mid-ocean ridges and by the dissolution of the oceanic crust. This is supported by the observation that most BIFs are remarkably free of terrestrial material. It is in contrast with modern ironstones which contain various degree of material from the continents, including the iron itself. The input of black smokers was especially strong with Algoma-type deposits and has diminished with time. The Rapitan-type deposits seem to reflect an average ocean water character at the time. Superior-type remains somewhat of a problem because these deposits are so vast, yet they are located far away from the mid-ocean ridges.

It seems likely that iron-rich water was brought to the continental shelves by upwelling which similarly has provided material for phosphorite deposits. Ferrous iron (Fe²⁺) brought up to the surface waters reacted with oxygen produced there by photosynthetic organisms, especially cyanobacteria. The oxidation of iron may also be a result of ultraviolet radiation inducing the oxidation of ferrous iron to ferric iron.

http://picasaweb.google.com/107509377372007544953/2015#6198018222631607026
Superior-type BIF from North America. Dark gray layers are composed of hematite. Red is jasper (hematitic chert). The rock is about three meters wide and weighs 8.5 tons. Photo by André Karwath. Licensed under CC BY-SA 2.5 via Commons.

The ocean was also an adequate source of silica to form chert layers because the seawater is believed to have been saturated with silica (120 mg/l) during most of the Archaean-Proterozoic². Currently seawater contains only less than 10 mg/l because modern oceans are home to several organisms (diatoms, radiolarians, sponges) that extract silica from the water. This is likely one of the reasons why BIFs can not form in modern conditions. The problem, however, is the precipitation mechanism. It has been suggested that perhaps the evaporation of seawater promoted local silica oversaturation which resulted in silica precipitating as a gel on the seafloor.

Another major problem is the banding of BIFs. We do not have an adequate explanation for that. These bands could represent seasonal cycles as modern varves do. Or it could be some other major cyclical change in ocean water chemistry or biology. It seems likely that there were some form of biological mediation and the changes in BIF composition reflect the cyclical changes in the numbers of respective organisms.

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Superior-type hematitic BIF from Kryvyi Rih, Ukraine. Width of sample 7 cm.

Yet another not fully explained aspect is the formation of BIFs in three distinct periods. This is generally believed to be the result of favorable conditions brought about by changes in tectonics, global climate, biological evolution, and ocean water chemistry. So the explanation needs to be fairly complex and is different for each type which is why we have no generally accepted detailed explanation of BIF formation.

The formation of Superior-type BIFs is closely related to the oxygenation of the atmosphere. Prior to that atmosphere contained little free oxygen because all the oxygen produced by marine photosynthetic organisms was consumed by iron dissolved in the seawater which subsequently after the oxidation settled to the seafloor. Free oxygen finally had a chance to start accumulating in the atmosphere after most of the dissolved iron from the ocean surface layer was precipitated. That took very long time, but finally gave us these vast deposits of iron-rich sediments we are so thankful for and even more importantly oxygen-rich atmosphere without which modern life could have never emerged.

The Rapitan-type BIFs seem to be associated with global ice age (Snowball Earth). The world ocean was almost completely covered by ice and therefore isolated from the atmosphere. That reintroduced reducing conditions in the water column similar to those that existed before the oxygenation of the atmosphere. This near global anoxia in seawater is generally believed to be the reason why BIFs reappeared as iron accumulated in the water and were later deposited when the ice age receded and the ocean was oxygenated again.

This sample, although somewhat similar to BIFs likely has a different genesis. The main constituents are magnetite, quartz and garnet. The latter indicates that the material is terrestrial and deposited either inland or near the coast. It seems to be a lithified and metamorphosed heavy mineral sand deposit. Varanger Peninsula, Norway. Width of sample 18 cm.

References

1. Misra, K. (1999). Understanding Mineral DepositsSpringer.
2. Robb, L. (2005). Introduction to Ore-Forming ProcessesBlackwell Science Ltd.

Hematite

June 13, 2025September 21, 2015 by Siim

Hematite (Fe₂O₃) is an iron oxide mineral. It is widespread in nature, especially in sedimentary environments. Hematite is one of the two principal iron ores. The other is magnetite, which is also an iron oxide mineral. The term ‘hematite’ itself might not be familiar to everyone, but its rusty red color definitely is. Sandstones are often reddish. This is because sand grains are coated with fine-grained hematitic powder. If soil is reddish, then it is also because of hematite. Even red granite owes its color to this mineral. Iron rust itself is similarly colored, but it is mostly composed of hydrated iron oxides.

Hematitic iron ore with goethite (yellow). Fine-grained hematite is dark red, but it has a silvery sheen when crystalline. These colors are both present in the photo above. Svinsås, Norway. Width of sample 13 cm.

Hematite as a widespread natural pigment has been used by man since the earliest times. Red color in cave paintings comes from fine hematitic powder which is known as red ochre. The use of it as a coloring agent continues to this day. Sweden is an especially noteworthy country in this regard because lots of wooden houses there are traditionally brownish red (Falu red). The leftovers of copper mining from Falun and other mines have been used for making red paint there since the 16th century.

But far more important is the use of hematite as an ore of iron. The majority of it is mined from banded iron formations (BIF). These are old (formed generally more than 2 billion years ago) layered metasedimentary iron-silica rocks where the iron-bearing layers are composed of either hematite or magnetite. BIF is very important rock not only as an iron ore, but also because its genesis is closely related to the oxygenation of the Earth’s atmosphere. It is believed that the world ocean used to contain much more dissolved iron which was precipitated as an insoluble iron oxide (and formed an iron-rich layer of BIF) when iron combined with oxygen produced by photosynthetic cyanobacteria. Oxygen started to accumulate in the atmosphere when the iron level in the ocean was reduced to much lower level. So the formation of hematitic iron ore was a necessary precursor that made multicellular oxygen-breathing life possible.

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Hematite is a very important mineral as a principal iron ore. Majority of it is mined from banded iron formations like the sample above from Kryvyi Rih, Ukraine. Dark layers are rich in hematite. Width of sample 7 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196875592624910482
Rich, layered sedimentary hematitic iron ore. Ores like that may form when the original silica has been leached. Width of sample 8 cm.

Hematite is structurally similar to ilmenite (FeTiO₃) which is a major titanium ore. There is a solid solution between the two, but only at high temperature (above 1000°C). But even at room temperature its crystals usually contain some titanium. Iron may also be partly replaced by aluminum or manganese.

Hematite is interestingly versatile in appearance. The rocks and minerals that contain it tend to obtain reddish color. And the streak on an unglazed porcelain is also always cherry red, but crystals of hematite are actually silvery gray with a strong metallic luster. Such crystals are known as specular hematite (also specularite and iron-glance) because of better light reflectance. The habit of crystals is versatile as well. It may form botryoidal kidney ores which have a radial fibrous structure (pencil ore) in the interior of the crystal. Hematite may be platy (micaceous hematite). Such crystals easily give off small reddish flakes which soil the fingers when handled. Hematite may be an alteration product of magnetite and it may retain its crystal shape. If this is the case the material is known as martite (hematite pseudomorphs after magnetite). Hematite itself may be altered to hydrous iron oxides although it is fairly stable in the weathering environment as is normal for a mineral which itself is usually a product of weathering.

http://picasaweb.google.com/107509377372007544953/2015#6196875604541289538
Crystals typically have a radial structure. Width of sample 6 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196875583753452386
Radial aggregates are known as pencil ore. Width of sample 5 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190951005693800162
Hematite may be platy. This sample gives off small micaceous flakes and it also shows radial structure in the middle. Width of sample from Morocco 8 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5848481675389848706
Crystal aggregates have a botryoidal and very reflective surface. Such crystals are known as iron-glance or specularite. Width of sample 6 cm.

Hematite may be weakly magnetic, but usually it does not react to a hand magnet at all. The best diagnostic feature is color. Although the crystals are steel gray the edges if thin enough still demonstrate reddish brown color and crystals that are large enough to show metallic luster are generally suitable for the streak test as well. Goethite is compositionally similar but has a duller brown streak. Cinnabar may be superficially similar because of red color, but it is rarer mineral with much restricted occurrence and it is heavier.

Although hematite is chiefly a mineral of sedimentary environments, it can also crystallize directly from late-stage magmatic fluids. Primary hematite usually occurs in felsic igneous rocks like syenite, granite, trachyte, and rhyolite. The majority of it occurs in (meta)sedimentary rocks like sandstone, banded iron formations, and quartzite.

http://picasaweb.google.com/107509377372007544953/Coll#5774367096818871522
Hematite is a mineral that gives a reddish color to the soil. Here is an outcrop of laterite in Northern Ireland near the Giant’s Causeway.
http://picasaweb.google.com/107509377372007544953/2015#6196128118857984114
Jasper is a reddish impure silica-rich rock. Red color is due to microscopic hematite impurities. Svinsås, Norway. Width of sample 13 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196128132572694994
Jasper with micaceous hematite and magnetite. Svinsås, Norway. Width of sample 10 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196875596238375810
Bauxite is a principal ore of aluminum. Hematite is not the main mineral phase here, but it is enough to give it a strongly reddish color. Width of sample 8 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196127754364659874
Igneous rocks like syenite are often red because alkali feldspar very often contains finely dispersed hematite inclusions. Kiruna, Sweden. Width of sample 10 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5848238745712314674
It is also responsible for the reddish brown coloration in obsidian. Width of sample 11 cm.
http://picasaweb.google.com/107509377372007544953/Sandstone#5781341969155840802
Sandstone contains various amounts of hematite which may give it a visibly layered appearance. Width of the sample from Scotland 7 cm.

Sand grains are usually composed of quartz which is generally not a colorful mineral. They look red because they are partly covered with a very fine-grained hematitic powder. This powder is composed of iron that was once in the crystal structure of minerals like pyroxenes and amphiboles, which are not stable in the weathering environment. Quartz and hematite as resistant minerals remain and form a very common mineral association. Sand sample is from Australia. Width of view 20 mm.

http://picasaweb.google.com/107509377372007544953/Rocks#5850032427024934130
Quartzite as a metamorphosed sandstone may be also reddish because of it. Width of sample 9 cm.

Reddish volcanic glass from California. Width of view 20 mm.

Sandstone outcrop in Estonia — Weakly cemented Devonian sandstone outcrop in Estonia has a reddish hue because of a small amount of hematite it contains.

http://picasaweb.google.com/107509377372007544953/Rocks#5851584483551973970
Red variety of chalcedony is known as carnelian. The sample from Kazakhstan is 14 cm in width.

Troctolite

June 12, 2025September 18, 2015 by Siim

Troctolite is an intrusive igneous rock consisting of plagioclase feldspar and olivine. It is a member of gabbroic rocks family. It is compositionally similar to gabbro. The main difference is that it does not contain pyroxene or contains very little while it is a major mineral in gabbro. It can be described as pyroxene-depleted gabbro.

Troctolite is an olivine-bearing gabbroic rock without pyroxene. Gray mineral is Ca-rich plagioclase, orange is olivine. Olivine has lost its original green color due to weathering. Orange spots are composed of various weathering products of olivine which is collectively known as iddingsite. Flakstadøya, the Lofoten Archipelago, Norway. Width of sample 15 cm.

http://picasaweb.google.com/107509377372007544953/2015#6195818050343685810
Classification diagram of olivine-bearing gabbroic rocks. The field of troctolite is annotated. The rock contains variable amount of plagioclase and olivine (10-90%) but very small amounts (less than 10%) of pyroxene¹.

The genesis of troctolite is also strongly tied to gabbro. These two rocks usually occur together in the same magma intrusion because troctolite can not form directly as magma crystallizes. There is no way how magma can crystallize into olivine and plagioclase without pyroxene. Pyroxene crystals must be separated from the melt by a mechanism that leads to the formation of cumulate rocks. In these rocks some minerals occur in much higher (or lower) concentration than expected. Anorthosite is an example of cumulate rock which is also a member of the same gabbro family. Anorthosite is composed of almost pure plagioclase. Troctolite may be similar to anorthosite and these rocks may smoothly grade into each-other as demonstrated below.

http://picasaweb.google.com/107509377372007544953/2015#6190952353277858754
Anorthosite on the left and troctolite on the right. Flakstadøya, the Lofoten Archipelago, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6195757889372414354
This sample can be described as coarse-grained leuco-troctolite. Flakstadøya, the Lofoten Archipelago, Norway. Width of sample 12 cm.
http://picasaweb.google.com/107509377372007544953/2015#6195757887502746626
Here is a sample of olivine gabbro from the same intrusion on Flakstadøya. The difference is that this rock contains lots of black pyroxene augite. Width of sample 12 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190952317337991970
An outcrop of mafic intrusion on Flakstadøya. The rock surface cropping out is troctolite. The layer of the rock is several hundred meters wide. It grades to anorthosite (in front) and gabbro (behind the scene).
http://picasaweb.google.com/107509377372007544953/2015#6190952319469711250
A closer look of the same outcrop reveals the spotted gray-orange appearance of the rocky surface.

Troctolite as demonstrated above has a speckled appearance which is the reason it carries such a name – troctolite means troutstone in Greek. ‘Troutstone’ is a synonym of troctolite in English and German (forellenstein) also².

References

1. Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.
2. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.

Granite

June 13, 2025September 14, 2015 by Siim

Granite is a crystalline igneous rock that consists largely of feldspar and quartz. These two are the most common minerals in the crust which means that granite too is among the most ubiquitous rock types, especially in the upper continental crust.

This picture of a granite pegmatite from northern Norway (Nyelv) is very coarse-grained for a normal granite and compositionally simpler than most granite samples, but it illustrates well what granite is. It is crystalline (composed of visible mineral grains) and always contains quartz (gray) and also feldspar (red). *Width of view is about 50 cm.*

Although it is indeed very widespread, not every rock that is named so is a true granite. This rock name may well be the most abused geological term in existence. This article aims to explain what really is granite and what it is not.

Classification

Granite is a coarse-grained quartzo-feldspathic igneous rock which contains 20-60% quartz of total quartz + feldspar (other minerals are neglected) and 35-90% of the feldspar is alkali feldspar (orthoclase or microcline). Rocks which fulfil the quartz content requirement but have a different ratio of feldspars have different names although they are known as granitoids or granitic rocks too. Granitic rocks which contain less than 10% plagioclase feldspar are named alkali feldspar granite and granitic rocks where the dominant feldspar is clearly plagioclase (over 65%) are named either granodiorite (65-90% of feldspar is plagioclase) or tonalite (over 90% of feldspar is plagioclase)². See the diagram below:

http://picasaweb.google.com/107509377372007544953/2015#6193580045622566322
Classification of granite (red field) and granitic rocks (red+yellow) is based on the content of quartz (Q) (in relation to feldspar) and the mineralogy of the feldspar group minerals (A and P).

These rocks are known as granitoids (rocks resembling granite) because it may not be easy to distinguish different feldspar group minerals in the hand sample. But it should remain just a field term. It makes sense to classify granitic rocks based on the feldspars because rocks from different tectonic regimes tend to have different feldspar content. True granites, for example, are virtually absent in the oceanic crust although there are granitic rocks which are rich in plagioclase (tonalite).

Similarly coarse-grained igneous rocks that contain less quartz are named syenite (feldspar is mostly alkali feldspar), monzonite (contains both feldspars in roughly equal amounts) and diorite (plagioclase-rich). Rocks that contain more than 60% quartz as an average of voluminous igneous intrusion are uncommon because silicic magma from which granite crystallizes almost always contains enough potassium, sodium and calcium to form lots of feldspars in addition to pure silica (quartz). Feldspars actually precede quartz in the crystallization order from magma. So quartz can form only if there is free silica left after other cations have already satisfied their need for silica. In the vast majority of cases it contains more feldspar than quartz.

http://picasaweb.google.com/107509377372007544953/2015#6190953010689058114
Trondhjemite is a leucocratic variety of tonalite. It is a granitic rock, but not true granite because the feldspar it contains is sodic plagioclase, not alkali feldspar. Width of sample from Norway is 10 cm.

Syenite may be similar to granite, but it does not contain enough quartz. This sample is quartz alkali feldspar syenite from Estonia. Width of sample 8 cm.

Texture

Granite is a coarse-grained igneous rock with average grain size ranging from 1 to 25 millimeters¹. These rocks crystallize from a very slowly cooling magma within the crust where they are well insulated. Time gives crystals a chance to grow. Volcanic rocks with a similar composition exist as well. These are known as rhyolite (volcanic equivalent of granite) and dacite (similar in composition to plagioclase-rich granitoids). The groundmass of these rocks is very fine-grained although they frequently contain phenocrysts (larger crystals that were already formed before the extrusion to the surface) embedded in the finer matrix. Granite that crystallized in a narrow dike may be fine-grained because the heat was rapidly lost to the adjacent rocks. Such granite is known as aplite. Sometimes granite is very coarse-grained. That too happens usually close to the margins of a granitic pluton, but the coarseness is mostly a result of volatiles in the magma which greatly reduce its viscosity and therefore enhance crystal growth. These coarse-grained granites are known as pegmatites.

It has often roughly uniformly sized crystals which show no preferred orientation but that is not always the case. Some granites just like rhyolites contain phenocrysts – crystals that are clearly bigger than the material surrounding it. These phenocrysts are usually feldspar crystals. The difference between granite and rhyolite is that in granite even the finer material is visibly crystalline, but in rhyolite the individual crystals within the groundmass are not visible to the naked eye. Elongated minerals may have a preferred orientation. Sometimes so much that it is no longer clear whether it is a granite or gneiss. Some granites have special textures like rims of one mineral around the other. Rapakivi granite is a notable example.

http://picasaweb.google.com/107509377372007544953/2015#6190951122739162034
Typical granite has visible mineral grains without any orientation. Orange mineral is alkali feldspar, gray is quartz. Picture taken in Norway. Width of view 20 cm.
http://picasaweb.google.com/107509377372007544953/2015#6194282136332693778
It may have a preferred orientation. Note how most of the biotite flakes are aligned sub-parallel to each-other. Lødingen, Hinnøya, Norway. Width of sample 15 cm.
http://picasaweb.google.com/107509377372007544953/Coll#5808110375348210322
Aplite in the lower half is much finer than granite above it. Their composition seems to be the same. There are larger phenocrysts of quartz and feldspar within the granite. Width of sample 12 cm.

Gneiss sample — Gneiss and granite may be the same compositionally, but elongated mineral grains are strongly oriented and segregated in gneiss. The sample of gneiss is from Karelia. Width of sample 11 cm.

Rhyolite and dacite are extrusive versions of granitic rocks. Their groundmass is very fine-grained, but they often contain phenocrysts, mostly feldspars. The sample from Scotland is 8 cm in width.

http://picasaweb.google.com/107509377372007544953/2015#6194282159317086338
Rapakivi texture (plagioclase rims around ovoidal phenocrysts of alkali feldspar) in granite. Luumäki, Finland. Width of sample 17 cm.
http://picasaweb.google.com/107509377372007544953/2015#6196126451189924914
Graphic texture (quartz intergrowth in feldspar that resembles runic script) is common in pegmatites. Quartz crystals are uniformly scattered in the feldspar crystals and not randomly oriented. These crystals grew simultaneously with alkali feldspar host and they are optically continuous (one crystal). The term “pegmatite” was originally used to describe graphic granites by Haüy in 1822. Graphic granites may indeed be described as pegmatites even according to current usage, but the term “pegmatite” nowadays has much wider meaning, encompassing also rocks which have no graphic texture nor do they have to be granitic in composition⁴. Evje, Norway. Width of sample 9 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190951568325813794
A contact between gneiss (granodioritic with migmatitic granite veins) and granite pegmatite. Nyelv, Norway.

Composition

Average chemical composition of granite determined by 2485 chemical analyses of granitic rocks³ (numbers are mass percents, recalculated volatile-free to total 100%):

SiO₂ — 71.84
TiO₂ — 0.31
Al₂O₃ — 14.43
Fe₂O₃ — 1.22
FeO — 1.65
MnO — 0.05
MgO — 0.72
CaO — 1.85
Na₂O — 3.71
K₂O — 4.10
P₂O₅ — 0.12

Notable is a very high SiO₂ which is the reason why it contains so much quartz. The amount of iron, magnesium and titanium are low when compared with basaltic rocks.

Minerals quartz and feldspar take most of the silicon, oxygen, potassium, sodium, aluminum, and calcium. Small amounts of other chemical elements will find their place in the crystal lattice of apatite (accommodates phosphorus), magnetite and ilmenite (iron and titanium), biotite and hornblende (iron, magnesium), muscovite (potassium, aluminum, fluorine), zircon (zirconium), titanite (titanium), monazite (rare earth elements, phosphorous). Some granites or granite-like rocks may contain pyroxene or even olivine which are rich in iron and magnesium.

Dark minerals are usually either biotite (mica) or hornblende (amphibole group mineral). It is usually biotite in potassium-rich true granites and more often hornblende in granodiorites and tonalites.

Pegmatites may be either simple (ordinary granite with large crystals) or complex. In the latter case granite has an elevated concentration of rare chemical elements which give rise to rocks with unusual composition. Minerals that often occur in granite pegmatites are tourmaline (contains boron), topaz (fluorine), spodumene (lithium), cassiterite (tin), fluorite (fluorine), lepidolite (lithium), zircon (zirconium). Complex pegmatites may be very valuable as mineral resources. Simple pegmatites may be mined as well, mostly because of large muscovite flakes they often contain.

http://picasaweb.google.com/107509377372007544953/2015#6194282129261240274
Granite pegmatite with tourmaline (schörl). Kaatiala, Finland. Width of sample 11 cm.
http://picasaweb.google.com/107509377372007544953/2015#6194282130920969538
Lepidolite (lithium mica) with cleavelandite (variety of albite) in a pegmatite. Haapaluoma, Finland. Width of sample 12 cm.

Types

Several classification have been propesed for granitic rocks. The most common way is to divide them into (I,S,M,A) types. Most of these letters refer to protoliths. I-type granite is the most common one which is believed to have an igneous protolith. S-type granites originate at least partly from a sedimentary source. M-type granites are rare and are supposed to have mafic protolith although this is somewhat problematic because mafic rocks do not yield granitic melt. ‘A’ means anorogenic. These are granites from plutons that were not associated with magmatism related to subduction processes and mountain-building. This classification scheme, although still widely in use, is somewhat problematic and inconsistent.

Newer and perhaps better or at least more easily quantifiable method is to use alumina saturation index. In this scheme the molecular ratio of Al₂O₃/K₂O + Na₂O + CaO is calculated. This ratio is useful because it is 1 in feldspars. So the excess or deficiency of aluminum must be accommodated by other minerals.

If granite is rich in aluminum, it contains muscovite in addition to feldspars plus other aluminum-bearing accessory minerals like corundum, tourmaline, cordierite, etc. These rocks are known as peraluminous granites.

Rocks which have Al₂O₃/K₂O + Na₂O + CaO below 1 but Al₂O₃/K₂O + Na₂O over 1 are metaluminous. These rocks are alumina deficient which means that minerals like biotite (Al-poor) and hornblende must crystallize in addition to feldspars.

Third category of granites (known as peralkaline) are even more deficient in alumina: Al₂O₃/K₂O + Na₂O < 1. These rocks contain alkali pyroxenes and amphiboles (aegirine, riebeckite) and even iron-rich olivine fayalite.

http://picasaweb.google.com/107509377372007544953/Rocks#5808533358262843506
Peraluminous granite with garnet phenocrysts. Granites that contain lots of aluminum may have a sedimentary protolith (S-type granites). Aluminum comes from pelitic (rich in clay minerals) sedimentary rocks. Width of sample from Estonia is 8 cm.
http://picasaweb.google.com/107509377372007544953/Rocks#5808533379571331314
Peraluminous granite from Estonia. The excess of aluminum is accommodated in light-colored mica muscovite. Width of sample 10 cm.
http://picasaweb.google.com/107509377372007544953/2015#6194386930081261026
Metaluminous granite containing lots of hornblende (black). Width of sample 9 cm.
http://picasaweb.google.com/107509377372007544953/2015#6194386929663937778
Peralkaline granite pegmatite. Black mineral is amphibole riebeckite. Width of sample 7 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190952952147907170
Peralkaline granite with iron-rich end-member of olivine group: fayalite. Southern Norway. Width of sample 18 cm.

Charnockite is an enigmatic rock which compositionally could be a subtype of orthopyroxene-bearing granite, but it probably formed as a result of metamorphic processes deep in the crust. It is perhaps better to classify such rocks as granulites. Flakstadøya, Lofoten Islands, Norway. Width of sample 9 cm.

Occurrence

Granite is one of the main ingredients of the continental crust. Although continental landmasses are composed of many different igneous, sedimentary and metamorphic rocks, originally they were derived from the mantle by a process called partial melting which will step-by-step lead to the generation of granitic magma. When rocks melt the liquid that forms has different composition than the original source rock. When peridotite melts, basaltic rocks form which contain more silicon and aluminum and less iron and magnesium. Basalt is because of its chemical composition more light-weight and moves upward and solidifies as a lava on the surface or as dikes in the crust. When basalt melts even lighter magma forms which will lead to the formation of granite. It is therefore a result of a remelting of the material from the continental crust.

Granite is so light-weight when compared to the original peridotite that it can not subduct back to the mantle. So it remains buoyant as a young continent. These continents will go through multitude of processes. Continents collide and metamorphose, they are worn down by weathering and igneous processes add new material. It has taken very long time. The process started probably right after the formation of the Earth as a planetary body. The cores of continents formed in the Archaean and they tend to grow bigger because the continental crust that has already formed can not sink to the mantle again. As a result continents are structurally complex mixtures of virtually every rock type known to us. Granite being perhaps the most important of them.

Not all of these rocks are granites in the strict petrological sense. Major part of this material is clearly metamorphosed and should be described as gneiss. However, our knowledge about the interior of the crust mostly comes from the seismic studies and for the seismic waves it does not make any difference whether it is granite or a granitic gneiss. This is why it is often assumed that the continental crust is granitic. Furthermore, the composition of this material is not necessarily granitic in the strict sense. Plagioclase feldspar is more common in the crust than alkali feldspar. So it is fair to assume that the dominant rock type in the continental crust is a metamorphic rock with a granodioritic composition.

True granites occur mostly in plutons which are pancake-shaped igneous intrusions in the upper crust. Sometimes they reach the surface when the material above them has been removed by the weathering processes. These plutons vary enormously in size (1-1,000,000 km³)¹. The mechanism how they formed has been very controversial. The dominant hypothesis has been that they are the result of igneous diapirism – igneous material rises through the crust like a hot balloon. The formation of some granite plutons may be partly explained that way, but it seems more likely that the migration of granitic melt took place in the network of narrower cracks in the rocks. It seems rather difficult to understand how can very large balloon of hot magma move through the cold and rigid rocks of the upper crust. The formation process of granitic melt is known as migmatization and the rocks that contain metamorphic rocks mixed with magmatic veins are known as migmatites.

http://picasaweb.google.com/107509377372007544953/Coll#5776562547968632530
Much of the ‘granitic’ continental crust is actually made of granodioritic gneisses like the example above. Karelia, Russia (Archaean Fennoscandian Shield). Width of sample 16 cm.
http://picasaweb.google.com/107509377372007544953/2015#6194335090254198626
Migmatite erratic on the northern coast of Estonia. Migmatites are mixtures of metamorphic and igneous rocks. Igneous material usually has a granitic composition and is either a result of partial melting in situ or is a magma that formed elsewhere and intruded into the rocks where we see it now. Migmatization seems to be the mechanism how granitic magma is formed and how it migrates in the crust.
http://picasaweb.google.com/107509377372007544953/2015#6194337066073112130
Outcrop of a granite intrusion north of the High-Atlas mountain range in Morocco.
http://picasaweb.google.com/107509377372007544953/2015#6190952280371307266
Granite cutting through calc-silicate schist. Gimsøy, Lofoten Archipelago, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6190952363734480450
Large granite dike (several meters wide). Å, Lofoten Archipelago, Norway.

Uses

Granite has several uses. It may contain valuable minerals. These may be either gemstones or industrial minerals. Pegmatites are especially rich source of both beautiful and rare crystals and minerals with useful properties. Zircon, beryl and tourmaline crystals come from pegmatites. Also industrial minerals like micas and feldspars are taken from pegmatites because large grain size makes the separation process easier. Zirconium and beryllium are extracted from granite pegmatites (minerals zircon and beryl, respectively). Granite may contain ore minerals. Tin and tungsten ores, for example, are hosted by granite.

Granite is an important construction material. It is hard and durable which makes it a very good material for aggregate. It is one of the most important dimension stones because of beautiful textures and colors. It is also massive and durable – ideal material for countertops. It must be said, however, that not all of these granites are true granites. Some of them are metamorphic although are granitic in composition. But sometimes rocks with much lower quartz content are named granite. For a geologists they may be diorite, monzonite, syenite, diabase, gabbro, etc. These rocks are also known as commercial granite.

http://picasaweb.google.com/107509377372007544953/2015#6190951401996398930
Rapakivi granite is a variety of granite widely used as a dimension stone. Luumäki, Wyborg batholith, Finland. Width of view 35 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190951563426090962
This ‘granite’ from Norway (Nyelv) is actually migmatitic gneiss, but such rocks are usually marketed as granite (commercial granite).
http://picasaweb.google.com/107509377372007544953/2015#6194393806570153730
Granite is an excellent material when hard and durable rocks are needed. These granitic rocks are used as a railroad ballast in Estonia.

References

1. Barker, Fred. (2007). Granite. In: McGraw Hill Encyclopedia of Science & Technology , 10th Edition. McGraw-Hill. Volume 8. 202-204.
2. Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.
3. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
4. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.

Diopside

June 13, 2025September 8, 2015 by Siim

Diopside is a mineral of the pyroxene group. It is closely related to the most common pyroxene augite. Composition of pure diopside is CaMgSi₂O₆ but there is a complete solid solution to hedenbergite CaFeSi₂O₆ and also to augite (Ca,Mg,Fe²⁺,Fe³⁺,Al)₂(Si,Al)₂O₆. These three are collectively named calcic clinopyroxenes and the boundaries between them are arbitrarily chosen to be percentages of Fe/Mg and Ca. How the classification works is shown on the diagram below.

http://picasaweb.google.com/107509377372007544953/Rocks#5851064555789504994
The classification principles of pyroxenes. Diopside contains more magnesium than iron and its Ca content is above 45% of the sums of Fe+Mg+Ca. This diagram does not have enough variables to display all the pyroxene group minerals. There are also solid solutions toward Na-clinopyroxenes (Na substituting for Ca) which gives rise to more members of the group. Wollastonite shown in the upper corner is not a member of the group. There is no solid solution between pigeonite and augite. The same diagram is available in PDF version also.

The diagram above may leave an impression that what is diopside and what is not is completely artificial but it is not that bad. All three end-members have clearly distinct rocks where they occur, although there are some overlaps – sometimes wrong minerals occur in the wrong rocks. Just in case someone thinks that I am serious I will explain: minerals never occur in the wrong rocks. It is our knowledge and understanding that is incomplete or our classification principles are inflexible and cumbersome. What I want to say is that although pyroxene in gabbro is usually augite, sometimes it is diopside. And pyroxene in skarn, although usually diopside, may sometimes turn out to be augite. But eventually we have to decide where to draw a line when there is a solid solution case and it is often decided that 50% or other round numbers are the way to go.

Diopside in an ultramafic rock wehrlite (peridotite). The yellowish green mineral is olivine, purple is pyrope. In mafic and especially ultramafic rocks it may contain significant amount of chromium which gives it intense green color. Åheim, Norway. Width of sample 16 cm.

Augite — Crystals of Ca-clinopyroxenes are short and not as slender as, for example, crystals of Na-pyroxene aegirine. This is a crystal of augite (35 mm across).

Pure diopside contains no iron which means that there are no chromophore elements in its composition and the crystal should be colorless. That is indeed the case, although pure diopside is somewhat rare. It usually does contain variable amounts of chromophores like iron and chromium, which give it a greenish hue. Mg-rich variety with well-developed crystal faces is a semi-precious gemstone as is the chromian variety because of its intense green color.

Density of the mineral is 3.19 g/cm³, but it increases with increasing iron content. Hardness is about 6 and the crystals have two cleavage planes intersecting at 93/87 degrees as is typical to all the pyroxenes.

Diopside occurs mostly in basic and ultrabasic igneous rocks or in metamorphic rocks which are compositionally mixtures of silicates and carbonates. The first group includes rocks from the mantle which may be colorful as are the example pictures of pyrope- and diopside-rich peridotites in this article. It also occurs in kimberlites and it is an indicator mineral in diamond prospecting because intensely green chromian varieties are easily spotted in the heavy mineral fraction of sand. It is a common mineral in these rocks because the mantle is rich in magnesium when compared to the crust. Calcic clinopyroxenes in mafic rocks like basalt and gabbro are usually augitic in composition, but some contain iron- and aluminum-rich diopside (especially alkali basalts).

The latter group of metamorphic diopside-bearing rocks includes skarns and some marbles. These rocks form when silicate magma reacts with surrounding carbonate rocks (dolomite, limestone, marble) or when carbonate rocks with a significant silicate component are thermally metamorphosed. Skarns frequently contain ore minerals like magnetite and various sulfides and are therefore often mined. Skarn is a Swedish mining term for the gangue minerals surrounding the valuable ore. It is a major mineral in many skarn samples.

Diopside just as most pyroxenes has few uses. It is a semi-precious gemstone and it also has been used in dental ceramics.

Diopside as a Ca-bearing silicate is a common mineral in skarns which form when silicate magma reacts with carbonate rocks. This sample is composed of calcite (pink), actinolite (black slender amphibole) and dull green diopside. Tapuli, Sweden. Width of sample 10 cm.

http://picasaweb.google.com/107509377372007544953/2015#6190954044428015794
Abundant pyrope (Mg-rich garnet) and chromian diopside in peridotite from Åheim, Norway. Width of view 20 cm.
http://picasaweb.google.com/107509377372007544953/2015#6190953185637630898
Diopside forming rims around pyrope in peridotite from Åheim, Norway. Width of sample 10 cm.
http://picasaweb.google.com/107509377372007544953/2015#6191004616559617970
Layered ultramafic intrusion where olivine-rich dunite alternates with pyrope-diopside pyroxenite layers. Width of view is about 30 cm. Åheim, Norway.
http://picasaweb.google.com/107509377372007544953/2015#6191004786537859586
Green mineral here is not diopside, but it is also monoclinic pyroxene and compositionally not far from it. This mineral is omphacite and the rock is eclogite. Nordfjord, Norway.

Barite

January 11, 2016September 4, 2015 by Siim

Barite (spelled also baryte) is a barium-bearing mineral (BaSO₄). It is mined both as an industrial mineral and as an ore of barium. This is somewhat unusual situation. Another well-known mineral mined as an ore and industrial mineral is zircon. Barite as the main barium-bearing phase in the crust is not uncommon mineral.

http://picasaweb.google.com/107509377372007544953/2015#6190279832136792882
Intergrown tabular crystals (rosette) from Morocco. Width of sample 10 cm.

Baryte is usually quite close to ideal composition although solid solution to celestine (SrSO₄) is possible. Pure mineral is colorless, white or gray, but impurities (mainly iron compounds but also sulfides and organic matter) often give it slight yellowish or reddish hue. Blue coloration is usually due to radiation by radium atoms (Ra may replace Ba in the crystal structure because they are similar in size).

Barite may occur in well-formed crystals grown in hydrothermal veins. In these veins it is often accompanied by galena and sphalerite — lead and zinc ores, respectively. Other minerals that commonly occur with baryte are pyrite, quartz, fluorite, carbonates and other sulfidic ore minerals.

In the majority of cases it is found in various mineral aggregates. The most notable ones are platy and intergrown rosettes which are called desert roses or barite roses (gypsum also forms similar rosettes). Barite from sedimentary rocks may resemble marble – light-colored, massive and crystalline, but it is easily recognizable by its weight. It is strikingly heavy (specific gravity about 4.5) for a mineral without metallic appearance (even the name baryte itself comes from Greek barys which means heavy). Crystals may be mistaken for feldspar, but again weight gives it away and it is also significantly softer mineral (hardness about 3 on Mohs scale). Calcite and barite crystals may look alike, but calcite reacts vigorously with dilute HCl while barite does not react. It also occurs in various sedimentary rocks as cavity-filling concretions. It may be a residual mineral in clayey sediments in weathered limestones (limestone dissolved and carried away, insoluble barite and clay left behind). Barite may be a cementing mineral in sandstones.

http://picasaweb.google.com/107509377372007544953/2015#6191013925389818002
Crystals may be mistaken for other more common minerals like feldspar or calcite, but barite is much heavier. Width of sample 8 cm.

Barite is the principal ore of barium. Barium has very high number of applications. It is known to have more than 2000 industrial uses. However, the majority of barite mined is not used to extract barium. It is valuable in its native form because of high density. It is mostly used by oil & gas industry as an ingredient of drilling mud. This slurry is pumped into the drill stem to prevent gas and other fluids from entering the wellbore during drilling operations. The drilling mud also lubricates and cools the drilling bit and carries rock cuttings back to the surface, but that could be accomplished by any drilling fluid. It is added to the mud only to increase its density.

http://picasaweb.google.com/107509377372007544953/2015#6191013925510729730
Barite may sometimes resemble marble. Width of the sample from Kazakhstan 14 cm.

One peculiar use is swallowing it in chemically purified form (blanc fixe) in substantial quantities to make the gastrointestinal tract (or throat) more visible in X-ray images. It is somewhat odd to think about that because barium compounds are usually very toxic. This practice is considered to be of low risk because barite is very insoluble and chemically inert mineral. Blanc fixe is also used as a filler in paper and cosmetics and as a pigment. Playing cards, for example, are filled with barite to make them heavier.

http://picasaweb.google.com/107509377372007544953/2015#6191111887168256306
Barite concretions from the Cretaceous marine shale (Bearpaw Formation, Saskatchewan, Canada). Largest concretion is slightly over 4 cm in diameter. Photo taken by Howard Allen (see the comments).

Rhomb-porphyry

January 11, 2016August 30, 2015 by Siim

Rhomb-porphyry is a porphyritic igneous rock with abundant wedge- or lens-shaped anorthoclase (feldspar) phenocrysts. Rhomb-porphyry is a rare rock type. The most well-known is the rhomb-porphyry from the Oslo Rift in Norway. Similar rocks are known to exist in only two other locations: The East African Rift Valley and the Antarctic¹. They are all volcanic areas with active or former (when the rocks formed) continental rifting.

Rhomb porphyry
Rhomb-porphyry has an attractive appearance and is thus famous not only among geologists. The rock is from the Oslo Rift, Norway.

Rhomb-porphyry is compositionally trachy-andesitic (latite) which corresponds to plutonic rock monzonite.

Although rare globally, these rocks are very common in Norway just south-east of the capital Oslo. There are numerous lava flows (over 70) and many dikes. Lava flows cover over ten thousand square kilometers and dikes may reach thicknesses up to 80 meters. The most voluminous flow unit consists of about 1,000 cubic kilometers of lava. This is huge amount which has no historic analogues although other pre-historic lava flows of that magnitude are known. Thicknesses of a single lava flows vary between 4 and 130 meters¹. The volcanism that produced rhomb-porphyry lava started about 295 million years ago and lasted for 20 million years². Rhomb-porphyry lava also has a plutonic version known as larvikite which is highly valued dimension stone because of large feldspar crystals with bluish iridescence.

Rhomb-porphyry should not flow easily given its high silica content and abundance of phenocrysts. Yet it covers huge area as both subaerial lava flows and dikes. Field observations indicate that rhomb-porphyry lava behaved pretty much as basaltic lava is supposed to behave. This is in stark contrast to modern volcanoes with trachy-andesitic composition which tend to erupt violently and produce more ash than lava. To this day it remains somewhat of a mystery. However, we do know that rhomb-porphyry has abnormally high fluorine (0.2-0.45 %) content which might have lowered the viscosity of the magma. Continental rifts are known to produce odd volcanic phenomena. Perhaps the most well-known is a carbonatitic magma which compositionally is more like a limestone than normal igneous rock. Rhomb-porphyry is just another rock from a continental rift with unusual characteristics.

Rhomb porphyry from Norway
Different lava flows have different geochemical fingerprints and they also differ in appearance. Not all of them have rhomb-shaped phenocrysts. Width of sample 7 cm.

Rhomb porphyry rock sample
It is actually easy to see that rhomb is not the best analogue to describe the shape of the phenocrysts. They are highly variable but none of them seems to be rhomb-shaped. I would say that the most beautiful ones resemble lenses or wedges.

Rhomb porphyry fresh surface
Fresh surface of a rhomb-porphyry sample from Norway. Width of sample 13 cm.

References

1. Ramberg, I. B. et al. (2008). The Making of a Land – The Geology of Norway. Geological Society of Norway.
2. Sundsvoll, B. et al. (1990). Age relations among Oslo Rift magmatic rocks: implications for tectonic and magmatic modelling. Tectonophysics. Volume 178, Issue 1.

Chalcopyrite

June 12, 2025August 27, 2015 by Siim

Chalcopyrite is the principal source of copper. It is also the most likely source material for many other notable copper-bearing minerals like turquoise, malachite, cuprite, azurite, etc. These minerals form in the supergene environment (close to the surface) where chalcopyrite is not stable in contact with groundwater. Chalcopyrite may also alter to iron-bearing minerals because it contains iron as much as copper. Its chemical formula is simple as is common among the sulfide mineral group: CuFeS₂.

Chalcopyrite and magnetite. Width of view 30 mm.

Chalcopyrite and magnetite. Width of sample 6 cm.

Pyrite and chalcopyrite — Pyrite is paler and greenish yellow on the left. Chalcopyrite next to it has much more intense yellow color. Reddish patch on the left is an igneous rock monzonite. Width of sample 8 cm. Hannukainen, Finland.

Chalcopyrite, as the name suggests, is somewhat similar to the most common sulfide mineral pyrite but the difference becomes obvious when we have a chance to see them next to each other:

Copper-minerals in a supergene zone — Greenish copper-bearing minerals on the wall of an abandoned copper mine in Norway. These minerals are alteration products of chalcopyrite. Dragset, Norway.

It is often said that pyrite is a fool’s gold but chalcopyrite is actually much better candidate to be mistaken for gold because of richer yellow tone. It is also significantly rarer mineral than pyrite which is chemically simply iron sulfide.

The source material of chalcopyrite comes from magma but it often crystallizes outside of the magma body with other ore minerals and of course quartz which forms the backbone of these mineral veins.

Chalcopyrite with quartz — Chalcopyrite and quartz precipitated from hot aqueous (hydrothermal) solution. Width of sample 12 cm.

Quartz with ore minerals chalcopyrite, pyrite, pyrrhotite, magnetite. Width of sample 11 cm.

Chalcopyrite may be an important constituent of skarn ores if the magmatic liquids were rich in copper. Skarn is a rock type that forms when hot silicate magmatic liquids are in contact with carbonate rocks (limestone, dolomite, marble). The result is a Ca-bearing silicate minerals, mostly certain amphiboles and pyroxenes but also Ca-garnets, epidote, wollastonite. These rocks are also often rich in ore minerals. The name ‘skarn’ itself is given to these rocks by Scandinavian miners. This is the useless part (gangue) of the ore body which is surrounding the richest mineralized vein. For the geologists and rockhounds, on the other hand, skarns are a rich source of many interesting and uncommon minerals.

Skarn sample with ore minerals — Here is a sample of skarn with ore minerals. Skarn minerals are Ca-rich amphibole actinolite and calcic clinopyroxene. Width of sample 8 cm. Hannukainen, Finland.

Polymetallic ore — A polymetallic ore sample with chalcopyrite, magnetite and pyrite being the main phases. Width of sample 9 cm. Hannukainen, Finland.

copper and iron ore — A closer view of the same sample. Gray is magnetite and bright yellow is chalcopyrite.

Chalcopyrite is a sulfidic mineral and very likely occurs with pyrite which is a well-known nuisance causing an array of environmental problems. These minerals decompose easily and turn groundwater acidic which also helps to leach out other heavy metals from minerals often associated with them.

Copper mine dumps — An abandoned copper mine. Even the air has a specific sour smell in these desolate places. The dumps of the Dragset copper-zinc mine, Norway.

Metamorphosed heavy mineral sand

December 2, 2015August 19, 2015 by Siim

Topics surrounding heavy minerals have been discussed here before but so far not as a rocky subject. I recently returned from Northern Norway, where I stumbled on a beautiful red-black-white folded metamorphic rock on a rocky coastline of the Varanger Peninsula. There were lots of interesting rocks, but something like this one really caught my eye.

http://picasaweb.google.com/107509377372007544953/2015#6191124032243014386
Magnetite, garnet and quartz in a metamorphosed placer deposit. Eastern coast of the Varanger Peninsula, Barents Sea, Northern Norway. Width of sample 18 cm.

http://picasaweb.google.com/107509377372007544953/2015#6190951699030876802
http://picasaweb.google.com/107509377372007544953/2015#6190951639222326754
More rocks from the same formation.

The rock is composed of quartz (white), magnetite (black) and red garnet (almandine). These minerals are very common ingredients of heavy mineral (placer) deposits. I cannot imagine any other genesis so I assume that this rock is a former heavy mineral concentrate which got buried and finally metamorphosed to a rock-type similar to quartzite. But obviously it is no quartzite because quartz forms clearly less than half of the composition. How to name it? I don’t know. I would like to know if some of my readers have an idea. That rock would be a nice example of a BIF (banded iron formation), but these rocks normally contain no garnet. It is still a potential iron ore because it is very rich in magnetite (strongly attracts a hand magnet) although I am afraid it does not form a deposit which is extensive enough.

UPDATE: Pedro Castiñeiras (@PetroMet) suggested Grt-Mag-Qtz banded granofels or Grt-Mag-Qtz metaplacer as a name for the rock. I like the latter more as it also gives information about the genesis, which in this case seems to be important.

This is how the source deposit of this rock might have looked like. This is a modern heavy mineral rich beach sand from the Pfeiffer Beach, California, USA.