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.

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.

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:

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.

http://picasaweb.google.com/107509377372007544953/2015#6190953263891228866
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.

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.

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.

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.

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.

http://picasaweb.google.com/107509377372007544953/2015#6199465205242293154
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.

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, 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:

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 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.

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.

Feldspar is the most abundant mineral group in the Earth’s crust. There are more feldspars (60%) than all the other minerals combined in the outer (13-17) km of the crust¹. Feldspars occur in most igneous and metamorphic rocks. They are less common in sedimentary rocks as they tend to break down to clay minerals in the weathering environment. Feldspars are broadly divided into two groups: alkali feldspars and plagioclase feldspars. Common alkali feldspars are orthoclase, microcline, adularia, and sanidine. Plagioclase feldspars are albite, oligoclase, andesine, labradorite, bytownite, and anorthite.

Feldspars occur in many rock types, but one rock type is composed of nearly pure feldspar – anorthosite. Very large feldspar crystals occur in pegmatites and well-developed crystals are found in hydrothermal veins.

Feldspars are hard (6-6.5 on Mohs scale) and have a glassy luster. They have a good cleavage (forming step-like surface in broken rock faces) which helps to distinguish them from quartz which is often associated with feldspars in rocks. Density of feldspar is 2.56 (K-feldspar) and ranging from 2.62-2.78 (albite to anorthite) which makes them relatively light-weight minerals among other silicate rock-forming minerals. Only Ba-feldspar (3.37) is clearly among the heavy minerals.

Feldspars are so common because their constituent elements (Si,Al,O,Na,K,Ca) are ubiquitous. Alkali feldspars are silicates that contain K and Na. That is why they are called alkali feldspars (Na and K are alkali metals). Plagioclase feldspars form a solid solution series between Na and Ca end-member.

Feldspars are structurally complex minerals. The complexity arises from the fact that Na, Ca, and K plus Si and Al do not easily replace each other in the lattice because of different ionic radii and charges. Exsolution is common because feldspar crystals that are stable at high temperature tend to break down when cooling. Feldspars are also very prone to twinning which in some cases makes the identification easier and may also provide a beautiful play of colors.

Granite with white sodium-rich plagioclase albite, pink K-feldspar, black biotite, gray quartz. Width of view 21 cm.

Composition

Feldspars include three compositional end-members: K-feldspar (KAlSi₃O₈), albite (NaAlSi₃O₈), anorthite (CaAl₂Si₂O₈). Abbreviations commonly used are Or (K-feldspar), Ab (albite), An (anorthite). ‘Or’ is an abbreviation of a common K-feldspar orthoclase, but sometimes it is used as a symbol of all the alkali feldspars.

Alkali feldspars are compositionally between Or and Ab end-members. Plagioclase feldspars are between Ab and An. There are no feldspars that are intermediate in composition between K (Or) and Ca (An) end-members because these ions have different ionic radii and charges which would make the structure unstable.

Some uncommon feldspars contain barium in the crystal structure. They are celsian (BaAl₂Si₂O₈) and hyalophane (compositionally between K-feldspar and celsian). They are also structurally similar to K-feldspars. These minerals are volumetrically very restricted, nowhere near as common as other feldspars.

Plagioclase feldspars are the most common feldspar minerals because calcium is somewhat more common in the crust than potassium (3.6 and 2.8 percent of the crust, respectively). Plagioclase feldspars form a continuous solid solution between Ab and An end-members at high temperature, but the replacement of ions needs to be coupled because of the charge difference between Na⁺ and Ca²⁺. The charge balance in maintained by substituting Al³⁺ for Si⁴⁺. This is why feldspars are named aluminosilicates – Al and Si replace each other in the structure. The amount of potassium that may enter the lattice is limited because of large difference in ionic radii.

The plagioclase series is arbitrarily divided into six minerals or compositional ranges: albite (An₀-An₁₀ or Ab₉₀-Ab₁₀₀), oligoclase (An₁₀-An₃₀), andesine (An₃₀-An₅₀), labradorite (An₅₀-An₇₀), bytownite (An₇₀-An₉₀), and anorthite (An₉₀-An₁₀₀). These boundaries have no structural significance. Their use is justified because plagioclase is very common mineral and occurs in a wide variety of rocks and the composition of plagioclase is rather predictable. For example, it is common to find sodic plagioclase (oligoclase) in granite, more calcium-rich varieties (labradorite) in mafic rocks like gabbro, and intermediate andesine in intermediate igneous rocks like andesite.

Anorthosite is predominantly composed of only one mineral – plagioclase feldspar. Rogaland, Norway. Width of sample 12 cm.

Unlike plagioclase feldspars, alkali feldspars are divided into separate minerals not only based on their chemistry, but more importantly, based on optical properties determined by a petrographic microscope. The terminology and nomenclature is complex and somewhat even contradictory. This problem arises from the fact that the terminology has evolved over centuries and what we know today were not known to our forerunners who had no way to study the submicroscopic structure of these minerals. Orthoclase which is to this day traditionally defined as a monoclinic mineral is actually submicroscopically triclinic, just as microcline². But traditions are here to stay so we continue to refer to orthoclase as a monoclinic K-feldspar.

Most alkali feldspars are compositionally closer to Or end-member which explains why we often refer to them as K-feldspars (potassium feldspars). Anorthoclase is an alkali feldspar that is sodic in composition.

Microcline is a triclinic K-feldspar and has a characteristic cross-hatched tartan pattern of twinning seen with a petrographic microscope. Orthoclase and sanidine are both optically monoclinic K-feldspars, but differ in acute angle between the optic axes (can be determined with a petrographic microscope). Sanidine occurs in volcanic and subvolcanic rocks. Orthoclase is more widespread in occurrence. Adularia is a K-feldspar produced by low-temperature hydrothermal (veins) or authigenic processes. It is structurally similar to microcline but without the cross-hatched twinning. Anorthoclase is the only alkali feldspar that is not K-feldspar. It is sodic in composition, optically triclinic and characterized by a twinning similar to microcline but on a smaller scale.

Trachyte with anorthoclase phenocrysts. La Palma, Canary Islands, Spain. Width of sample 6 cm.
Rhomb-porphyry from Norway with abundant anorthoclase phenocrysts. Oslo rift, Norway.
Well-developed low-temperature alkali feldspar crystals. Width of sample 14 cm.
Trachyte with sanidine phenocryst. Drachenfels (type locality of trachyte), Germany. Width of sample 10 cm.

The structural state which is the basis of the alkali feldspar classification, is dependent on the temperature of crystallization and subsequent cooling history. Volcanic feldspars tend to retain their high temperature ordering because the cooling is very rapid (sanidine). Low-temperature feldspars either crystallized at lower temperatures (adularia) or had ample amount of time to cool down (microcline).

Exsolution

Most feldspars are not compositionally homogenous. Alkali feldspars usually have separated into potassium- and sodium-rich phases. Sodium-rich albite lamellae in K-feldspar is known as a perthitic and the opposite (K-fsp in Ab) as an antiperthitic texture. These textures develop because of differences in ionic radii of K and Na.

It is primarily the rate of cooling that determines how extensive the perthitic texture can be. Rapid cooling of volcanic rocks allow only microscopic exsolution to develop, but very slow cooling in deep-seated plutons may yield perthitic microcline where the exsolution lamellae are easily seen with a naked eye. Microcline is much more prone to develop easily seen perthitic texture than orthoclase and sanidine.

Visible perthitic exsolution lamellae of albite in microcline. Evje, Norway. Width of sample 6 cm.

Moonstone is an alkali feldspar that displays beautiful iridescence produced by light interacting with submicroscopic exsolution lamellae that act as diffraction grates. Similar effect also occurs in plagioclase feldspars. Iridescence in plagioclase is known as labradorescence or schiller. This is also produced by the light reflecting back from separate exsolution lamellae which are submicroscopic (in size comparable to the wavelength of visible light) and therefore allow the reflecting light to combine (this phenomenon is known as interference of light waves) and form beautiful bluish play of colors. Some moonstones owe their spectacular appearance to thin hematite flakes or copper platelets in a particular crystallographic orientation that give the rock rosy or gold (hematite) or pink schiller (copper)¹.

The reason for exsolution in plagioclase is different, however. It is primarily ordering in the crystals, not the difference in the ionic radii. The exsolution in plagioclase is also more subtle. They are usually not seen even with a microscope, let alone macroscopically.

Twinning

Twinning deserves a special attention because no other minerals have as ubiquitous and complex twinning as feldspars. Twinning in crystals means that two or more crystal segments of the same mineral are symmetrically intergrown. Twinning is never random intergrowth, it always follows the laws of symmetry. Crystals may be twinned by reflection (there is a mirror plane between the two segments), rotation (one crystal is usually turned 180° to the other) or inversion (two segments are symmetrical to each other through the center of the crystal).

Twins may be simple (just two symmetrical segments) or multiple. The latter is especially common in plagioclases where many parallel segments are mirrored. This is known as polysynthetic twinning. It is easily seen with an optical microscope, but very often also macroscopically with a hand lens and even with a naked eye if you have a good eyesight. This is an important aspect because looking for parallel striations or grooves on the feldspar surface is one of the very few methods to determine whether it is a plagioclase or alkali feldspar. Twinning in alkali feldspars is common too, but does not have such a diagnostic value. Common twins are rotational Carlsbad twins, which are also penetrative (twin segments appear to be intergrown). There are about 20 different twin laws identified in feldspars although only few of them are common.

Geologists are relying on these long, straight and parallel grooves on the feldspar crystals to tell apart plagioclase from alkali feldspars in the field. Unfortunately, it is not always so easy to see as here. The oligoclase (Ab20) sample above is from a pegmatitic granitoid from Evje, Norway. Length of sample 12 cm.

Color

Feldspars have no color of their own because they lack chromofore chemical elements in their structure. But they are almost never transparent either. Internal reflections from exsolution lamellae, inclusions, cleavage surfaces, etc. make the pure feldspar white in color. Albite (from the Latin albus) is even named so because of its characteristic white color. Other plagioclase feldspars are also commonly white, although many other shades are possible. Even almost black color due to Fe-Ti inclusions is not uncommon.

K-feldspars are often pink because of finely dispersed iron oxide mineral hematite. Some microclines are blue because of lead and even have a special name amazonite because of this color. Small amounts of three-valent iron gives them yellowish hue.

Feldspars may be partly weathered or altered to clay minerals, sericite (fine-grained muscovite mica), saussurite (mixture of albite, epidote and others – disintegration product of calcic plagioclase) which gives them somewhat dirty and worn-out appearance. Plagioclase is more susceptible to weathering than K-feldspar and Ca-rich anorthite is the least resistant. This is one of the reasons why K-feldspar is more common in sand than plagioclase. The other major reason is that although plagioclase is very common in the crust, a major part of it is in the oceanic, not in the continental crust. The source of sand is mostly the latter.

Green amazonite in microcline. This crystal of microcline is truly gigantic – tens of centimeters across. Evje, Norway. Width of view 13 cm.

Occurrence

Igneous rocks that contain no or little feldspar are rare. These are ultramafic (olivine-pyroxene) rocks and alkaline rocks where feldspars are replaced by feldspathoids. All the other major igneous rocks like granite, syenite, gabbro, diorite, andesite, pegmatite, etc. not only contain feldspars, but in most cases feldspars make up more than half of their composition. This is the reason why we have chosen feldspars as the backbone of the classification schemes of igneous rocks. For example, granite is a rock that contains lots of alkali feldspar and quartz. Gabbro contains lots of plagioclase and pyroxene. Granodiorite is similar to granite but contains more plagioclase than alkali feldspar.

It is very useful to distinguish between alkali feldspars and plagioclase when classifying igneous rocks because alkali feldspars are clearly more widespread in felsic rocks (granite, syenite, granitic pegmatite, rhyolite) and plagioclase occurs mostly in igneous rocks intermediate to mafic in composition (andesite, basalt, gabbro). Alkali feldspar in plutonic rocks is either orthoclase or microcline. Felsic or intermediate volcanic rocks contain sanidine, orthoclase and anorthoclase.

Feldspars are also common in metamorphic rocks (gneiss, amphibolite, schist) and in hydrothermal veins. Feldspars are not very common in sedimentary rocks because they are not as resistant to weathering as are quartz and clay minerals (these are mainly the product of chemical weathering of feldspars). But still, in arenaceous (sandy) sediments feldspars are second in abundance after quartz. They are less important in mudstones-shales and carbonate rocks.

Feldspar-rich quartzitic rock – metamorphosed arkose. Aust-Agder, Norway. Width of sample 19 cm.
Pegmatite with graphic granite (intergrown quartz and alkali feldspar) and plagioclase. Evje, Norway.

Cleavelandite is a lamellar variety of almost pure albite. It forms as a late-stage mineral in pegmatites. Width of sample from Ontario is 5 cm.
Basalt dike in albitite. Albitite is composed of albite. Plagioclase feldspars are also major constituents of basaltic rocks, but they are usually too fine-grained to be seen. La Palma, Canary Islands, Spain. Width of sample 15 cm.

Mylonite with large (largest is over 10 cm) K-feldspar porphyroclasts. Sør-Trøndelag, Norway.
White plagioclase in norite. Rogaland, Norway. Width of sample 8 cm.
Wyborgite is the most classic rapakivi from the type locality in SE Finland. It is characterized by large orthoclase ovoids mantled by sodic plagioclase.
White calcium-rich plagioclase (bytownite) in gneissic granulite. Bergen, Norway. Width of sample 12 cm.
Trondhjemite is a granitoid (tonalite) that contains lots of plagioclase and quartz and the only mafic mineral is biotite. Hordaland, Norway. Width of sample 10 cm.
Larvikite (monzonite) is an interesting rock not only because of iridescent colors, but also because it contains all three feldspars (K-feldspar, albite, plagioclase). Larvik, Norway. Width of sample 17 cm.
Porphyritic texture is usually given to the rocks by large feldspar phenocrysts. Rhomb porphyry (latite) is an extrusive equivalent of larvikite. It is also ternary like larvikite – containing all three feldspars. Width of sample 13 cm.

These mountains in southwestern Norway are composed of almost pure feldspar – anorthosite as a rock type. Rogaland, Norway.
Rocky coast of Åland Islands in Finland is composed of feldspar-rich reddish granite.

Uses

Feldspar-rich rocks are used as an aggregate. Clay deposits are derived primarily from feldspar-rich rocks. Feldspars are raw materials for glass and ceramic industries. Ca-rich plagioclase feldspar has some potential as an aluminum ore, but currently it is more economical to extract aluminum from bauxite. They are also used in metallurgy. Some iridescent feldspars are valued as gemstones and many feldspar-rich rocks are valued building and monument stones.

Railroad track ballast has to be made of hard rocks. Feldspar-rich crushed rocks are commonly used for that purpose. Picture taken in Estonia.

Tombstone in Norway made of feldspar-rich monzonitic iridescent rock larvikite.
Road dam between rocky islands in Åland, Finland. Note how red the road is. This is because the aggregate used is largely composed of alkali feldspars.

References

1. Ribbe, Paul H. (2007). Feldspar. In: McGraw Hill Encyclopedia of Science & Technology, 10th Edition. McGraw-Hill. Volume 7. 45-49.
2. Nesse, William D. (2011). Introduction to Mineralogy, 2nd Edition. Oxford University Press.
3. Deer, W. A., Howie, R. A. & Zussman, J. (1996). An Introduction to the Rock-Forming Minerals, 2nd Edition. Prentice Hall.
4. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.

Magnesite is a magnesium carbonate mineral (MgCO₃). Unlike related carbonates calcite (CaCO₃) and dolomite (CaMg(CO₃)₂), it is not a major rock-forming mineral. It most commonly occurs in metamorphosed igneous rocks which are rich in magnesium. These are ultramafic rocks like dunite, pyroxenite and peridotite. These rocks commonly metamorphose to serpentinite.

Low- or medium-grade metamorphism of ultramafic rocks may also yield magnesite if there is enough carbon dioxide available which is needed to form the carbonate ion. But it is not restricted to these often beautiful green-white assemblages. It also occurs in hydrothermal veins and in sedimentary rocks. Sometimes sedimentary dolomite is replaced with it (calcium in the lattice is replaced with magnesium) in which case the magnesite is said to be diagenetic. It may infrequently precipitate directly from salty brine.

Magnesite is usually white, although it may be yellow or brown if some of its magnesium is replaced with iron. There is a complete solid solution between siderite (FeCO₃) and magnesite just as there are similar solid solutions possible in many other iron-magnesium minerals, for example olivine.

Magnesite, like dolomite, reacts with dilute hydrochloric acid only if powdered or if the acid is heated. Unlike calcite it very rarely forms well-developed crystals. This is why its crystals are highly valued by mineral collectors, although the mineral itself is not particularly rare. Beware of cheaters if you want to buy these crystals. They may be actually calcite (iceland spar) which is similar in appearance. Real magnesite crystals usually come from Brumado, Brazil.

The mineral is usually earthy or granular. It may be very fine-grained, sometimes said to be amorphous. This is incorrect because it is still composed of crystalline matter (‘amorphous’ means without crystal structure), these crystals are just too small to be seen. Magnesite is denser (specific gravity 3-3.2, depending on the composition) than many other common rock-forming minerals and it is also relatively soft (3.5-4.5 on Mohs scale).

It could be used as an ore of magnesium, but usually magnesium is extracted from brines and seawater. Every cubic meter of seawater contains more than one kilogram of magnesium. So there is really no shortage of this metal and never will be. River water usually contains very small amounts of magnesium (only about 4 ppm), but it gets concentrated in seawater because rivers constantly carry more and more of it but there are no easy ways out. It is actually a minor biomineral. Some cyanobacteria are involved in biologically induced magnesite formation, but this is relatively minor part of the magnesium cycle.

Magnesite is mined but rarely as an ore of magnesium. Instead, it is heated (625…643°C is required) to produce MgO (periclase as a naturally occurring mineral, but as a manufactured white powder it is known as magnesia or dead-burned magnesite) which is used mainly in the manufacture of refractory materials. More than half of produced magnesia is used by the refractory industry, but magnesia has many other industrial applications. It is even used as a plant fertilizer and as a desiccant in libraries.

http://picasaweb.google.com/107509377372007544953/2015#6220284766086874994
Hydromagnesite sand grains from Lake Salda in Turkey. Green is serpentine. Width of view 15 mm.

http://picasaweb.google.com/107509377372007544953/2015#6220284765868137362
Massive variety (sometimes said to be amorphous) from the Ural Mountains in Russia. Width of sample 12 cm.

http://picasaweb.google.com/107509377372007544953/2015#6220284763813136098
Magnesite from Satka (Satkinskoye) in Russia. Satka is the largest sedimentary deposit (dolomite replaced by magnesite) of magnesite in the world. Width of sample 11 cm.

Olivine is a very common silicate mineral that occurs mostly in dark-colored igneous rocks like peridotite and basalt. It is usually easily identifiable because of its bright green color and glassy luster.

Olivine is a common mineral in dark-colored igneous rocks because these rocks are rich in iron and magnesium (rocks rich in iron-bearing minerals tend to be either black or at least dark-colored). These chemical elements (Mg and Fe) are the essential components of olivine which has the following chemical formula: (Mg,Fe)₂SiO₄. Magnesium and iron can replace each other in all proportions. There are specific names for compositional varieties, but most of them are rarely used. Only forsterite (more than 90% of the Mg+Fe is Mg) and fayalite (similarly iron-rich endmember) are used more often. The vast majority of all the samples are forsteritic or compositionally close to it.

Olivine is a nesosilicate. It means that silica tetrahedra (which is the central building block of all silicate minerals) are surrounded from all sides by other ions. Silica tetrahedra are not in contact with each other. It implies relatively low content of silicon which is indeed the case. It is a silicate mineral that uses silicon very conservatively. On the other end of the spectrum is mineral quartz which is pure silica (SiO₂) without any other constituents. Other well-known nesosilicates are garnet, zircon, topaz, kyanite, etc.

Silicate minerals that crystallize from magma have a higher melting/crystallization temperature if the content of silica is lower and the content of Mg+Fe is higher. Hence, olivine has a high crystallization temperature and is therefore one of the first minerals to start crystallizing from a cooling magma. It takes silica out of magma relatively conservatively, as already mentioned. So the concentration of silica rises as olivine crystals form and next silicate minerals to crystallize (which are pyroxenes) are already somewhat richer in silica. This sequential order of crystallizing silicate minerals from olivine to quartz is known as the Bowen’s reaction series after a Canadian geologist Norman Bowen who first described it. It is one of the most important concepts every geology student is taught during the petrology course.

Dunite xenolith in basaltic lava from Hawaii. The sample is 8 cm in width.

Bowen’s series or order of minerals in this series (olivine -> pyroxene -> amphibole -> biotite -> K-feldspar -> muscovite -> quartz) is a really useful one to memorize and there are several properties of these minerals that generally follow the same order. Olivine and its close neighbors are darker, contain iron and magnesium, and have a high melting temperature. Quartz, muscovite and K-feldspars are generally much lighter in color and weight, they melt at lower temperatures, and they contain no iron and magnesium. Another interesting fact is that the order of susceptibility to weathering and metamorphic alteration is exactly the reverse. It is readily altered or weathered while quartz is extremely resistant to any kind of change. All other minerals in the series are somewhere in the middle. In the correct order, of course.

Important aspect that rises from this series is the explanation why certain minerals typically form assemblages while others are almost never found together. Olivine is typically with pyroxenes (in basalt, for example) and quartz + K-feldspar with micas (biotite and muscovite) is a typical composition of granite. But there are no such rock types that are composed of olivine plus quartz. Granite and similar rocks are said to be felsic (composed of feldspar and silica) and basaltic rocks are referred to as mafic rocks (magnesium + ferric).

Olivine is a common rock-forming mineral in mafic and ultramafic igneous rocks, but it also occurs in impure metamorphosed carbonate rocks (picture below). It is a very common mineral in the mantle. Some xenoliths from the mantle are almost entirely composed of this mineral. Such a rock type is known as dunite. Olivine occurs as a groundmass mineral but also as distinct phenocrysts in many basaltic rocks. These rocks need not to be basalts in the strict sense. They may be picrites, basanites, etc. but all of them may be very similar to each other as boundaries between them are arbitrary. So it is frequently impossible to say for sure before chemical analysis is made.

Olivine is very susceptible to weathering. Bright green mineral loses its appeal rapidly in the weathering environment. It becomes dull, earthy, and yellowish brown. This material is usually a mixture of clay minerals and iron hydroxide goethite and it is known as iddingsite. It also demonstrates very little resistance to hydrothermal metamorphism. Hot and chemically aggressive fluids quickly alter olivine-rich igneous rocks into metamorphic rock known as serpentinite. It is also an important constituent of many stony and mixed meteorites. Especially beautiful is pallasite. It is a mixture of iron and olivine and is thought to represent a core-mantle boundary of a disintegrated asteroid. Perhaps the core-mantle transition of our own home planet looks something like that too.

However, there is one little thing to remember. The mantle is indeed most likely compositionally close to it, but most of it is not composed of this exact mineral. Olivine tolerates well pressures in the crust and in the upper mantle, but at 350 km depth its crystal structure starts to break down. The composition remains, but it takes a new and more compact form. It is not technically olivine anymore because minerals have a definite crystal structure.

http://picasaweb.google.com/107509377372007544953/Rocks#5852183063536948050
Olivine is not just an igneous mineral. It also occurs in impure metamorphosed carbonate rocks. Here olivine crystals are found in a sample of calcitic marble. Some crystals even possess a typical crystal faces which are usually lacking in igneous rocks because olivine grains are often corroded (they reacted with the melt surrounding them). Width of sample is 9 cm.
http://picasaweb.google.com/107509377372007544953/Tenerife#5841862778873146786
Phenocrysts in ultramafic picritic rock from La Palma, Canary Islands. Width of sample is 5 cm.

http://picasaweb.google.com/107509377372007544953/2015#6190951168417918930
Olivine (yellow) with pyrope (purple) and chromian diopside (green) in peridotite. Åheim, Norway. Width of view 25 cm.