Feldspar

June 13, 2025July 22, 2015 by Siim

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.

This rock sample (alkali feldspar syenite) does not contain much more than red-colored alkali feldspar. The width of the sample from Estonia is 8 cm.

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.

Microcline in microscope. Twinning pattern. — Microcline twinning pattern seen with a microscope. Microcline grain in a coarse-grained sandstone from Estonia. Width of view 3 mm.

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.

Plagioclase may be almost black in color and demonstrate beautiful iridescence (labradorescence). The plagioclase as a whole is compositionally labradorite although it has separated into segments with different chemistry. The width of the rock sample from Finland is 17 cm.

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.

Plagioclase twinning is easily seen with a polarizing microscope. Plagioclase is striped black-gray-white. A longer description of this thin section is in the article about gabbro.

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.

Diabase — Plagioclase crystals (here within an igneous rock diabase) are often white. Mushroom Rock, Death Valley, USA. Width of view 25 cm.

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.

Sand arctic Canada — *Feldspar grains (red and yellow) are common in sand, especially in cold climate where weathering is slower. Coronation Gulf, Nunavut, Canada. Width of view 10 mm.*

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.

*Alkali feldspar in pegmatite. Nyelv, Finnmark, Norway. Width of view 50 cm.*

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.

*Plagioclase often forms a porphyritic texture in mafic volcanic rocks if the lava contained plagioclase crystals formed prior to the eruption. The Isle of Mull, Scotland. Width of sample 8 cm.*

White plagioclase in a gabbroic pegmatite. Plagioclase is very common mineral in the oceanic crust. This rock is just that – part of a world-famous Troodos Ophiolite in Cyprus.

Ca-rich plagioclase in troctolite (gabbro without pyroxene). Orange is iddingsite (weathered olivine). Flakstadøya, the Lofoten Archipelago, Norway. Width of sample 15 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.

Clay has many uses. Clay minerals are mostly weathering products of feldspars. The picture was taken in a clay quarry 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.

Monomineralic rocks

January 11, 2016July 20, 2015 by Siim

Monomineralic rocks, as the name suggests, are rocks that are composed of only one mineral. Majority of rocks are composed of several different minerals and absolutely pure monomineralic rocks do not exist in nature anyway. But some of them are reasonably ‘pure’ to be called monomineralic.

Dunite is composed of almost pure forsterite. Gusdal Quarry, Møre og Romsdal, Norway. Width of sample 9 cm.

How pure exactly do they need to be is something that is not set in stone but in several cases 10% is arbitrarily chosen as the lower purity standard. Especially among igneous rocks where the term ‘monomineralic’ is most often used. Examples of monomineralic igneous rocks are dunite (more than 90% olivine) and anorthosite (more than 90% plagioclase feldspar).

Common monomineralic metamorphic rocks are marble and quartzite although they do not need necessarily to be monomineralic. Similar is the situation with their sedimentary protoliths – limestone and sandstone which may be very pure. This is the most important difference between most igneous monomineralic rocks (which need to be more than 90% pure to earn its name) and sedimentary and metamorphic rocks. There are some exceptions, though. A rare igneous rock carbonatite may be almost monomineralic but is often not. It only needs to be more than 50% carbonate to be named that way. So it is very similar to sedimentary carbonate rock limestone that is also at least 50% carbonate. Pyroxenite is another example of often almost monomineralic igneous rock although it may contain up to 40% of olivine in addition to pyroxene group minerals.

So we can talk about ‘true’ monomineralic rocks that need to be pure and monomineralic rocks that only may be pure (their classification principles allow that) but do not have to be.

Dunite is an igneous rock made of olivine (over 90%). Dunite is relatively rare rock type on the surface but very common deep in the mantle. Dunite may be mined as a mineral resource because olivine has some industrial applications. Especially useful is its high melting temperature which makes it a good refractory material. The sample above is collected from a working olivine quarry.

Anorthosite. Rogaland, Norway. Width of sample 13 cm.

Anorthosite is a common rock type on the Moon where the highlands surrounding the darker maria are composed of this rock type. But it also occurs here on Earth although it is not distributed as widely here. The existence of this rock type is no accident because its constituent mineral plagioclase feldspar is the most common mineral in the Earth’s crust.

Carbonatite (sövite). Alnö Island, Sweden. Width of sample 8 cm.

Carbonatite is a rare igneous rock with an exotic composition. It is composed of carbonate minerals (>50%) which makes it compositionally very similar to sedimentary limestone and metamorphic marble. Its appearance may also very much resemble that of marble.

Pyroxenite. Aust-Agder, Norway. Width of sample 8 cm.

Pyroxenite is an ultramafic rock just as dunite. It may contain significant amount of olivine (up to 40%) but the sample above is mostly composed of almost pure clinopyroxene.

Quartzite. Telemark, Norway. Width of sample 9 cm.

Quartzite is a common metamorphic rock just as its sedimentary protolith sandstone. Many quartzite samples are composed of almost pure interlocking mass of fused quartz grains just as the sample above.

Marble. Baikal, Siberia. Width of sample 12 cm.

Marble is a metamorphosed limestone that may contain significant amount of impurities but almost pure marbles are common also.

Chalk. Width of sample 6 cm.

Chalk is a type of limestone that is composed of calcitic exoskeletons of coccolithophores.

Mylonite: A Comprehensive Overview with Photographic Examples

June 11, 2025July 8, 2015 by Siim

Mylonite is a foliated metamorphic rock that is composed of intensely flattened minerals in a fine-grained streaked matrix. Mylonites form deep in the crust where temperature and pressure are high enough for the rocks to deform plastically (ductile deformation). Mylonites form in shear zones where rocks are deformed because of the very high strain rate. Mylonites contain porphyroclasts. These are crystals that existed before the process of mylonitization began, but have survived with lots of strain damage. They are ovoidal or stretched and are surrounded by finer and even more deformed mineral grains. Mylonitic rocks with porphyroclasts are often described as having a flaser fabric.

*Mylonite is a fine-grained rock that containes intensely flattened minerals. A boulder on the coast of Varanger Peninsula in Norway.*

Mylonites are foliated, but the foliation is generally passive. It means that the foliation is not controlling how the rock breaks. In this way mylonitic rocks resemble gneiss and are distinct from schist, slate, and phyllite which tend to break along the foliation plane.¹

The rock is called protomylonite if the porphyroclasts make up more than half of the rock volume and ultramylonite if almost all the porphyroclasts are gone (less than 10% have survived). In the latter case the matrix grains are so small that the rock may even resemble chert.

Mylonitic fabric forms as a result of both destructive and constructive processes. The first is responsible for the plastic deformation and the latter for the recrystallisation which gives an impression that the rock has been semi-molten and flowing under very heavy stress. Mylonites may occur as thin sheets in relatively unmetamorphosed rocks (see the pictures below) or they may encompass broad zones several kilometers thick.

‘Mylonite’ as a term is not new. It was coined by Charles Lapworth in the late 1800s who described mylonitic rocks from the Moine thrust in Scotland. But the interpretation how these rocks were formed has changed. It was generally believed that mylonite is a result of intense brecciation (brittle pulverization). The name originates from the Greek language (mylon – to mill). Nowadays we believe that instead of milling mylonitization is a result of ductile deformation and the discovery of plate tectonics has given us a framework to understand where and how these rocks can form – deep in the crust where rocks are hot and there are movements between crustal blocks, but the rocks are too plastic to deform by faulting. The process higher in the upper crust where rocks are brittle is called cataclasis. That we can indeed describe as a certain type of brecciation that results in lots of broken rock fragments and generally increased volume of the fractured rock body because of induced voids between the fragments.

Mylonites with larger grain size are sometimes described as gneisses and very fine-grained ultramylonites may resemble slate. Mylonites usually have a quartzo-feldspathic protolith (like granite or gneiss) although they may originate from any rock type. The temperature has to be at least 250-300°C for the mylonitization to take place (for quartzo-feldspathic rock). This is enough for quartz to flow while feldspar still resists dislocation creep and slide and retains its former shape more successfully as strained porphyroclasts. 450°C is enough for feldspar to go through dynamic recrystallization as well.²

The photos below are from western Norway near the small village of Flatraket. There is an igneous intrusion surrounded by gneisses and also eclogites here and there. Especially eclogites were the reason why I visited this part of Norway and I managed to get some really nice pictures which I hope to demonstrate in the near future. The igneous intrusion is composed of mangerite according to the Norwegian bedrock map⁴. Mangerite is an orthopyroxene-bearing monzonite, but in this case it seems to contain a little too much quartz and alkali feldspar which makes it actually quartz syenite. It may be completely inappropriate to use the term mangerite in this case because according to Lappin et. al.³ this rock contains clinopyroxene, not orthopyroxene.

But in this article the composition of the protolith is not that important. What struck me when I walked into a roadside quarry (where I just hoped to find a good hand sample of mangerite) were narrow and very intense zones of mylonitization. In the end, I got no proper mangerite, but found something entirely different but no less interesting. Here are the pictures from the Flatraket quarry:

Large orthoclase ovoids in a matrix composed of plagioclase, quartz, clinopyroxene, garnet, and ore minerals.³
It turned out that the rock has a rapakivi texture. At least some orthoclase ovoids are surrounded by plagioclase rims – a texture very familiar to me as the type locality of these rocks is in southeastern Finland. Not far from my home.
Here is a larger view with a hammer for scale.
Something is going on here. The rock is clearly oriented and orthoclase porphyroclasts have tails (recrystallized mineral matter). The ovoids have even grown bigger which makes this rock a blastomylonite (recrystallization has increased the volume of porphyroclasts).
More intense mylonitization…
and even more. Now we are really talking about heavily stretched rocks.

http://picasaweb.google.com/107509377372007544953/2015#6190951215832700402
Some mylonite bands are so fine-grained that are approaching ultramylonite in appearance.

Mylonitic zones may be only few centimeters in width.

http://picasaweb.google.com/107509377372007544953/2015#6190951194612010802
Another narrow band of mylonite within an almost unstrained igneous rock.

And more ultramylonite (or approaching that) in the lower part of the deformed zone.
Quartz syenite with mylonite.

Mylonites from other localities

http://picasaweb.google.com/107509377372007544953/2015#6191004916488584034
Mylonite with large (largest is over 10 cm) K-feldspar porphyroclasts. Sør-Trøndelag, Norway.

References

1. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
2. Simpson, Carol (2007). Mylonite. In: McGraw Hill Encyclopedia of Science & Technology , 10th Edition. McGraw-Hill. Volume 11. 585-586.
3. Lappin, Michael A., Pidgeon, Robert T. & Van Breemen, Otto (1979). Geochronology of basal gneisses and mangerite syenites of Stadlandet, west Norway. Norsk Geologisk Tidsskrift. Vol. 59, pp. 161-181.
4. Bedrock map of Norway.

Ingredients of pegmatite

June 11, 2025July 4, 2015 by Siim

Pegmatites are unusually coarse-grained igneous rocks. They also often contain exotic minerals but even the really common ones are spectacular because of their size. Here you can see four very common minerals that in many cases make up the majority of the rock.

The ingredients of a common pegmatitic rock: quartz (W), muscovite mica (N), plagioclase with oligoclase composition (O), and microcline (S). These crystals are large (each one roughly 10 cm across) and they all come from a pegmatite from the same place. Evje, Norway.

Plagioclase and microcline are both feldspars and muscovite is a very common mica. Actually I should have added biotite here as well. This is dark mica which is also often in the mix. The four minerals above were together in one rock and there were no biotite. But few hundred meters away was another quarry that had beautiful biotite flakes as well. So I will add another photo to do justice to iron mica which is rarely missing in a classic pegmatitic rock.

Biotite from a granitic pegmatite. Width of sample 11 cm. Evje, Norway.

And here they are all together in one pegmatitic rock. Almost all, muscovite is missing:

Pegmatite with a monzogranitic composition. Evje, Norway.

Magnesite

June 7, 2025June 28, 2015 by Siim

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.

*Magnesite (white) with serpentine from Norway. Width of sample 24 cm.*

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.

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Massive variety (sometimes said to be amorphous) from the Ural Mountains in Russia. Width of sample 12 cm.

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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.

Chert and chalk of Stevns Klint

June 11, 2025June 25, 2015 by Siim

About a month ago I was invited to Denmark to speak about geology and rocks of Stevns Klint to young students of Copenhagen Estonian School (Københavns Estiske Søndagsskole). The school had a field trip with bicycles to interesting locations, including the coastal cliff which is world-famous among geologists. Why famous? It is one of the best K/T boundaries and it was the site where the radical theory of meteorite-driven end-Cretaceous extinction event was developed.

The cliff is composed of chalk and bryozoan limestone (lower and upper part of the klint, respectively) and a hard to notice narrow band of clayey material between them which happens to be abnormally rich in iridium. This is a metal that is rare on Earth but abundant in iron meteorites. The meteorite itself hit the Earth in Mexico but the consequences were truly global as demonstrated by this UNESCO World Heritage Site in Denmark.

The rock face also contains lots of chert (flint) nodules which are very common in Cretaceous chalks. The entire rocky coastline is composed of them because they are very resistant to abrasion, especially when compared to much softer limestones. But hardness does not mean that they are hard to break into pieces. Actually, it is quite easy and the result is lots of knife-sharp edges which were exploited by our stone age ancestors. Playing with these rocks was much more fun to most of the children than paying attention to stories about dinosaurs and iridium-rich clay.

All of the boulders here are made of chert but you have to break one into pieces to see the smooth and curving fresh surface with sharp edges. White material is chalk.

Stevns Klint in Denmark. If one would make a list of places every geologist should try to visit, Stevns Klint would surely be a strong candidate to be included.

I had my children with me as well. Here is my son looking for fossils in chalk in an abandoned chalk quarry. This chalk is very rich in fossils (brachiopods, sea urchins, bryozoans, corals, etc.) which made everyone busy and happy 🙂

Graphic granite

December 13, 2015June 24, 2015 by Siim

I recently returned from Norway with lots of rock samples which will be photographed in the near future. The photos will be added mostly to the old posts where appropriate but this way my regular readers will not see them. And I also miss an opportunity to learn something new from comments. So I thought that perhaps I should start writing shorter posts in addition to longer overviews of rock types.

Here is the first one of the new series which will contain mostly rock samples but also other interesting geological photos taken during my geotrips.

http://picasaweb.google.com/107509377372007544953/2015#6196126451189924914
Graphic granite from Evje, Norway. Width of sample 9 cm.

Width of view 14 cm.

Graphic granite is a granitic pegmatite where large alkali feldspar crystals contain angular, linear, or wedge-shaped intergrowths of quartz. These intergrowths may resemble cuneiform writings, hence the name.

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 composition¹.

Another sample of graphic granite from the same locality. Evje, Norway.

References

1. Jackson, J. A. (1997). Glossary of Geology, 4th Edition. American Geological Institute.

Discovering Norway

June 11, 2025June 15, 2015 by Siim

I have been discovering Norway for a week now by driving from one interesting location to another. So far it has been a highly successful trip. I have seen and photographed many outcrops and managed to collect beautiful rock samples.

Today and also tomorrow I will visit outcrops around Bergen with a Norwegian geologist Ole Tjugen. He has read my blog before and kindly invited me to stay at his home and organized a field trip to interesting places around.

It is my first visit of this kind – to be hosted by someone who is a reader of my blog. I am glad to have that opportunity. It is a great chance to really see the best of what particular location has to offer and of course I can not deny that it is enjoyable to sleep in a bed after spending several days in a tent.

Siim Sepp Ole Tjugen
Here we are in front of an outcrop of metamorphosed anorthosite.

Quartzite conglomerate
We saw some wild geology today. Here is a very beautiful outcrop of conglomerate from the Ordovician. The clasts are slightly flattened but compositionally there is little change because the clasts are made of quartzite.

Map of Africa in rocks

June 11, 2025June 4, 2015 by Siim

While hiking under the magnificent Imi n’Ifri natural bridge in Morocco I was stopped by two local boys who pointed at something behind me and said few explaining words in French. My French is extremely poor but the word Afrique was still understandable. Here is what I saw:

Contour of Africa under Imi n'Ifri natural bridge
Well, it is not a perfect shape of Africa but the resemblance is still quite astonishing.

This is something that could potentially bring quite a number of tourists to this spot but unfortunately the locals have made no efforts to exploit it. At least no intelligent efforts like information boards, websites, etc. Except these two boys who, by the way, did not ask any money for it (which is somewhat unusual in this part of the world).

Imi n’Ifri natural bridge is made of limestone. There is a river flowing under the bridge and the bridge itself is pretty high. Perhaps 50 meters from the river to the arch or maybe a bit less but it was beautiful and well worth a visit. A roundtrip down and back up should take about an hour and you do not need a guide 🙂

River under the natural bridge of Imi n'Ifri in Morocco
Here is a closer look of the limestone and the river cutting its way through it.

Nautiloids of Morocco

June 11, 2025May 31, 2015 by Siim

I recently returned from a three week geological field trip to Morocco. This country is full of very versatile geology. Lots of structures (especially nicely folded sedimentary rocks), minerals, and rock types. And of course also fossils. Morocco is famous for its ammonites but there are also lots of trilobites and nautiloids from older Paleozoic rock strata.

Today I am focusing on the latter. These nautiloid fossils are locally known as Orthoceras although I am a bit sceptical that this is indeed the correct name. According to the Treatise on Invertebrate Paleontology, real Orthoceras (Orthoceras regulare) comes from my home country Estonia and adjacent areas. The coiled-shape fossils are often called ammonites but in this case it can not be because true ammonites did not occur before the Mesozoic. But they are cephalopods for sure. If someone more familiar in paleontology happens to read it, I would gladly welcome more specific names given to these fossils.

Dark gray rock containing these fossils is limestone from the Ordovician. The width of view is from 40 cm (closer shots) to about two meters. Yellow material in the cracks is desert sand. These photos were taken about 20 km away from the Algerian border in the Sahara desert. Closest Moroccan settlement is Erfoud.

Nautiloid (cephalopod) fossils in Ordovician limestone in Morocco

Local traders on bikes arrive fast as soon as we arrived. The same pattern repeated itself in many places. The goods they are offering are local and genuine (actually made of the very same limestone we are standing on) and pretty inexpensive according to European standards. Of course you can not accept the price they are initially asking. It takes some practice but generally it is easy to agree for about 50% of the initial price or even less. I bought the plate with a nice nautiloid fossil right next to the man for six euros. I am sure it was a good trade for both of us although he asked for 12 euros as a starting price.