Minerals


Mineral is a naturally occurring crystalline solid with a definite composition. There are almost 5,000 minerals described so far2.

Trying to familiarize ourselves with all of them would be a hopeless task. Luckily, this is not necessary if your goal is to understand how our home planet works and what is it made of. About 200 minerals are called the rock-forming minerals. These are the minerals that are listed and described below. More than 99.9% of the Earth’s crust is composed of these minerals.

My goal is to compose articles about every mineral mentioned below. Needless to say, it is a huge amount of work which will likely keep me busy for years to come. The style of entries is a compromise between encyclopedia-like article and a narrative blog post. I pay almost no attention to crystallography or optical mineralogy. I think there are plenty of other options (both printed and web-based) available for such information but you will find here photos and some insights about the geological background of the minerals described.

More than 90% on the crust is composed of silicate minerals. Most abundant silicates are feldspars (plagioclase (39%) and alkali feldspar (12%)). Other common silicate minerals are quartz (12%) pyroxenes (11%), amphiboles (5%), micas (5%), and clay minerals (5%). The rest of the silicate family comprises 3% of the crust. Only 8% of the crust is composed of non-silicates — carbonates, oxides, sulfides, etc1.

1. Silicates

Silicates are minerals that contain SiO4 tetrahedra in their crystal structure.

1.1 Framework silicates (tectosilicates)

This is volumetrically the most important group of silicate minerals. More than two thirds of the crust is composed of framework silicates. Each oxygen ion of the SiO4 tetrahedron is shared with another tetrahedron. Hence, three-dimensional framework forms which gives name to the group. Si:O ratio in silicon tetrahedra is 1:2 or 0.50 (Si may be replaced with Al. In this case Si=Si+Al).

1.1.1 Silica group

Minerals that belong to this group share the same chemical composition — SiO2. By far the most important mineral in this group is quartz.


Quartz Very abundant (12% of the crust) and widespread. Occurs in many igneous, metamorphic, and sedimentary rocks. Quartz is the principal component of most sand samples. SiO2

Chalcedony To be precise, chalcedony is not a mineral. It is composed of fine intergrowth of fibrous quartz and moganite and it usually contains water. Moganite has the same composition as quartz but is composed of alternating sheets of right- and left-handed quartz. Moganite is recognized as a separate mineral.

Opal Opal is also not a mineral in the strict sense. It is composed of crystalline and amorphous silica with water. SiO2·nH2O

1.1.2 Feldspar group

Feldspars are the most abundant minerals in the crust. There are two subgroups: alkali feldspars and plagioclase feldspars. Alkali feldspars are usually Potassium-rich. Hence, they are often named K-feldspars (orthoclase, microcline, and sanidine belong to this group). Plagioclase feldspars form a solid solution between Na- and Ca-rich end-members.

Plagioclase Plagioclase feldspar is the most abundant mineral group in the crust. It occurs chiefly in igneous and metamorphic rocks. Plagioclase may be abundant in sand and sedimentary rocks but it is not as common there as K-feldspar because its resistance to weathering processes is not as good. NaAlSi3O8 (albite) — CaAl2Si2O8 (anorthite)

Orthoclase (K,Na)AlSi3O8

Microcline (K,Na)AlSi3O8

Sanidine (K,Na)AlSi3O8

1.1.3 Feldspathoid group

Feldspathoids are minerals that somewhat resemble feldspars and usually form instead of feldspars if there is not enough silica available. Compared to feldspars, feldspathoids are relatively rare minerals.

Nepheline

Leucite

Sodalite Sodalite can be considered a mineral group that also includes haüyne and nosean. Sodalite group minerals occur mostly in silicon-deficient, sodium-rich igneous rocks but also in some contact metamorphosed carbonate rocks. Sodalite: Na8Al6Si6O24Cl2

1.1.4 Zeolite group

Zeolites are the largest group of silicate minerals but not the most abundant. There are more than 80 naturally occurring zeolites. These minerals are widely used because they contain voids in the structure which makes them capable of carrying many chemical compounds. Zeolites are mostly too fine-grained to be identified without sophisticated analytical tools.

Stilbite

Natrolite Natrolite forms radiating aggregates of acicular (needle-like) crystals in the vesicles of mafic volcanic rocks.

1.1.5 Other framework silicates

There are two silicate minerals (cordierite and beryl) that are usually considered to be ring silicates but they could also be included here because there are three-dimensional network of silicon tetrahedra in addition to ring structure. I decided to keep them among ring silicates as it is usually done because the Si:O ratio puts them firmly into the ring silicate group. So, scapolite is the only one here that is abundant enough to be worth mentioning.

Scapolite Scapolite is found in various metamorphic rocks. Most commonly it is found in metamorphosed carbonate rocks (skarns) and amphibolite.

1.2 Sheet silicates (phyllosilicates)

An important group of minerals, especially within the topmost 20 kilometers of the crust. All sheet silicates are hydrous. That’s why they are important in the upper part of the crust. There is too high pressure for them in the lower crust — water gets pushed out of the crystal structure. Silicon tetrahedra are joined to form parallel sheets. Si:O ratio is 2:5 (0.40).

1.2.1 Serpentine group

Serpentine group minerals are an alteration products of olivine and pyroxene (ultramafic rocks).

Chrysotile Chrysotile is the most widely used asbestos mineral. Mg3Si2O5(OH)4

Antigorite & lizardite Antigorite and lizardite are macroscopically indistinguishable. Therefore, I decided to describe them together in one post. Mg3Si2O5(OH)4

1.2.2 Mica group

Micas comprise about 4% of the crust.

Muscovite Muscovite is a very common mica found in many rock types. KAl2(AlSi3O10)(OH)2

Biotite

Lepidolite

Glauconite Glauconite is a green-colored mineral (it shares characteristics of both micas and clay minerals) found mostly in marine sandstones. The chemical composition of glauconite is complex because of numerous possible replacements.

1.2.3 Chlorite group

There are many chlorite group minerals which are generally indistinguishable from each other by their physical and optical properties. Sometimes chlorite is included in the clay minerals group but I decided to keep it separately because chlorite is also an important metamorphic mineral which clay minerals are not.

Chlorite Chlorite group minerals are generally green in color and occur mostly in low- or medium-grade mafic or pelitic metamorphic rocks (greenstone, chlorite schist) or in soil as a weathering product of mafic minerals.

1.2.4 Clay minerals

These minerals are so fine-grained that you have no hope to see individual crystals with a naked eye. They are very important for us because they occur in the upper parts of the crust and are widespread (think about mud which is in large part composed of clay minerals). Clay minerals are weathering products of other silicate minerals.

Kaolinite Kaolinite is mostly a weathering and low-temperature hydrothermal alteration product of felsic rocks (granite and similar). Al2SiO2O5(OH)4

Smectite

Illite

1.2.5 Other sheet silicates

Talc Talc is a metamorphic mineral. It is an alteration product of Mg-rich rocks (mostly serpentinite and impure dolostone). Mg3Si4O10(OH)2

Stilpnomelane

Prehnite

Chrysocolla

Rare sheet silicates I have written about: zinnwaldite.

1.3 Chain silicates (inosilicates)

Chain silicates are common minerals in mafic igneous (especially pyroxenes) and metamorphic (especially amphiboles) rocks. Their structure is composed of many parallel chains (double chains in amphiboles) of silicon tetrahedra. That’s why the crystals of chain silicates tend to be elongated. Pyroxenoids are relatively scarce minerals that are structurally similar to pyroxenes. Si:O ratio is 1:3 (pyroxenes and pyroxenoids) or 4:11 (amphiboles) (0.33 or 0.36).

1.3.1 Pyroxene group

Augite Augite is the most common pyroxene. Augite occurs mostly in mafic igneous rocks.

Orthopyroxene Named so because they are orthorhombic (crystal system). Other pyroxenes are monoclinic. There used to be several different names for various orthopyroxenes (enstatite, bronzite, hypersthene, etc.) but the use of these names is no longer recommended. Orthopyroxenes occur mostly in mafic and ultramafic ignaous rocks (norite, pyroxenite). (Mg,Fe)2Si2O6

Omphacite

Spodumene

1.3.2 Pyroxenoid group

Wollastonite Wollastonite occurs mostly in metamorphosed carbonate rocks with other calc-silicate minerals like Ca-garnets, calcite, tremolite, diopside, etc. CaSiO3.

Rhodonite

1.3.3 Amphibole group

Amphiboles are common minerals in metamorphic rocks (amphibolite, glaucophane schist) and some igneous rocks (especially diorite). Amphiboles are hydrous minerals. Therefore, amphiboles are not stable in anhydrous and high-temperature conditions where they tend to transform to pyroxenes.

Hornblende Hornblende is not a single mineral. It is the most abundant mineral group within amphiboles. Hornblende occurs in many igneous (diorite, andesite, granitoids) and metamorphic rocks (amphibolite, hornblende gneiss). The chemical composition of hornblende is interesting because it contains all eight chemical elements that are most abundant in the Earth’s crust (O, Si, Al, Fe, Ca, Na, K, Mg).

Glaucophane

Riebeckite Riebeckite is a sodic amphibole that occurs mostly in acidic Na-rich igneous rocks. Fibrous asbestiform variety of riebeckite is known as Crocidolite (on the picture). Crocidolite occurs in metamorphosed iron formations.

Arfvedsonite

Rare chain silicates I have written about: charoite.

1.4 Disilicates (sorosilicates)

The structural element that defines this group is a double silicon tetrahedron that looks like a bow tie (two tetrahedrons share the same oxygen ion). There are not too many disilicates but they tend to resist weathering quite well and are therefore often abundant in sand. Si:O ratio is 2:7 (0.29).

1.4.1 Epidote group

Epidote Epidote is the most abundant disilicate. Epidote occurs mostly in metamorphic rocks. Epidote is also a common mineral in sand.

Zoisite

Allanite

1.4.2 Other disilicates

Vesuvianite Vesuvianite is not particularly common mineral but it occurs sometimes in metamorphosed limestone, nepheline syenite, and altered mafic rocks.

1.5 Ring silicates (cyclosilicates)

A small group of minerals that contain 6-fold rings of silicon tetrahedra that are on top of each other to form columns. Si:O ratio is 1:3 (0.33).

Beryl Beryl occurs mostly in granitic pegmatites where it may form beautiful crystals. Beryl is a valued gemstone (emerald and aquamarine). Al2Be3Si6O18

Cordierite

Tourmaline

Eudialyte

1.6 Orthosilicates (nesosilicates)

Silicon tetrahedra are isolated from each others in orthosilicates. Orthosilicates is a diverse and widespread group of minerals. Si:O ratio is 1:4 (0.25).

1.6.1 Garnet group

There are two subgroups of garnets: pyralspites and ugrandites. Pyralspites (pyrope, almandine, spessartine) are reddish and occur in aluminium-rich metamorphic rocks or igneous rocks. Ugrandites (uvarovite, grossular, andradite) are brownish and occur mostly in calc-silicate rocks.

Pyrope Pyrope is a garnet that occurs in ultramafic rocks and serpentinites (metamorphosed ultramafic rocks). Mg3Al2(SiO4)3

Almandine

Spessartine

Andradite Andradite is the most common garnet in skarns.

Grossular

1.6.2 Aluminum silicates

Aluminium silicates andalusite, kyanite, and sillimanite share the same composition but differ structurally. They are metamorphic minerals that transform from one form to another with changing pressure and temperature.

Kyanite Kyanite is a high-pressure polymorph of aluminium silicates. It occurs mostly in aluminium-rich (pelitic) metamorphic rocks. Al2SiO5

Andalusite Al2SiO5

1.6.3 Other orthosilicates

Zircon Zircon is widely distributed in igneous rocks but usually it occurs in very small crystals. Zircon is highly resistant to weathering and alteration. Hence, it is a common mineral in sediments and metamorphic rocks. Zircon is an especially useful mineral in geological research. ZrSiO4

Staurolite

Olivine

Chloritoid

Topaz Topaz is sometimes found in pegmatites along with other minerals hosting incompatible chemical elements.

Titanite Not exactly about the mineral itself but about the issues surrounding its name: sphene or titanite.

2. Carbonates, sulfates, phosphates, and borates

These minerals are classified according to anionic groups: CO32- (carbonates), SO42- (sulfates), PO43- (phosphates), etc.

2.1 Carbonates

Carbonates are an important group of minerals that are most widespread in sedimentary environments, evaporite deposits, and hydrothermal veins. These are environments where carbon dioxide is generally available to form the fundamental building block of carbonate minerals — the carbonate ion.

Calcite Calcite is a very common mineral, especially in sedimentary environments.

Dolomite Dolomite occurs mostly in dolostones (often named dolomites as well) which were originally limestone formations. CaMg(CO3)2

Magnesite

Rhodochrosite

Malachite Malachite is a green copper-bearing hydrated carbonate mineral (Cu2CO3(OH)2).

Aragonite

Strontianite

Azurite

2.2 Sulfates

Most sulfates occur in evaporite deposits (gypsum, for example) or hydrothermal veins (barite).

Gypsum Gypsum is a common marine evaporite mineral. It is often associated with other evaporites like halite and sylvite. CaSO4·2H2O

Anhydrite

Barite

Celestine Celestine is a strontium sulfate mineral that is used extensively in pyrotechnics.

2.3 Phosphates

Phosphates are widespread although not very abundant minerals because phosphorous comprises only 0.1% of the Earth’s crust.

Apatite Apatite occurs as an accessory mineral in many igneous rocks. It is also an important biomineral. Tooth enamel, for example, is made of apatite. Several marine animals use apatite to build their shells. Ca5(PO4)3(OH,F,Cl)

Monazite

Rare phosphate minerals I have written about: variscite.

2.4 Borates

Borates are almost exclusively evaporite minerals found in saline lakes. Borate minerals are not found in regions with wet climate because they are moderately soluble in water.

Colemanite An evaporite mineral found in the deposits of saline lakes. It forms as a secondary mineral at the expense of other borates ulexite and borax. CaB3O4OH3·H2O

Ulexite

3. Oxides, hydroxides, and halides

These minerals are grouped together because they all have so-called simple anions.

3.1 Oxides

Oxides are compounds of oxygen and one or more metal cations. Oxygen is the most abundant chemical element in the crust but much of it is already tied up in silicates (silicon is number 2 in abundance after oxygen). Oxides typically do not form the major part of rocks but they are very widely distributed. Rocks that contain high concentration of oxide minerals are possible metal ores.

3.1.1 Spinel group

Spinel group contains three subgroups or series: spinel series, magnetite series, and chromite series. They are all structurally similar but differ in composition. Each series has one important mineral that also gives name to the series. They are the minerals that are described below.

Spinel Spinel is a colorful mineral that occurs mostly in metamorphic rocks. Spinel is relatively resistant to weathering. So, it is often found in sand. MgAl2O4

Magnetite Magnetite in the most abundant mineral of spinel group. It is found in many igneous and metamorphic rocks but also forms an important component of detrital sediments. Fe3O4

Chromite Chromite is the principal ore of chromium. It occurs mostly in ultramafic cumulate rocks. FeCr2O4

3.1.2 Other oxides

Cuprite Cu2O

Hematite Fe2O3

Corundum Al2O3

Ilmenite FeTiO3

Rutile TiO2

Cassiterite Cassiterite is an oxide of (SnO2). It is the principal ore of tin. Cassiterite crystals have a strong luster and they are often twinned (elbow twins).

3.2 Hydroxides

Hydroxides are common minerals that occur as fine-grained (earthy) aggregates which are often mixtures of several minerals. Limonite, for example, is not a distinct mineral species because it contains several fine-grained minerals, most of them hydroxides. Hydroxides are very common minerals in soil.

Goethite Goethite is a common brown-colored iron hydroxide that is an important weathering product of many iron-bearing minerals. Goethite is a chief component of limonite. FeO(OH)

Brucite Mg(OH)2

Gibbsite Al(OH)3

3.3 Halides

Halides are minerals where anion is one of the halogen elements (Cl, F, Br, I). Only three halides are common minerals. Halite and sylvite are very common evaporites and fluorite is a common hydrothermal mineral.

Halite Halite is well-known as ordinary table salt. Halite is very important evaporite mineral. Every liter of seawater with average salinity contains 35 grams of dissolved halite. NaCl

Sylvite KCl

Fluorite CaF2

4. Sulfides and similar minerals

Sulfides is a very large mineral group (nearly 600 minerals). Only few sulfides are really abundant but many of them are important ore minerals. Sulfides occur mostly in hydrothermal veins.

4.1 Sulfides

Sphalerite

Galena

Chalcopyrite

Cinnabar

Pyrite Pyrite is the most abundant sulfide mineral. Pyrite is found in hydrothermal veins, igneous rocks (as an accessory mineral), some metamorphic rocks, and sedimentary rocks as well (slate, coal, etc.). Pyrite is also an important biomineral, it is often formed in biologically controlled processes. FeS2

Marcasite

Molybdenite Molybdenite is steel gray and very soft mineral. Molybdenite is the principal source of molybdenum.

Covellite

Orpiment Orpiment is a golden yellow arsenic-bearing mineral. As2S3

4.2 Sulfarsenides

Sulfarsenides are minerals where both sulfur and arsenic are anions. Only one sulfarsenide is common.

Arsenopyrite Arsenopyrite is the most common arsenic-bearing mineral. It forms in hydrothermal veins. Sometimes arsenopyrite is found in metamorphic rocks. In hydrothermal veins, arsenopyrite is often associated with gold. FeAsS

5. Native elements

Native elements are rare in the crust. Graphite is the most common of them and forms sometimes significant parts of rocks. Other native elements are found in small quantities and they are often very valuable mineral resources.

5.1 Metals

Metals rarely occur as native elements.

Gold Gold is usually found in hydrothermal sulfide deposits or in quartz veins in greenstones. Gold is also found as detrital grains in sand. Gold is also a biomineral. Several microbes are capable of extracting gold from solutions and precipitating it inside the cell. Gold in numbers is an article that might be interesting for those interested in gold. Au

Copper Cu

5.2 Nonmetals

Sulfur and graphite are the most common non-metal native elements in the crust. Diamond is very rare. It is definitely no rock-forming mineral but I think it is appropriate to include it here because diamond is highly sought after jewel and important for industry because of its extreme hardness.

Sulfur Sulfur occurs around fumaroles and volcanic vents, that’s where it is most easily noticeable. However, largest concentrations of native sulfur are found in the upper part of gypsum-bearing (which provides the sulfur) salt domes that are associated with hydrocarbons (crude oil and natural gas). Sulfur there is a waste product of anaerobic sulfur-reducing bacteria.

Graphite Graphite is a pure carbon which we often use as a pencil lead.

Diamond Diamond is the hardest mineral and used extensively in industry as well as a jewel.

References

1. Klein, C., Hurlbut, C. S. (1993). Manual of Mineralogy, 21st Edition. John Wiley & Sons.
2. Commission on New Minerals, Nomenclature and Classification