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.

You will find here photos and some insights about the geological background of the minerals described. Please note that the pictures shown below do not necessarily display an isolated crystal of the mineral species. Instead I like to display them in their natural environment – within rocks or sediments.

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 grains in sand 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.

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 crystals in diabase 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
Perthitic exsolution in microcline Microcline (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.

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

1.2 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 grains from sand 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

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 schist 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 clay 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

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.

Spodumene

1.3.2 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

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

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

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

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

Andradite Andradite is the most common garnet in skarns.

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

1.6.3 Other orthosilicates

Zircon concentrate 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.

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 Calcite is a very common mineral, especially in sedimentary environments.
Dolomites in Italy Dolomite Dolomite occurs mostly in dolostones (often named dolomites as well) which were originally limestone formations. CaMg(CO3)2

Magnesite

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

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

Barite

2.3 Phosphates

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

Monazite Monazite is an accessory mineral in felsic igneous rocks like granite, pegmatite, and syenite. It is also found in some metamorphic rocks (gneiss, mica schist, metamorphosed dolomite). Monazite is resistant to weathering and is therefore frequently present in sand.

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.

Ulexite Ulexite is found in saline lakes with other evaporite minerals. Most notable localities of beautiful crystals are California, Chile, and Kazakhstan.

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)

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 grains Fluorite CaF2

4. Sulfides

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.

Chalcopyrite
Pyrite crystals in a quartz vein 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

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.

Graphite Graphite is a pure carbon which we often use as a pencil lead. Graphite is a common mineral in some metamorphic rocks that originally as sediments contained carbonates or organic matter.

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