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.

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.

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.

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.

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.

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.

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).

1.2.2 Mica group

Micas comprise about 4% of the crust.

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.

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.

1.2.5 Other sheet silicates

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

1.3.2 Pyroxenoid group

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.

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

1.4.2 Other disilicates

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).

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.

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.

1.6.3 Other orthosilicates

2. Carbonates, sulfates, phosphates, and borates

These minerals are classified according to anionic groups: CO₃^2- (carbonates), SO₄^2- (sulfates), PO₄^3- (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.

2.2 Sulfates

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

2.3 Phosphates

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

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.

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.

3.1.2 Other oxides

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.

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.

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

4.2 Sulfarsenides

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

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.

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.

References

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