Minerals in the Earth's Crust

There are more than 3000 known minerals (the number is still growing), but of these only about 20 are very common, and only 9 of these constitute 95% of the crust. These 9 minerals are all silicates, and are called the rock forming minerals. They can be subdivided into two groups, the mafic and felsic minerals.

Mafic Minerals

The term mafic is used for silicate minerals, magmas, and rocks which are relatively high in the heavier elements (dominated by Fe, Mg, Ca, Al. Ma stands for magnesium and F stands for iron). The minerals are:

BIOTITE (mica)

Didier Descouens / CC BY-SA

K (FE, Mg)3 AlSi3 O10 (F, OH)2,

Potassium iron magnesium aluminium silicate

hydroxide fluoride.

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AMPHIBOLE c.q. HORNBLENDE

Creator:Robert Lavinsky / CC BY-SA

Hornblende Ca2(Mg, Fe, Al)5(Al, Si)8O22(OH)2,

Calcium Magnesium Iron Aluminium Silicate Hydroxide.

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PYROXENES c.q. AUGITE

Rock Currier / CC BY

Augite (Ca, Na)(Mg, Fe, Al)(Al, Si)2 O6,

Calcium Sodium Magnesium Iron Aluminium Silicate

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OLIVINE

(Mg, Fe)2SiO4, Magnesium iron silicate.

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Ca-PLAGIOCLASE (feldspar)

Public domain

CaAl2 Si2O8,

Calcium aluminium silicate.

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Of these minerals, the first four are of dark (almost black) to greenish colour, and the last one (Ca-Plagioclase) is light to transparent. Thus, mafic rocks are overall of dark colour. Mafic magmas are usually produced at spreading centres, and represent material which is newly differentiated from the upper mantle. Common mafic rocks include basalt and gabbro.

Felsic Minerals

Felsic is a term used for silicate minerals, magmas, and rocks which have a higher percentage of the lighter elements, such as silica and oxygen, aluminium, and potassium. The term is a combination of FEL (for feldspar; in this case the potassium-rich variety) and SIC (indicating a higher percentage of silica). The minerals are:

QUARTZ

Public domain

SiO2

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MUSCOVITE (mica)

James St. John / CC BY

KAl2(AlSi3O10)(F, OH)2,

Potassium aluminium silicate hydroxide fluoride.

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ORTHOCLASE (feldspar)

Didier Descouens / CC BY-SA

KAlSi3O8,

Potassium Aluminium Silicate.

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Na-PLAGIOCLASE/ALBITE (feldspar)

Marie-Lan Taÿ Pamart / CC BY

NaAlSi3O8,

Sodium aluminium silicate.

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Felsic minerals are light in colour and felsic rocks are therefore typically of light colour. The most common felsic rocks are granite and rhyolite. Rocks that are intermediate in composition between these two groups are also called the intermediate rocks. All of these minerals form through crystallization from silicate melts in the crust and mantle.

 

Minerals can form under a variety of conditions, such as:

A) during the cooling of molten materials (steel, from lavas, igneous rocks).

B) during the evaporation of liquids (salt, sugar, reference to evaporites)

C) the cooling of liquids (saturated solution)

D) at high temperatures and pressures new crystals may grow in solid materials (diamonds from coal, metamorphism)