Minerals: The Chemical and Structural Foundations of Geology

In the study of geology and mineralogy, a mineral is defined as a naturally occurring solid substance characterized by a well-defined chemical composition and a specific crystal structure. While the term is often used interchangeably with "rock" in casual conversation, the two are scientifically distinct. A rock is a bulk geologic material that may be composed of a single mineral or an aggregate of several different mineral phases.

: Crystals of serandite, natrolite, analcime, and aegirine from Mont Saint-Hilaire, Quebec, Canada

Key Facts

• Composition: Minerals have a specific chemical formula and a regular internal atomic arrangement.
• Silicates: This group makes up approximately 90% of the Earth's crust.
• Hardness: The Mohs scale is a standard method for measuring mineral scratch resistance, ranging from 1 (talc) to 10 (diamond).
• Biomineralization: Some minerals, like calcite or hydroxylapatite, are produced by living organisms.
• Classification: Major systems include the Dana and Strunz classifications, primarily based on chemical constituents.

Mineral Classification and Chemistry

Minerals are categorized based on their chemical makeup. The most prevalent group is the silicates, which are defined by their silicon-oxygen structures. Other significant groups include:

• Native Elements: Substances composed of a single pure element, such as gold.
• Sulfides: Compounds containing sulfur, such as galena (PbS).
• Oxides: Compounds containing oxygen, such as quartz (SiO2).
• Halides: Minerals like halite (NaCl).
• Carbonates: Minerals such as calcite (CaCO3).
• Sulfates: Examples include gypsum (CaSO4·2H2O).
• Phosphates: Characterized by the [PO4] tetrahedral unit, including the apatite group.

: Aegirine, an iron-sodium clinopyroxene, is part of the inosilicate subclass.

The Silicate Subclasses

Silicates are further divided by how their silicon-oxygen tetrahedra are bonded:

• Inosilicates: Chain silicates, which can be single-chain (pyroxenes) or double-chain (amphiboles).
• Cyclosilicates: Ring silicates, such as the tourmaline group, often forming strong, elongated crystals.
• Phyllosilicates: Sheet silicates, such as muscovite.
• Tectosilicates: Framework silicates.
• Sorosilicates: Disilicates, featuring a 2:7 silicon-to-oxygen ratio, such as the epidote group.
• Orthosilicates: Also known as nesosilicates.

: Muscovite, a mineral species in the mica group, within the phyllosilicate subclass

: Asbestiform tremolite, part of the amphibole group in the inosilicate subclass

: An example of elbaite, a species of tourmaline, with distinctive colour banding.

: Epidote often has a distinctive pistachio-green colour.

Physical Properties and Identification

Geologists use several diagnostic properties to identify minerals in the field and laboratory.

Crystal Structure and Habit

The crystal habit refers to the characteristic external shape of a mineral crystal. This shape is dictated by the internal crystal system. Common systems include Isometric, Tetragonal, Orthorhombic, Hexagonal, Monoclinic, and Triclinic.

: Topaz has a characteristic orthorhombic elongated crystal shape.

: Contact twins, as seen in spinel

: Natrolite is a mineral series in the zeolite group; this sample has a very prominent acicular crystal habit.

Hardness and the Mohs Scale

Hardness measures a mineral's resistance to being scratched. While the Mohs scale is a relative ranking, it is a fundamental tool for identification. Note that hardness can vary depending on the crystallographic direction; for example, kyanite is harder in one direction than another.

The Mohs Scale of Mineral Hardness

Hardness	Mineral	Chemical Formula
1	Talc	Mg3Si4O10(OH)2
2	Gypsum	CaSO4·2H2O
3	Calcite	CaCO3
4	Fluorite	CaF2
5	Apatite	Ca5(PO4)(OH,Cl,F)
6	Orthoclase	KAlSi3O8
7	Quartz	SiO2
8	Topaz	Al2SiO4(OH,F)2
9	Corundum	Al2O3
10	Diamond	C

: Diamond is the hardest natural material, and has a Mohs hardness of 10.

: Mohs Scale versus Absolute Hardness

Specific Gravity and Lustre

Specific gravity is the ratio of a mineral's density to the density of water. It is a vital diagnostic tool; for instance, metallic minerals like galena or native gold exhibit much higher specific gravities than non-metallic minerals. Lustre describes how light reflects off a mineral's surface, ranging from metallic to dull.

: Pyrite has a metallic lustre.

: Galena, PbS, is a mineral with a high specific gravity.

: Native gold. Rare specimen of stout crystals growing off of a central stalk, size 3.7 x 1.1 x 0.4 cm, from Venezuela.

Cleavage and Fracture

Cleavage is the tendency of a mineral to break along smooth, parallel planes of atomic weakness. If a mineral breaks irregularly without following planes, it is said to exhibit fracture.

: Perfect basal cleavage as seen in biotite (black), and good cleavage seen in the matrix (pink orthoclase).

: When minerals react, the products will sometimes assume the shape of the reagent; the product mineral is termed a pseudomorph of (or after) the reagent. Illustrated here is a pseudomorph of kaolinite after orthoclase. Here, the pseudomorph preserved the Carlsbad twinning common in orthoclase.

Metamorphism and Mineral Changes

Minerals can change through chemical reactions, often driven by heat and pressure during metamorphism. For example, under low-grade metamorphic conditions, kaolinite can react with quartz to form pyrophyllite. As metamorphic grades increase further, pyrophyllite may react to form kyanite and quartz.

: Schist is a metamorphic rock characterized by an abundance of platy minerals. In this example, the rock has prominent sillimanite porphyroblasts as large as 3 cm (1.2 in).

: Sphalerite crystal partially encased in calcite from the Devonian Milwaukee Formation of Wisconsin

: Pink cubic halite (NaCl; halide class) crystals on a nahcolite matrix (NaHCO3; a carbonate, and mineral form of sodium bicarbonate, used as baking soda).

: Gypsum desert rose

: Hübnerite, the manganese-rich end-member of the wolframite series, with minor quartz in the background

: Carnotite (yellow) is a radioactive uranium-bearing mineral.

: Black andradite, an end-member of the orthosilicate garnet group.

: Red cinnabar (HgS), a mercury ore, on dolomite.

Frequently Asked Questions

What is the difference between a mineral and a rock?

A mineral is a substance with a specific chemical composition and crystal structure. A rock is a solid mass that is usually made up of an aggregate of one or more minerals.

How are minerals classified?

Minerals are primarily classified by their chemical composition using systems like the Dana or Strunz classifications. The most common group is the silicates.

What does "cleavage" mean in mineralogy?

Cleavage refers to the tendency of a mineral to break along specific planes of weakness in its crystal structure, resulting in smooth, flat surfaces.

Can living organisms create minerals?

Yes, some minerals are biogenic, meaning they are produced by living organisms. Examples include calcite and hydroxylapatite, which are found in various biological structures.

Why do some minerals have higher specific gravity than others?

Specific gravity is influenced by the atomic mass of the elements within the mineral. Minerals containing heavy elements, such as lead in galena or gold in native gold, will have a much higher specific gravity.