Mineral

Understanding Minerals: A Geological Journey

‘What is a mineral, and how does it differ from a rock?’ These are questions that have puzzled geologists for centuries. In the realm of geology and mineralogy, a mineral is defined as a solid substance with a well-defined chemical composition and crystal structure occurring naturally in pure form.

The concept of minerals differs significantly from rocks, which can be homogeneous or composed of multiple minerals. For instance, opal and obsidian, while not crystalline like most minerals, are still classified as mineraloids due to their natural occurrence in pure form.

The International Mineralogical Association (IMA) recognizes 6,118 official mineral species. These minerals vary widely in chemical composition, with some allowing for impurities or different crystal structures. Descriptions often include physical properties such as habit, hardness, color, and specific gravity. Minerals are classified by key chemical constituents using systems like the Dana and Strunz classification.

One of the most intriguing aspects of mineralogy is the debate over biogenic crystalline substances. While Skinner views all solids as potential minerals, including biominerals created by metabolic activities of organisms, the IMA’s decision to exclude these from its definition has sparked controversy.

The Formation and Classification of Minerals

Minerals are formed through natural geological processes, with a well-defined crystallographic structure or ordered atomic arrangement. They must have a fairly well-defined chemical composition, allowing for some variability. The IMA has 6,118 approved mineral species as of January 2025.

The abundance and diversity of minerals are controlled by their chemistry, which is dependent on elemental abundances in the Earth. Eight elements account for most of the key components of minerals: oxygen, silicon, aluminium, iron, magnesium, calcium, sodium, and potassium. These eight elements sum to over 98% of the crust by weight.

Changes in temperature and pressure alter mineralogy, while changes in composition can occur through weathering or hydrothermal alteration. This process is related to the rock cycle and can result in identical or similar bulk rock chemistry without similar mineralogy. Mineral reactions can be illustrated by examples such as the transformation of orthoclase feldspar into kaolinite and pyrophyllite, and quartz into various polymorphs under high temperatures and pressures.

Physical Properties and Classification

The physical properties used for classification include crystal structure and habit, hardness, lustre, diaphaneity, colour, streak, cleavage and fracture, specific gravity, and others. Crystals are restricted to 32 point groups based on their symmetry, classified into six crystal families described by the relative lengths of the three crystallographic axes and angles between them.

Chemistry and crystal structure define minerals, which can have identical crystal structures despite different chemistries (e.g., halite, galena, periclase). Polymorphism occurs when minerals share a chemical formula but differ in structure (e.g., pyrite and marcasite), or when the coordination of elements differs within the same structure. Differences in crystal structure affect physical properties, such as those seen between diamond and graphite due to differences in bonding.

Twinning occurs when two or more crystals of a single mineral species intergrowth, controlled by the mineral’s symmetry. Reticulated twins, geniculated twins, penetration twins, cyclic twins, and polysynthetic twins are different types of twinning in minerals. Crystal habit includes acicular, dendritic, equant, prismatic, botryoidal, fibrous, tabular, and massive forms.

Specific Gravity and Other Properties

Specific gravity numerically describes the density of a mineral. It’s defined as the density of the mineral divided by the density of water at 4 °C and is a dimensionless quantity. Specific gravity can be measured using the quotient of the mass of the sample and difference between its weight in air and weight in water.

High specific gravity is diagnostic for minerals. Variations in chemistry correlate with changes in specific gravity. Oxides and sulfides tend to have higher specific gravities due to their high atomic mass. Minerals with metallic or adamantine lustre often have higher specific gravities than those without.

The Mineral Kingdom

Minerals are defined by unique chemical and physical properties, such as formula and crystalline structure. The Dana and Strunz classifications rely on composition and structure, assigning numbers to mineral species based on their characteristics.

Silicates dominate the Earth’s crust in terms of rock formation and diversity, while non-silicate minerals are economically important as ores. Non-silicate minerals can be classified by their dominant chemistry into categories such as native elements, sulfides, halides, oxides, hydroxides, carbonates, nitrates, borates, sulfates, phosphates, and organic compounds.

The most abundant mineral group is silicates, which make up over 95% of rocks and 90% of the Earth’s crust. Silicate minerals have a base unit of [SiO4]4− tetrahedron, with silicon in four-fold coordination with oxygen. They can form various structures such as one-dimensional chains, two-dimensional sheets, and three-dimensional frameworks through polymerization.

Common Mineral Groups

The aluminosilicates (bkyanite, andalusite, sillimanite) have one [SiO4]4− tetrahedron and one Al3+ in octahedral coordination. The olivine structure features magnesium-rich forsterite and iron-rich fayalite with iron and magnesium in octahedral coordination. Other mineral species having this structure exist, such as tephroite, Mn2SiO4.

The garnet group has a general formula of X3Y2(SiO4)3, where X is a large eight-fold coordinated cation, and Y is a smaller six-fold coordinated cation. There are six ideal endmembers of garnet, split into two groups: the pyralspite garnets (pyrope, almandine, spessartine) and the ugrandite garnets (uvarovite, grossular, andradite).

While there are two subgroups of garnet, solid solutions exist between all six end-members. Other orthosilicates include zircon, staurolite, and topaz. Zircon is useful in geochronology as U6+ can substitute for Zr4+. Staurolite has a complicated crystal structure that was only fully described in 1986. Topaz is a common gemstone mineral.

Native Elements and Sulfides

Native elements are those that are not chemically bonded to other elements, such as gold, silver, and copper. The sulfide minerals are chemical compounds of one or more metals or semimetals with a chalcogen or pnictogen, of which sulfur is most common. Sulfides tend to be soft, brittle minerals with a high specific gravity.

Related to sulfides are the rare sulfosalts, in which a metallic element is bonded to sulfur and a semimetal such as antimony, arsenic, or bismuth. Oxide minerals are divided into three categories: simple oxides, hydroxides, and multiple oxides. Simple oxides have O2− as the main anion and primarily ionic bonding.

Halides, Carbonates, and Sulfates

Halide minerals are compounds in which a halogen (fluorine, chlorine, iodine, or bromine) is the main anion. These minerals tend to be soft, weak, brittle, and water-soluble. Examples include halite, sylvite, and fluorite.

Carbonate minerals are those in which the main anionic group is carbonate, [CO3]2−. Carbonates tend to be brittle with rhombohedral cleavage, and all react with acid. The reaction of acid with carbonates relates to the dissolution and precipitation of the mineral.

Organic Minerals

Organic minerals contain organic carbon but can be formed by geologic processes. An example is whewellite, CaC2O4⋅H2O, which can be deposited in hydrothermal ore veins. Recent advances include the addition of an organic class to mineral classification schemes and the adoption of a hierarchical scheme for naming and classification of minerals.

These advancements have led to seven commissions and four working groups being established to review and classify minerals. Astrobiology suggests that biominerals could be important indicators of extraterrestrial life, particularly on Mars. NASA’s Curiosity and Opportunity rovers are searching for evidence of ancient life, including a biosphere based on autotrophic microorganisms.

The search for evidence of habitability, taphonomy, and organic carbon on the planet Mars has become a primary NASA objective. Understanding minerals is crucial not only in geology but also in fields like astrobiology, where they can provide clues to life beyond Earth.

Understanding minerals opens up a vast world of geological wonders, from the formation of rocks to the search for extraterrestrial life. Whether you’re a student or an enthusiast, delving into the intricacies of mineralogy reveals the beauty and complexity of our planet’s natural resources.