Mineral Formation: Silicates, Crystal Systems & Mohs Scale

Ninety Percent of Earth's Crust Is Made of One Group

Silicate minerals — those built around the silicon-oxygen tetrahedron (SiO4⁴⁻) — constitute approximately 90% of Earth's continental crust by volume. Feldspar alone makes up roughly 60% of the exposed crust; quartz adds another 12%. Of the approximately 5,800 known mineral species recognized by the International Mineralogical Association, the vast majority of economically and geologically significant minerals belong to just a few structural families. Understanding mineral classification by chemistry and structure is the foundation of petrology, mining, materials science, and a growing part of the clean energy supply chain.

Silicate vs. Non-Silicate Classification

The primary division in mineralogy runs between silicate and non-silicate groups. Silicates share the fundamental SiO4 tetrahedral building block; non-silicates are classified by their dominant anion or anionic group.

Formation Environments

Minerals form in distinct geological environments, and their chemistry reflects the physical conditions — temperature, pressure, and fluid chemistry — present at formation. Geologists use mineral assemblages as thermometers and barometers of ancient conditions.

• Igneous minerals: Crystallize from silicate melts (magma/lava) as temperature drops. Bowen's Reaction Series predicts the order: olivine and pyroxene crystallize at high temperatures (~1,200°C–1,400°C); feldspars crystallize in the 800°C–1,100°C range; quartz, muscovite, and potassium feldspar last at ~650°C–800°C. Deep plutonic bodies (granites) cool slowly and form large crystals (coarse-grained texture); lava flows cool rapidly and form tiny crystals or glass (fine-grained or aphanitic texture).
• Sedimentary minerals: Form by precipitation from solution (evaporites: halite, gypsum), by biochemical processes (calcite in shells, silica in chert from diatom tests), or as weathering residues (kaolinite from feldspar hydrolysis, iron oxides from pyrite oxidation). Evaporite basins — where seawater or lake water evaporates — produce gypsum first (at ~3.35 salinity), then halite, then potassium and magnesium salts at extreme concentration.
• Metamorphic minerals: Form when pre-existing minerals recrystallize under elevated temperature and pressure without melting. Index minerals defined by Barrow's Zones track metamorphic grade: chlorite (lowest grade) → biotite → garnet → staurolite → kyanite → sillimanite (highest grade). Garnet pressure-temperature stability makes it a crucial geothermobarometer.

Crystal Systems

Crystals are classified into seven systems based on the symmetry of their unit cell — the smallest repeating structural unit. Every mineral belongs to exactly one system, and crystal habit (visible shape) often reflects internal symmetry.

Mohs Hardness Scale

Friedrich Mohs devised his relative hardness scale in 1812, ranking ten reference minerals from softest to hardest based on scratch resistance. The scale remains the standard field tool for mineral identification because hardness depends on bond strength and crystal structure — properties intrinsic to each mineral.

• 1 — Talc: Scratched by a fingernail; used in cosmetics and as a lubricant filler.
• 2 — Gypsum: Barely scratched by a fingernail; major component of drywall (calcium sulfate dihydrate).
• 3 — Calcite: Scratched by a copper coin; dissolves in dilute hydrochloric acid (diagnostic).
• 4 — Fluorite: Scratched by a steel knife; the eponym of fluorescence and the fluorine supply chain.
• 5 — Apatite: Scratched by a steel knife with difficulty; primary mineral in tooth enamel and bone.
• 6 — Orthoclase feldspar: Scratches glass; defines the hardness of most glass (~5.5).
• 7 — Quartz: Scratches steel; the hardness benchmark for most sand, meaning it abrades most materials.
• 8 — Topaz: Scratches quartz; used as gemstone.
• 9 — Corundum: Ruby and sapphire; used as industrial abrasive (emery).
• 10 — Diamond: Scratches all other minerals; 58x harder than corundum on an absolute scale (Vickers hardness ~10,000).

Rare Earth Elements and Their Mineral Hosts

The 17 rare earth elements (REEs) — the 15 lanthanides plus scandium and yttrium — are not geochemically rare in absolute terms (cerium is more abundant in Earth's crust than copper), but they concentrate into economically minable deposits in only a few mineral phases. Bastnäsite (a REE fluorocarbonate) hosts the majority of global REE production; monazite (a REE phosphate) and xenotime (yttrium phosphate) are significant secondary hosts.

China controls approximately 60% of global REE mine production and ~85% of REE processing capacity (USGS 2023 Mineral Commodity Summaries). REEs are indispensable in neodymium-iron-boron (NdFeB) permanent magnets used in EV motors and wind turbines, europium and terbium in phosphor lighting and displays, and lanthanum in fluid catalytic cracking catalysts. The supply concentration has driven a global scramble for REE projects in Australia (Mt. Weld, operated by Lynas), Canada, and the United States (Mountain Pass, California — reopened in 2017).