3.1 Mineralogy: properties, crystallography & silicate classification

What Defines a Mineral

A mineral is a naturally occurring, inorganic solid with a definite (though not fixed) chemical composition and an ordered internal atomic arrangement (a crystal structure). Five criteria must all be met: naturally occurring, inorganic, solid at surface conditions, definite chemical composition, and an ordered crystalline structure. Substances that fail one or more test are mineraloids: obsidian (volcanic glass, no ordered structure), opal (amorphous), and coal (organic). The ASBOG Fundamentals (FG) and Practice (PG) exams lean heavily on identifying rock-forming minerals from their diagnostic physical properties, so master the properties below.

Physical Properties

• Hardness — resistance to scratching, measured on the Mohs scale (1 to 10), a relative, ordinal scale: 1 talc, 2 gypsum, 3 calcite, 4 fluorite, 5 apatite, 6 orthoclase, 7 quartz, 8 topaz, 9 corundum, 10 diamond. Useful field references: fingernail ~2.5, copper penny ~3, steel knife or glass plate ~5.5, steel file ~6.5. The scale is nonlinear — the absolute hardness jump from corundum (9) to diamond (10) is far larger than any step below it.
• Cleavage vs. fracture — cleavage is the tendency to break along planes of weak bonding, giving flat, reproducible surfaces described by the number of directions and the angles between them. Micas show 1 perfect direction; feldspar and pyroxene show 2 directions at ~90 degrees; amphibole shows 2 directions at ~56/124 degrees; halite and galena show 3 at 90 degrees (cubic); calcite shows 3 not at 90 degrees (rhombohedral); fluorite shows 4 (octahedral). Fracture is breakage not controlled by structure — quartz breaks with a curved conchoidal fracture.
• Luster — how a surface reflects light: metallic (galena, pyrite) versus nonmetallic (vitreous/glassy, resinous, pearly, silky, adamantine, earthy/dull).
• Streak — the color of the powdered mineral rubbed on unglazed porcelain; more reliable than color. Hematite looks steel-gray or red in hand sample but always leaves a red-brown streak.
• Specific gravity (SG) — density relative to water. Most silicates cluster near 2.6 to 3.0 (quartz is 2.65), whereas metallic ore minerals are far denser (galena 7.5, native gold 19.3).
• Other diagnostics — color (often unreliable, altered by trace impurities), crystal habit, tenacity, magnetism (magnetite), effervescence in dilute HCl (calcite), and taste (halite).

Crystallography: The Seven Crystal Systems

Crystalline solids are grouped by symmetry into seven systems, each defined by crystallographic axis lengths (a, b, c) and the angles between them:

Monoclinic and triclinic symmetries are the most common among rock-forming silicates.

Silicate Classification

The building block of every silicate is the silica tetrahedron, (SiO4) with a 4-minus charge: one small silicon cation bonded to four oxygen anions. Classes are defined by how tetrahedra polymerize (share corner oxygens), which sets the Si:O ratio and controls physical behavior:

• Nesosilicates (isolated tetrahedra) — no shared oxygens, Si:O = 1:4; olivine (Mg,Fe)2SiO4, garnet, zircon. Cations link isolated tetrahedra, so no cleavage dominates.
• Sorosilicates (paired) — two tetrahedra share one oxygen, Si2O7; epidote.
• Cyclosilicates (ring) — Si6O18; beryl, tourmaline.
• Inosilicates (chain) — single chains give pyroxene (Si:O 1:3, cleavage at ~90 degrees); double chains give amphibole (Si4O11, cleavage at ~56/124 degrees).
• Phyllosilicates (sheet) — Si2O5; one perfect cleavage from stacked sheets. Micas (biotite, muscovite), clays, chlorite, talc, and serpentine.
• Tectosilicates (framework) — every oxygen shared, Si:O = 1:2. Quartz (SiO2), feldspars, and feldspathoids; feldspars alone make up roughly half the crust.

Non-Silicate Mineral Classes

Although silicates dominate the crust, the ASBOG exams also test the major non-silicate classes, each defined by its characteristic anion or anionic group:

• Native elements — a single element (gold Au, copper Cu, sulfur S, and both carbon polymorphs diamond and graphite).
• Oxides — metal plus oxygen (hematite Fe2O3, magnetite Fe3O4, corundum Al2O3); important ore and accessory minerals.
• Sulfides — metal plus sulfur (pyrite FeS2, galena PbS, sphalerite ZnS, chalcopyrite); the principal metallic ore minerals.
• Sulfates — the sulfate group (gypsum CaSO4 with water, anhydrite CaSO4, barite BaSO4).
• Carbonates — the carbonate group (calcite and aragonite CaCO3, dolomite); calcite effervesces in dilute HCl.
• Halides — halogen salts (halite NaCl, fluorite CaF2, sylvite KCl).

Polymorphs are minerals with identical chemistry but different crystal structures, and because properties flow from structure they can differ sharply: diamond (hardness 10) versus graphite (hardness 1), both pure carbon; calcite versus aragonite, both CaCO3; and the aluminosilicate trio kyanite, andalusite, and sillimanite, whose stability fields are used as pressure-temperature indicators.

As polymerization increases from neso- to tectosilicates, silicon and oxygen content rises, and the minerals become more felsic and more resistant to weathering. This trend mirrors the crystallization order of Bowen's reaction series covered in the next section, tying mineral structure directly to petrology.