Geology tricks that make minerals memorable

Full mnemonic: 'The Girls Can Flirt And Other Queer Things Can Do' — Talc (1), Gypsum (2), Calcite (3), Fluorite (4), Apatite (5), Orthoclase/Feldspar (6), Quartz (7), Topaz (8), Corundum (9), Diamond (10). Mohs scale is relative (ordinal), not linear — diamond is ~1,500× harder than corundum in absolute terms. Fingernail ≈ 2.5, copper penny ≈ 3, glass ≈ 5.5, steel file ≈ 6.5. Hardness test: scratch unknown mineral with known one. Industrial: diamond (cutting), corundum (sandpaper/rubies/sapphires), quartz (glass/electronics).

Color: unreliable alone (quartz can be clear, purple, pink, white). Streak: color of powder on unglazed porcelain — diagnostic (hematite: red-brown streak regardless of surface color). Luster: metallic vs non-metallic (vitreous/glassy, resinous, pearly, silky, adamantine/diamond). Hardness: Mohs test. Cleavage: flat breaks along atomic planes — number of directions and angles (mica: 1 direction perfect; halite: 3 at 90°). Fracture: conchoidal (quartz), hackly. Specific gravity: density relative to water. Special properties: magnetism (magnetite), effervescence with HCl (calcite), fluorescence (fluorite), taste (halite).

SiO₄ tetrahedron: 4 oxygen around 1 silicon — fundamental building block. Structures: isolated (olivine, garnets — nesosilicates), single chain (pyroxenes), double chain (amphiboles), sheet (micas, clay minerals), framework (quartz, feldspars — every Si-O shared). Framework silicates are most stable/common. Feldspars: most abundant mineral group in crust — plagioclase (Na-Ca) and K-feldspar. Quartz: pure SiO₂, very common, resistant to weathering. Micas: perfect basal cleavage (sheet structure). Olivine: mantle mineral, least stable, weathers first (Bowen's Series).

Cleavage: preferential breakage along planes of weak atomic bonding. Number of directions: mica (1), feldspar and calcite (2–3), halite (3 at 90°). Angles between cleavage planes: diagnostic. Calcite: 3 cleavage at 75° → rhombohedra. Halite: 3 at 90° → cubes. Amphibole vs pyroxene: both have 2 cleavages, but angle differs (amphibole ~60/120°, pyroxene ~90°) — diagnostic for thin section. Fracture types: conchoidal (quartz, obsidian — curved shell-like), hackly (native metals), uneven/irregular. Obsidian cleavage: none (fractures conchoidally — sharp edges, used as cutting tools).

N.L. Bowen (1922): two branches crystallizing from basaltic magma. Discontinuous branch (iron/magnesium): olivine → pyroxene → amphibole → biotite (each replaces previous as temp drops). Continuous branch: Ca-plagioclase → Na-plagioclase (composition shifts as temp drops). Both converge at K-feldspar → muscovite → quartz (last to crystallize, most stable at surface). Goldich Dissolution Series: minerals crystallizing FIRST (high temp, deep) are LEAST stable at surface → weather first. Quartz: crystallizes last, most stable — forms beach sand. Olivine: weathers fastest.

Carbonates: calcite (CaCO₃, reacts with HCl → CO₂), dolomite (CaMg(CO₃)₂), aragonite. Form limestone and marble. Sulfides: pyrite (FeS₂, 'fool's gold' — metallic luster, cubic crystals), galena (PbS, lead ore), chalcopyrite (CuFeS₂, copper ore), sphalerite (ZnS). Oxides: hematite (Fe₂O₃, red streak, iron ore), magnetite (Fe₃O₄, magnetic, iron ore), corundum (Al₂O₃, rubies/sapphires). Sulfates: gypsum (CaSO₄·2H₂O, wallboard), anhydrite, barite. Halides: halite (NaCl, table salt), fluorite (CaF₂, fluorescence). Native elements: gold, silver, copper, sulfur, graphite, diamond.

Seven systems based on symmetry of unit cell: Cubic (isometric): 3 equal axes at 90° — garnet, halite, pyrite, diamond, magnetite. Hexagonal: 4 axes, 3 equal at 60° + vertical — quartz, calcite, graphite, ice. Tetragonal: 3 axes at 90°, 2 equal — zircon, rutile. Orthorhombic: 3 unequal axes at 90° — olivine, aragonite, topaz. Monoclinic: 3 unequal axes, one not at 90° — gypsum, augite, hornblende. Triclinic: all unequal, none at 90° — plagioclase, kyanite. Crystal habit: shape of crystal (prismatic, tabular, acicular, bladed). Crystal system informs cleavage angles.

Diamond: pure carbon, cubic, hardest mineral (10 Mohs). Formed at >150 km depth, brought up in kimberlite pipes. 4Cs: cut, clarity, color, carat. Ruby: corundum (Al₂O₃) + Cr³⁺ impurity → red. Sapphire: corundum + Fe/Ti → blue (also yellow, pink, orange). Myanmar (Burma) best rubies, Kashmir best sapphires. Emerald: beryl (Be₃Al₂Si₆O₁₈) + Cr → green. Colombia best. Aquamarine: beryl + Fe → blue. Amethyst: quartz + Fe + irradiation → purple. Opal: hydrated amorphous silica (not crystalline). Pearl: calcium carbonate from molluscs. Value = rarity + hardness + optical properties.

Ore deposit: mineral concentration economically viable to extract. BIFs (Banded Iron Formations): 3.8–1.8 Ga, Precambrian, source of most iron ore (Pilbara, Labrador). Porphyry copper: large-volume, low-grade Cu deposits associated with intrusive magmatism (Chile — Escondida, Chuquicamata). Hydrothermal veins: hot water deposits minerals in fractures (gold, silver, quartz). Evaporites: evaporation deposits halite, gypsum, potash (fertilizer). Placers: heavy minerals concentrate in streams (gold, tin, diamonds). Critical minerals: lithium (pegmatites, brines — EV batteries), cobalt, rare earth elements (REE (Rare Earth Elements) — wind turbines, electronics). Mining impacts: tailings, acid mine drainage.

Feldspars: most abundant group — K-feldspar (orthoclase, sanidine, microcline) in granite; plagioclase (albite→anorthite solid solution) in most igneous rocks. Distinguish: K-feldspar has pink tint, Carlsbad twinning; plagioclase has striations (albite twinning). Quartz: SiO₂, clear/white/gray, conchoidal fracture, no cleavage, hardness 7. Micas: perfect basal cleavage, flexible sheets — muscovite (light, felsic rocks) and biotite (dark, mafic). Pyroxenes: dark, 2 cleavages at ~90°, in gabbro and basalt. Amphiboles: dark, 2 cleavages at ~60/120°, hornblende in andesite/diorite. Olivine: green, granular, in basalt and mantle.

Magmatic: crystallize from cooling melt — silicates (Bowen's Series). Temperature determines which minerals form. Hydrothermal: hot water (100–500°C) carries dissolved minerals → precipitate in fractures as water cools. Many ore deposits (gold, copper, silver veins). Sedimentary: evaporation (halite, gypsum), biochemical precipitation (calcite in shells → limestone), chemical (chert, BIF). Weathering: decomposition of existing minerals → clay minerals. Metamorphic: existing minerals recrystallize under heat + pressure without melting → new minerals (garnet, staurolite, kyanite, sillimanite index minerals of metamorphic grade).

Native metals: gold (Au, inert — never tarnishes, found in streams and veins), silver (Ag, tarnishes), copper (Cu, oldest metal smelted ~7000 BCE), platinum (rare, high melting point). Native nonmetals: sulfur (yellow, around volcanic vents), diamond (C, cubic, hardest), graphite (C, hexagonal, softest conductor — same element as diamond, different crystal structure). Carbon allotropes: diamond (sp³ bonds, 3D framework → hardest) vs graphite (sp² bonds, flat sheets → soft, conducts electricity). Bismuth, arsenic, antimony also occur as native elements. Native elements ≈ 20 minerals — small group but highly valuable.