The classic 1–10 mineral hardness scale — and the mnemonic that makes it permanent
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).
The systematic properties used to identify any mineral — without a chemistry lab
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).
Color
Unreliable alone
Luster
Metallic vs non-metallic
Hardness
Mohs scratch test
Streak
Powder color — more reliable
Cleavage
Flat breaks along planes
Specific gravity
Density vs water
Silicates
Silicates: SiO₄ tetrahedra — isolated, chains, sheets, frameworks. 90% of Earth's crust. Quartz = 3D framework.
Silicate Minerals
The dominant mineral group in Earth's crust — built from the silicon-oxygen tetrahedron
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).
The most diagnostic physical property of many minerals — how they break
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).
Bowen's Reaction Series
Bowen's Series: olivine → pyroxene → amphibole → biotite → K-feldspar → muscovite → quartz. First in, first to weather.
Bowen's Reaction Series
The order in which minerals crystallize from cooling magma — and why it also predicts weathering sequence
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.
How minerals form — four processes that create all natural inorganic compounds
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 Elements
Native elements: pure single-element minerals. Gold, silver, copper, sulfur, graphite, and diamond are all native elements.
Native Elements
Minerals made of just one element — from precious metals to graphite and diamond
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.
Mnemonic
What it means
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🎓 Common Exam Questions
Q: What does CLHSC stand for — explain each mineral identification property.
A: CLHSC = Color, Luster, Hardness, Streak, Cleavage. Color: least reliable property — a single mineral can occur in many colors due to impurities (quartz: clear, purple amethyst, pink rose quartz, yellow citrine). Luster: how a mineral reflects light. Metallic (gold, galena), vitreous/glassy (quartz, feldspar), pearly (talc, muscovite), silky (gypsum fibers), resinous (sulfur), adamantine (diamond). Hardness: resistance to scratching — Mohs scale 1 (talc) to 10 (diamond). A mineral cannot be scratched by anything softer than itself. Streak: color of the powdered mineral rubbed on an unglazed porcelain streak plate — more reliable than color (hematite is red-brown streak regardless of specimen color). Cleavage: tendency to break along flat planar surfaces reflecting weak atomic bonds. Number of cleavage directions (1 = mica; 3 at 90 degrees = halite; 3 not at 90 degrees = calcite) and their angles are diagnostic.
Q: Explain Bowen's Reaction Series — what determines the order of mineral crystallization?
A: N.L. Bowen (1922) experimentally determined the crystallization sequence from cooling basaltic magma. Two branches crystallize simultaneously: Discontinuous (Ferromagnesian) branch: olivine (highest melting point, ~1200 degrees C) → pyroxene → amphibole → biotite. Each mineral reacts with remaining melt to form the next — if reaction is incomplete (rapid cooling), both old and new minerals coexist. These are Mg-Fe rich, dark-colored (mafic). Continuous (Plagioclase) branch: calcium-rich plagioclase (anorthite) → gradually becomes sodium-rich (albite). Composition changes continuously. At the bottom: K-feldspar, muscovite, then quartz crystallize last at about 600-700 degrees C. The series explains: why granite (quartz + feldspar) and gabbro (olivine + pyroxene) are so different despite deriving from similar basaltic parent magma. Early minerals = mafic/ultramafic; late minerals = felsic. Fractional crystallization (removing early minerals) progressively evolves the melt composition.
Q: What are silicates and what determines the different silicate mineral structures?
A: Silicates are built around the SiO4 tetrahedron — silicon bonded to 4 oxygen atoms. About 90% of Earth's crust. The structure depends on how tetrahedra are linked: Isolated (nesosilicates): tetrahedra not connected to each other. Example: olivine (Mg,Fe)2SiO4 — dense, high melting point, early in Bowen's Series. Single chains (inosilicates): tetrahedra share 2 oxygens. Example: pyroxene (SiO3 repeat unit) — 2 cleavage directions at 90 degrees. Double chains: tetrahedra share alternately 2 and 3 oxygens. Example: amphibole — 2 cleavage directions at 60/120 degrees (diagnostic difference from pyroxene). Sheet silicates (phyllosilicates): all 4 oxygens of each tetrahedron shared with 3 others in a flat sheet. Examples: micas (muscovite, biotite), clay minerals, talc. Perfect 1-direction cleavage. Framework (tectosilicates): all 4 oxygens shared — 3D network. Examples: quartz (SiO2), feldspar. Very stable, no cleavage (quartz) or complex cleavage (feldspar). Structure = bonds = cleavage = physical properties.
Q: What are BIFs (Banded Iron Formations) and REEs (Rare Earth Elements) — why are they economically significant?
A: BIFs (Banded Iron Formations): Precambrian sedimentary deposits of alternating iron-rich layers (hematite Fe2O3, magnetite Fe3O4) and silica (chert). Formed 2.5-1.8 Ga when: (1) Early oceans were rich in dissolved Fe2+ (no oxygen to oxidize it). (2) Cyanobacteria evolved oxygenic photosynthesis, releasing O2. (3) O2 reacted with Fe2+ to form insoluble Fe3+ oxides that settled to the seafloor. BIFs mark the Great Oxidation Event. Economically: world's primary iron ore deposits — Pilbara (Western Australia), Carajas (Brazil), Mesabi Range (Minnesota). REEs (Rare Earth Elements): 17 elements (La through Lu lanthanides, plus Sc and Y). Not truly rare in crust, just rarely concentrated. Critical for clean energy technology: Nd and Dy for permanent magnets in EV motors and wind turbines, Eu and Tb for LED and fluorescent screens, Ce for catalytic converters. Strategic importance: China controls ~60% of global production and more of processing — supply chain vulnerability for clean energy transition.
Q: How do the six crystal systems relate to mineral properties?
A: Crystal systems are based on the symmetry of the unit cell (the repeating structural unit). Six systems: Cubic (isometric): 3 equal axes at 90 degrees. Highest symmetry. Minerals: halite, garnet, pyrite, diamond, galena, fluorite. Equidimensional habit, perfect cubic or octahedral cleavage. Tetragonal: 2 equal horizontal axes, 1 different vertical axis, all 90 degrees. Minerals: zircon (prisms with pyramidal terminations), cassiterite. Orthorhombic: 3 unequal axes, all 90 degrees. Minerals: olivine, barite, sulfur. Hexagonal: 3 equal axes at 60 degrees + vertical axis perpendicular. Minerals: quartz (well-formed 6-sided prisms), calcite, apatite, tourmaline. Includes Trigonal subsystem. Monoclinic: most common system for rock-forming minerals. 3 unequal axes, one angle is not 90 degrees. Minerals: orthoclase feldspar, amphiboles, pyroxenes, micas, gypsum. Triclinic: lowest symmetry. All axes unequal, no 90 degree angles. Minerals: plagioclase feldspar, kyanite. Crystal system controls: cleavage directions, optical properties (birefringence), and habit (outward shape).