Mineral Study Guide

📖 Core Concepts Mineral – naturally occurring solid with a definite chemical composition and a specific crystal structure. Mineraloid – solid lacking a regular crystal lattice (e.g., opal, obsidian). Mineral Species vs. Variety – different crystal structures = different species (quartz vs. stishovite); same composition but different colour/impurities = variety (amethyst). Mineral Group – a set of species with continuous compositional ranges (e.g., olivine group \((CaxMgyFe{2‑x‑y}SiO4)\)). Solid‑Solution Series – end‑member minerals form a continuous range of compositions by ionic substitution of similar size/charge (e.g., albite ↔ anorthite). Silica Tetrahedron – fundamental SiO₄⁴⁻ unit; its linkage (isolated, chain, sheet, framework) creates the major silicate families. International Mineralogical Association (IMA) Criteria – natural occurrence, solid state, ordered crystal structure, and defined chemistry are required for a mineral to be recognized. Rock vs. Mineral – rock = aggregate of minerals/mineraloids; mineral assemblage = the set of minerals present in a rock (equilibrium vs. non‑equilibrium). --- 📌 Must Remember Silicates make up 90 % of the crust; the 8 most abundant elements are O, Si, Al, Fe, Mg, Ca, Na, K (>98 % of crust weight). Dana & Strunz are the two main classification schemes; both sort minerals first by dominant chemistry. Hardness is measured on the Mohs scale (talc = 1, diamond = 10). Cleavage directions: basal (mica), prismatic (pyroxenes, amphiboles), cubic (galena, halite), rhombohedral (calcite), octahedral (fluorite, diamond), dodecahedral (sphalerite). Polymorphs share formula but differ in crystal system (e.g., quartz vs. stishovite). Streak is a reliable diagnostic for metallic minerals; it is the colour of the powdered mineral. Acid test – carbonates fizz in dilute HCl (calcite instantly, dolomite only when powdered). Magnetism – magnetite = strong; pyrrhotite/ilmenite = weak. --- 🔄 Key Processes Identify a mineral Observe colour, luster, hardness → narrow group. Test cleavage/parting → determine crystal family. Perform streak, acid, magnetism tests → confirm. Classify silicates Count shared O atoms per SiO₄ tetrahedron → determine family: 0 shared → orthosilicates (nesosilicates) 1 shared → sorosilicates (disilicates) 2 shared → inosilicates (chains) → single‑chain (pyroxene) or double‑chain (amphibole) 3 shared → phyllosilicates (sheets) 4 shared → tectosilicates (framework) Determine solid‑solution membership Compare ionic radius & charge of substituting ions. If size/charge similar → substitution possible (e.g., K⁺ ↔ Na⁺). --- 🔍 Key Comparisons Cleavage vs. Parting – Cleavage: systematic, crystallographic planes; Parting: defect‑controlled, varies between crystals. Polymorph vs. Variety – Polymorph: same composition, different crystal structure (e.g., quartz vs. stishovite); Variety: same structure, different colour/impurity (e.g., amethyst vs. clear quartz). Silicate vs. Non‑Silicate – Silicate: SiO₄ tetrahedra dominate; Non‑silicate families (oxides, sulfides, carbonates, etc.) have other anionic groups. Basal vs. Prismatic Cleavage – Basal: one direction (mica); Prismatic: two directions at 90° (pyroxenes, amphiboles). --- ⚠️ Common Misunderstandings Colour is diagnostic – many minerals (e.g., quartz) have wide colour ranges due to trace impurities; rely on hardness, cleavage, etc. All quartz is the same – high‑pressure polymorph stishovite has octahedral Si coordination, completely different properties. Mineraloids are “soft” minerals – mineraloids lack crystal structure but can be hard (e.g., obsidian is glassy yet relatively hard). All sulfides are ore minerals – pyrite is abundant but not an iron ore; only specific sulfides (galena, sphalerite, etc.) are economic ores. --- 🧠 Mental Models / Intuition “Tetrahedron‑linkage ladder” – imagine SiO₄ tetrahedra as building blocks; each type of linkage (corner‑sharing, edge‑sharing) adds a rung to the ladder that defines the silicate family. “Size‑charge match” – substitution works best when the incoming ion fits the “parking spot” (similar ionic radius) and pays the same “toll” (same charge). “Crystal family = cleavage pattern” – the symmetry of the crystal system predicts likely cleavage directions (e.g., cubic → three mutually perpendicular perfect cleavages). --- 🚩 Exceptions & Edge Cases Stishovite – Si adopts six‑fold (octahedral) coordination under high pressure, unlike all other silicates. Dolomite acid reaction – does not fizz unless powdered; the surface layer resists acid until exposed. Barite‑group sulfates – require 12‑fold coordination; CaSO₄ (anhydrite) is excluded because Ca is only 8‑fold coordinated. Polysynthetic twinning in plagioclase – may be misread as multiple minerals; it is a single species with repeated twin law. --- 📍 When to Use Which Use cleavage pattern to distinguish mica (basal) from amphibole (prismatic). Apply acid test when a carbonate is suspected; choose dilute HCl for quick field work. Select magnetic test for sulfides: strong magnetism → magnetite; weak → pyrrhotite/ilmenite. Choose hardness test for metallic vs. non‑metallic lusters: metallic minerals often have low Mohs (e.g., galena ≈ 2.5). Use X‑ray diffraction (XRD) when polymorph identification is required (e.g., quartz vs. stishovite). --- 👀 Patterns to Recognize Consistent 2‑direction cleavage → amphibole/pyroxene (check for prismatic vs. two‑plane angles). Perfect basal cleavage + perfect basal parting → mica group. High specific gravity + metallic lustre + cubic cleavage → galena (PbS). Conchoidal fracture + hardness 7 → quartz. Twinning style – polysynthetic twins → plagioclase; Carlsbad twins → alkali feldspar. --- 🗂️ Exam Traps “All quartz is transparent” – amethyst and smoky quartz are coloured; colour alone is not definitive. “Presence of SiO₄ tetrahedron means silicate” – many non‑silicate minerals also contain Si (e.g., feldspathoids), but they form in silica‑deficient settings and never coexist with quartz. “Any mineral that effervesces in HCl is calcite” – dolomite can also effervesce when powdered; check hardness and crystal habit. “Stishovite is a common quartz polymorph” – it only forms at high‑pressure metamorphic or impact conditions; ignore on most sedimentary/igneous exam questions. “All minerals with perfect cleavage are easy to break” – tenacity is separate; fluorite has perfect octahedral cleavage but is brittle, whereas mica has perfect basal cleavage yet is flexible.