Mineral Classes

Silicates make up about 90% of Earth's crust: Basic Structure: Silicates contain silicon and oxygen, forming the fundamental building block - the SiO₄ tetrahedron. Four oxygen atoms surround one silicon atom in a tetrahedral arrangement. Subclasses: - Nesosilicates: Isolated tetrahedra (olivine, garnet, zircon) - Sorosilicates: Double tetrahedra (epidote, vesuvianite) - Cyclosilicates: Ring structures (beryl, tourmaline) - Inosilicates: Chain structures - single chains (pyroxenes) or double chains (amphiboles) - Phyllosilicates: Sheet structures (micas, clays, talc) - Tectosilicates: Framework structures (quartz, feldspars, zeolites) Common Silicates: Quartz, feldspars, micas, pyroxenes, amphiboles, olivine, and garnet are all silicates. These are the rock-forming minerals. Properties: Silicates vary widely in properties. Framework silicates (quartz, feldspars) are hard and common. Sheet silicates (micas) have perfect cleavage. Chain silicates (pyroxenes, amphiboles) form prismatic crystals. Formation: Form in igneous, metamorphic, and some sedimentary environments. Most common minerals in all rock types.

Minerals containing the carbonate ion (CO₃²⁻): Common Carbonates: - Calcite (CaCO₃): Most common carbonate. Forms limestone and marble. Reacts with acid (effervesces). - Dolomite (CaMg(CO₃)₂): Similar to calcite but with magnesium. Forms dolomite rock. - Aragonite (CaCO₃): Different crystal structure than calcite. Less stable, converts to calcite over time. - Siderite (FeCO₃): Iron carbonate, often brown. - Rhodochrosite (MnCO₃): Manganese carbonate, often pink to red. Properties: Generally soft (3-4 on Mohs scale), react with acid, often have rhombohedral cleavage. Many are colorful. Formation: Form in sedimentary environments (limestone, dolomite), metamorphic environments (marble), and some hydrothermal environments. Economic Importance: Limestone is used for cement, construction, and many industrial applications. Marble is valued for sculpture and building stone. Identification: Acid test (dilute HCl) causes effervescence. This is a key identifying feature.

Minerals where oxygen is combined with metals: Important Oxides: - Quartz (SiO₂): Technically an oxide, but usually classified separately. Most common mineral. - Hematite (Fe₂O₃): Iron oxide, red to black. Important iron ore. - Magnetite (Fe₃O₄): Magnetic iron oxide. Black, strongly magnetic. - Corundum (Al₂O₃): Very hard (9). Ruby and sapphire are corundum. - Rutile (TiO₂): Titanium oxide. Often forms needle-like crystals in quartz. - Ilmenite (FeTiO₃): Iron-titanium oxide. Important titanium ore. Properties: Wide range of properties. Some are very hard (corundum), others soft. Many are important ores. Formation: Form in various environments - igneous, metamorphic, sedimentary, and through weathering. Many form through oxidation. Economic Importance: Many oxides are important ores (iron, aluminum, titanium). Gem varieties (ruby, sapphire) are highly valued. Special Properties: Some oxides have special properties - magnetite is magnetic, some are fluorescent, others have interesting colors.

Minerals containing sulfur combined with metals: Common Sulfides: - Pyrite (FeS₂): "Fool's gold." Metallic luster, brassy yellow. Cubic crystals common. - Galena (PbS): Lead sulfide. Very dense, perfect cubic cleavage. Important lead ore. - Sphalerite (ZnS): Zinc sulfide. Variable color, resinous luster. Important zinc ore. - Chalcopyrite (CuFeS₂): Copper-iron sulfide. Brassy yellow, tarnishes to iridescent colors. Important copper ore. - Arsenopyrite (FeAsS): Iron-arsenic sulfide. Silvery, often forms distinctive crystals. Properties: Generally have metallic luster, are opaque, and many are important ores. Often form in cubic or other geometric shapes. Formation: Primarily form in hydrothermal environments. Often associated with igneous activity and ore deposits. Economic Importance: Many sulfides are important ores. Galena (lead), sphalerite (zinc), chalcopyrite (copper) are major sources of these metals. Identification: Metallic luster, often brassy or silvery colors, and association with ore deposits are key characteristics. Some have distinctive crystal habits. Caution: Some sulfides can be toxic or produce acid when exposed to air and water. Handle with care, especially in collections.

Additional important mineral classes: Sulfates: Contain the sulfate ion (SO₄²⁻) - Gypsum (CaSO₄·2H₂O): Soft (2), forms crystals and massive forms (alabaster, selenite) - Barite (BaSO₄): Very dense, often forms tabular crystals - Anhydrite (CaSO₄): Gypsum without water. Harder than gypsum Halides: Contain halogen elements (F, Cl, Br, I) - Halite (NaCl): Table salt. Cubic crystals, perfect cubic cleavage, salty taste - Fluorite (CaF₂): Often colorful, cubic crystals, fluorescent. Important industrial mineral - Sylvite (KCl): Potassium chloride, similar to halite Phosphates: Contain the phosphate ion (PO₄³⁻) - Apatite (Ca₅(PO₄)₃(F,Cl,OH)): Common in igneous rocks, source of phosphorus - Turquoise (CuAl₆(PO₄)₄(OH)₈·4H₂O): Valued gemstone, blue to green Native Elements: Elements found in pure form - Gold (Au): Precious metal, often in nuggets or flakes - Silver (Ag): Precious metal, often tarnishes black - Copper (Cu): Native copper, often in distinctive shapes - Diamond (C): Pure carbon, hardest mineral - Graphite (C): Also pure carbon, but very soft Other Classes: Nitrates, borates, tungstates, molybdates, and others. Less common but include interesting and valuable minerals.

How minerals are organized: Dana Classification: Developed by James Dana in the 1800s. Organizes minerals by chemical composition and crystal structure. Still widely used, especially in the United States. Strunz Classification: Developed by Hugo Strunz. Also based on chemical composition. More commonly used in Europe. Organizes minerals into 10 classes. Chemical Classification: Groups minerals by their dominant anion (negative ion) or anionic group. This is the basis for most classification systems. Crystal System: Minerals are also classified by their crystal system (cubic, tetragonal, etc.). This is a secondary classification. Practical Use: Classification helps organize knowledge, predict properties, and identify minerals. Understanding classes helps narrow down identification possibilities. Evolution: Classification systems evolve as new minerals are discovered and our understanding improves. New minerals are added to appropriate classes. For Rockhounds: Understanding mineral classes helps you identify finds, understand where to look for specific minerals, and appreciate the diversity of the mineral world.