Physical Properties of Minerals

Resistance to scratching: Mohs Hardness Scale: Relative scale from 1 (softest, talc) to 10 (hardest, diamond). Each mineral scratches those below it and is scratched by those above it. Scale Values: 1. Talc 2. Gypsum 3. Calcite 4. Fluorite 5. Apatite 6. Orthoclase feldspar 7. Quartz 8. Topaz 9. Corundum 10. Diamond Testing: Use common objects or mineral picks: - Fingernail: ~2.5 - Copper penny: ~3.5 - Glass: ~5.5 - Steel file: ~6.5 - Quartz: 7 Directional Hardness: Some minerals have different hardness in different directions (kyanite: 5 parallel to length, 7 perpendicular). This is rare but important when it occurs. Practical Use: Hardness is one of the most useful identification properties. Quick to test and often distinctive.

How minerals break: Cleavage: Tendency to break along specific planes, creating smooth surfaces. Results from weak bonds in the crystal structure. Cleavage Quality: - Perfect: Smooth, mirror-like surfaces (micas, halite) - Good: Clear cleavage but not perfect (feldspars, calcite) - Fair: Cleavage present but not prominent - Poor: Barely visible cleavage - None: No cleavage (quartz, garnet) Cleavage Directions: Number of directions and angles between them are diagnostic: - One direction: Micas (basal cleavage) - Two directions at 90°: Pyroxenes, feldspars - Two directions not at 90°: Amphiboles (~56° and 124°) - Three directions at 90°: Halite, galena (cubic) - Three directions not at 90°: Calcite (rhombohedral, ~75°) - Four directions: Fluorite (octahedral) - Six directions: Sphalerite (dodecahedral) Fracture: Breaking in directions other than cleavage: - Conchoidal: Curved, shell-like (quartz, obsidian) - Uneven: Irregular, rough - Splintery: Like wood (some fibrous minerals) - Hackly: Jagged, like metal (native copper) Identification Value: Cleavage is often the most diagnostic property. Combined with other properties, it quickly narrows identification.

Visual properties for identification: Color: Often the first property noticed, but can be unreliable for identification. Many minerals occur in multiple colors. Some minerals have characteristic colors (malachite is always green), others are variable (quartz can be many colors). Idiochromatic Minerals: Color is due to essential elements in the composition. Always the same color (or narrow range). Examples: malachite (green), azurite (blue), cinnabar (red). Allochromatic Minerals: Color is due to trace elements or defects, not essential composition. Can be many colors. Examples: quartz (many colors), corundum (ruby red, sapphire blue, etc.), fluorite (many colors). Pleochroism: Some minerals show different colors when viewed from different directions. Strong pleochroism is diagnostic (cordierite, tanzanite). Streak: Color of the powdered mineral. More reliable than body color because it eliminates effects of surface features. Test by rubbing on unglazed porcelain (streak plate). Streak Colors: - Metallic minerals: Usually dark (hematite: red-brown, magnetite: black) - Non-metallic minerals: Usually light or colored (sulfur: yellow, malachite: green) Luster: How light reflects from the surface: - Metallic: Like polished metal (pyrite, galena, native metals) - Non-metallic: Vitreous (glass-like), resinous, pearly, silky, adamantine (diamond-like), dull Identification Use: While color alone is unreliable, combined with streak and luster, it's very useful.

Weight relative to water: Specific Gravity (SG): Ratio of mineral's density to water's density. Water has SG = 1.0. Most minerals have SG between 2 and 4. Common Values: - Light minerals (SG < 2.5): Halite (2.1), gypsum (2.3) - Medium (2.5-3.5): Quartz (2.65), feldspars (~2.6), calcite (2.7) - Heavy (3.5-5): Pyrite (5.0), barite (4.5), galena (7.5) - Very heavy (>5): Native metals (gold: 19.3, silver: 10.5) Testing: Can be estimated by heft (how heavy it feels) or measured precisely using water displacement (hydrostatic weighing). Factors Affecting SG: Chemical composition (heavier elements = higher SG), crystal structure (closer packing = higher density). Identification Value: Very useful, especially for distinguishing similar-looking minerals. Galena feels much heavier than similar-looking minerals. Gold is unmistakably heavy. Practical Use: Often the first clue that a mineral is unusual. Very heavy or very light minerals stand out.

External shape of minerals: Crystal Form: The geometric shape resulting from the crystal structure. Well-formed crystals show the symmetry of the crystal system. Common Forms: - Cubic: Halite, pyrite, fluorite - Octahedral: Diamond, magnetite, spinel - Prismatic: Quartz, tourmaline, beryl - Tabular: Barite, wulfenite - Acicular: Needle-like (rutile, some zeolites) - Bladed: Kyanite, some gypsum - Botryoidal: Grape-like clusters (hematite, malachite) - Reniform: Kidney-shaped (hematite) - Dendritic: Tree-like (manganese oxides) Crystal Habit: The typical appearance of a mineral, including form and how crystals grow together: - Massive: No distinct crystals - Granular: Aggregates of grains - Fibrous: Thread-like (asbestos, some zeolites) - Platy: Thin plates (micas) - Radiating: Crystals radiating from center Twinned Crystals: Two or more crystals grown together in specific orientations. Can be diagnostic (staurolite cross, feldspar twinning). Identification Value: Well-formed crystals are very diagnostic. Habit can be distinctive even when crystals aren't perfect.

Additional properties for identification: Magnetism: Some minerals are magnetic (magnetite is strongly magnetic, pyrrhotite is weakly magnetic). Can be tested with a magnet. Fluorescence: Some minerals glow under ultraviolet light. Can be very distinctive (fluorite, some calcites, scheelite). Some minerals fluoresce different colors under different UV wavelengths. Phosphorescence: Continues to glow after UV light is removed. Less common than fluorescence. Taste: Some minerals have distinctive tastes (halite: salty - but don't taste unknown minerals, some are toxic!). Feel: Some minerals have distinctive textures (talc: greasy, graphite: greasy, kaolinite: earthy). Reaction to Acid: Carbonates effervesce (fizz) when touched with dilute hydrochloric acid. Very diagnostic for calcite and dolomite. Tenacity: How minerals behave when deformed: - Brittle: Breaks easily (most minerals) - Malleable: Can be hammered flat (gold, silver) - Sectile: Can be cut into shavings (gypsum) - Elastic: Bends and springs back (micas) - Flexible: Bends but doesn't spring back (some clays) Transparency: Transparent (light passes through), translucent (some light passes), or opaque (no light passes). Important for gem materials. Using Properties: No single property identifies all minerals. Use combinations of properties. Start with most diagnostic (cleavage, hardness) and add others to narrow down.