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Physical Properties: Streak, Cleavage & Fracture

Continuing our exploration of physical properties, streak, cleavage, and fracture provide crucial diagnostic information. Streak Streak is the color of a mineral's powder, typically observed by rubbing the mineral across an unglazed porcelain plate streak plate . Why Streak Matters - Streak is often more consistent than mineral color - Hematite can be black, red, or silver, but always has a red-brown streak - Magnetite and hematite look similar but have different streaks black vs. red-brown Streak Testing Tips - Works best for minerals softer than the streak plate hardness 6.5 - Hard minerals may scratch the plate instead of leaving a streak - Metallic minerals often have distinctive, diagnostic streaks | Mineral | Color | Streak | |---------|-------|--------| | Hematite | Black/Silver/Red | Red-brown | | Magnetite | Black | Black | | Pyrite | Brass yellow | Greenish-black | | Chalcopyrite | Golden | Greenish-black | Cleavage Cleavage is the tendency of minerals to break along flat planes of weakness, determined by crystal structure. Describing Cleavage - Number of planes : 1, 2, 3, 4, or 6 directions - Quality : Perfect, good, poor, or absent - Angles : Measured between cleavage planes Common Cleavage Patterns - 1 direction basal : Micas - sheets peel off easily - 2 directions at 90° : Feldspars - blocky breaks - 2 directions at 56°/124° : Pyroxenes - 2 directions at 60°/120° : Amphiboles - 3 directions at 90° cubic : Halite, galena - 3 directions not at 90° rhombohedral : Calcite Fracture When minerals break along surfaces that are NOT cleavage planes, the break is called fracture. Fracture Types - Conchoidal : Curved, shell-like breaks quartz, obsidian - Fibrous/Splintery : Breaks into fibers - Hackly : Jagged, irregular native metals - Uneven/Irregular : Rough, random surfaces