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Crystal Systems & Habits

Crystal structure determines many physical properties and provides key identification clues. Understanding crystal systems and habits will elevate your mineral ID skills. The Six Crystal Systems All minerals belong to one of six crystal systems based on their internal atomic arrangement: 1. Isometric Cubic - Three equal axes at 90° angles - Examples: Halite cubes , garnet dodecahedrons , pyrite cubes with striations 2. Tetragonal - Two equal horizontal axes, one different vertical axis, all at 90° - Examples: Zircon, rutile, chalcopyrite 3. Orthorhombic - Three unequal axes at 90° - Examples: Olivine, topaz, sulfur 4. Hexagonal - Three equal horizontal axes at 120°, one vertical axis - Examples: Quartz, beryl emerald, aquamarine , apatite 5. Monoclinic - Three unequal axes, only two at 90° - Examples: Orthoclase, gypsum, augite 6. Triclinic - Three unequal axes, none at 90° - Examples: Plagioclase, kyanite, microcline Crystal Habit Habit describes the typical external shape or form of a mineral as it occurs in nature: - Prismatic : Elongated crystals with parallel faces tourmaline - Tabular : Flat, plate-like crystals barite - Bladed : Flat and elongated kyanite - Acicular : Needle-like crystals rutile - Fibrous : Hair-like fibers asbestos, chrysotile - Botryoidal : Grape-like rounded masses malachite - Dendritic : Branching tree-like patterns pyrolusite - Massive : No visible crystal form many ore minerals - Drusy : Small crystals coating a surface Twinning Twinning occurs when crystals intergrow in specific geometric relationships: - Contact twins : Crystals share a plane gypsum swallowtail - Penetration twins : Crystals appear to grow through each other staurolite cross - Polysynthetic twins : Repeated parallel twins causing striations plagioclase