Key Identification Terms
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Habit
The characteristic external shape a mineral typically adopts. Determined by the dominant crystal forms present and the conditions of growth.
Form
A set of crystal faces related by the symmetry of the system. A closed form (e.g. cube, octahedron) encloses space alone; an open form (e.g. prism, pinacoid) requires other forms to close a crystal.
Cleavage
The tendency to break along flat planes parallel to possible crystal faces, reflecting planes of weak atomic bonding. Described by direction and quality (perfect, good, distinct, poor).
Striations
Fine parallel lines on crystal faces caused by oscillatory growth between two forms (e.g. prism ↔ pyramid), or by polysynthetic twinning. Often diagnostic for a specific system or mineral.