In crystallography, having half the number of faces that a fully formed crystal would have, with the same symmetry properties.
From Greek 'hemi-' (half) + 'hedra' (face, side). The term emerged in the 1800s when scientists studying crystal formation noticed some crystals displayed only partial faceting compared to the ideal geometric form.
Crystals are nature's way of organizing atoms in perfect patterns, but sometimes they "cheat"—a hemihedral crystal has only half the faces you'd expect, yet it's still beautifully symmetric! This helped mineralogists understand that crystal form depends on growth conditions, not just the rules of geometry.
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