# Symmetry - Symmetry Mineral = &quot;. a highly ordered...

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Symmetry Mineral = “… a highly ordered atomic arrangement.” - external ordering – crystal faces, only some minerals - internal ordering – atomic arrangements, all minerals Crystal = “a homogeneous solid with long-range, three dimensional order” • All minerals are crystals, not all crystals are minerals • Modifiers: - euhedral = well developed faces - subhedral = imperfect faces - anhedral = without faces Crystalline = any solid with an ordered arrangement of atoms Amorphous = solid lacking ordered arrangements. Examples: Mineraloids, glass • Processes - Metamict: destruction of structure through radiation - Fulgarite: melting from lightning strike - Clinker: melting from burning coal beds 1

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Symmetry: • result of ordered arrangements • reflected in crystal faces • reflected in internal structure Constant for any individual mineral!!! What is symmetry? Symmetry: • correspondence in size, shape and position of parts on opposite sides of a dividing line or median plane or about a center or axis • a rigid motion of a geometric figure that determines a one-to-one mapping onto itself, i.e. a symmetry operation Two types of symmetry operations: - translational (through a volume) - repetition (around a point) Translation in a plane: Translation in a plane • Involves repetition of a single point 2
- one and two directions • Resulting pattern = plane lattice - extends to infinity in the plane • Each spot = lattice node Results of symmetry operations: • Only five different plane lattices produced by translation in 2 dimensions • Only four fundamentally different shapes, called- Unit meshes • Primitive (p) – nothing at center of mesh

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## Symmetry - Symmetry Mineral = &quot;. a highly ordered...

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