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Crystalline substances such as minerals have characteristic planar features that include: (1) crystal faces that develop during growth, (2) cleavage surfaces that develop during breakage, and (3) crystal lattice planes that reflect X‐rays and other types of electromagnetic radiation. All these types of planes possess a number of shared properties.

ssss1 (a) Common open forms: pedions, pinacoids, domes, sphenoids, and pyramids. (b) Different types of prisms that characterize the orthorhombic, trigonal, tetragonal, and hexagonal systems. The illustrated prisms are bounded by pinacoids at the top and bottom.

Source: Klein and Hurlbut (1985). © John Wiley & Sons.

Each type of plane is part of a large set of parallel lattice planes of which it is representative. As a mineral with a particular crystal form grows freely it may be bounded by a sequence of planar faces. When it stops growing, it is bounded by crystal faces that are parallel to many other lattice planes that bounded the mineral as it grew over time. When a mineral cleaves, it breaks along a specific set of parallel planes of relative weakness, but these cleavage planes are parallel to large numbers of planes of weakness or potential cleavage surfaces in the mineral structure along which the mineral did not happen to rupture. When X‐rays are reflected from a reflecting plane, they are reflected simultaneously from all the planes in the crystal that are parallel to one another to produce a “reflection peak” that is characteristic of the mineral and can be used to identify it.