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twinned crystalsTwins

twin law

composition surfacescomposition planescontact twinspenetration twins

simple twinsmultiple twinspolysynthetic twins

ssss1 Examples of twinned crystals: (a) swallowtail twins in gypsum; (b) polysynthetic albite twins in plagioclase; (c) penetration twins in galena; (d) penetration twins in pyrite; (e) penetration twins in staurolite; (f) Carlsbad twins in potassium feldspar.

Source: Wenk and Bulakh (2004). © Cambridge University Press.

growth twinsmechanical twins

crystal defects

Frenkel defectsSchottky defectspyrrhotite1–X1‐x

Point defects can occur on still smaller scales. In some cases electrons are missing from a quantum level, which produces an electron hole in the crystal structure. In others, an electron substitutes for an anion in the crystal structure. As with other point defects, the existence of electron holes plays an important role in the properties of the crystalline materials in which they occur. In most minerals, as temperature increases, the number of omission defects tends to increase. This allows minerals to deform more readily in a plastic manner at higher temperatures.