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ssss1 (a) Deformation map showing the significant role of omission defects and dislocations in the high temperature plastic deformation of crystals.

Source: Modified from Davis et al. (1996). © John Wiley & Sons.

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polymorphismpolymorphs

geotherms

ssss1 Important rock‐forming mineral polymorphs.

Chemical composition Common polymorphs 3 Calcite and aragonite Carbon (C) Diamond and graphite 2 α‐quartz, β‐quartz, tridymite, cristobalite, coesite, stishovite 4 Andalusite, kyanite, sillimanite 38 Orthoclase, microcline, sanidine 2 Pyrite, marcasite

ssss1 Phase stability diagram showing the conditions under which graphite, the low pressure polymorph of carbon, and diamond, the high pressure polymorph of carbon, are stable beneath continental lithosphere.

Reconstructive transformationsmetastable

displacive transformations

Alpha quartz (low quartz) is generally stable at lower temperatures than beta quartz (high quartz). Although α‐ and β‐quartz have different structures, the structures are so similar (ssss1) that the conversion of one to the other is a displacive transformation. It is not at all unusual, especially in volcanic rocks, to see quartz crystals with the external crystal form of β‐quartz but the internal structure of α‐quartz. These quartz crystals are interpreted to have crystallized at the elevated temperatures at which β‐quartz is stable and to have been diplacively transformed into the α‐quartz structure as they cooled, while retaining their original external crystal forms.