Twinning and cleveage
Primary (growth) twining
Developed during mineral growth and is facilitated if the mineral growth free in a magma or in a vug.
Secondary twining / deformation twins / mechanical twining
Induced by the application of an external force (shearing, pressure, solid solution unmixing can also induce sufficient stresses)
Tend to be flame-shaped (irregular and discontinuous)
Twin lamellae – parallel orientation of twining, often observed in soft minerals (e.g., sulfide) to which pressure has been applied
Translation twining – as deformation progresses, translation occurs along the secondary twining planes
Primary cleveage
Very specific, usual to identify mineral phases
e.g., galena - triangular cleveage pits
Crenulation cleveage
Due to deformation applied to phyllosilicate-rich rocks.
The crenulation cleveage can be symmetric or asymmetric, depending on the angle between the initial cleavage/foliation plane and the stress field.