cleavage 1. In
minerals, cleavage is evident when crystals split along planes of weakness inherent in the structure of their atomic lattices. Cleavage is described by an adjective, e.g. good, poor, etc., and by referring to its crystallographic direction, plane, and degree of perfection, the resulting digits being contained in braces ({}) to distinguish them from descriptions of crystals. See
MILLER INDICES.
2. The formation of a set of fractures along closely spaced, parallel surfaces in a rock (the term is usually applied to low-grade
metamorphic rocks) by the alignment of various mineralogical and structural elements during
metamorphism and deformation, e.g. in
slates, where cleavage is due to a parallel arrangement of minerals. The
fabric generally gives rise to a preferred direction of fracturing, broadly analogous to mineral cleavage. Rock cleavages may be divided into two groups: (
a) continuous cleavages, e.g. ‘
slaty cleavage’ (synonymous with
schistosity and
foliation in high-grade
metamorphic rocks, see
METAMORPHIC GRADE) which, with further deformation, may be superimposed and cross-cut by a secondary crenulation cleavage; (
b) spaced cleavages, either crenulation or disjunctive, e.g.
fracture cleavage. Crenulation cleavages form by the microfolding of a pre-existing
anisotropic fabric. Disjunctive cleavages require no such primary fabric. Compare
FOLIATION.
