ABSTRACT
The terms foliation and cleavage in metamorphic rocks have formal definitions, but their colloquial use in the field is commonly haphazard and interchangeable. Foliation refers to plane-parallel alignment of planar or plate-shaped features in a rock that developed during metamorphism. For minerals the word ‘cleavage’ refers to parallel planes of easy breaking. These are controlled by planes of particularly weak atomic bonds in the crystal lattice, spaced at an atomic scale. For rocks, cleavage in its most primitive sense refers to similar parallel-aligned planes or curved surfaces along which the rock breaks easily. Slate (Fig. 2.2) and flagstone (Fig. 2.7), are examples of rocks with good cleavage. In modern, colloquial use the term ‘cleavage’ has been expanded to include parallel sets of planar surfaces that developed during metamorphism, but along which rocks may not, in fact, easily break. That is, perhaps, the root of the foliationcleavage terminology problem.
