cleavage in minerals

cleavage in minerals Cleavage is the property of a mineral to split along smooth, well-defined planes that are planes with a high density of atoms and are parallel to possible faces in the crystal. Cleavage arises because the bonds between the atoms are less strong across the planes. Graphite, for example, has a sheet structure with its carbon atoms strongly bonded within the planes but with only feeble residual forces between the planes; as a result, it has a perfect cleavage.

Cleavage, defined as imperfect, distinct, perfect, or eminent, according to its quality, usually gives rise to smooth, glistening faces (in contrast to the external faces of crystals, which may be dull, etched, or uneven). The number of cleavage planes in a mineral depends on the symmetry of the structure. The cleavage may consist of either one form or set of planes (as, for example, in fluorite (4 planes, octahedral), galena (3, cubic), andalusite (2, prismatic), gypsum (1, pinacoidal), or there may be several sets in one mineral (as in barite with one set of two planes (prisms) and two sets of one each (pinacoids), i.e. four cleavages in total. Some minerals (e.g. quartz) have no cleavage but a fracture which is irregular. The orientation of cleavage planes is defined by using Miller indices.

Cleavage may give useful clues to identification, as in the case of the pyroxenes and amphiboles. These rock-forming minerals are similar chemically and in colour and have two cleavages, which in the pyroxenes intersect at an angle of 87° but in the amphiboles at 124°.

R. Bradshaw