390 METAMORPHIC ROCKS/Classification, Nomenclature and Formation Table Terms for structures partially used in the rock definitions of this chapter Structure: The arrangement of the parts of a rock mass irrespective of scale, including spatial relationships between the parts, their relative size and shape and the internal features of the parts Dependent on the scale of the feature, the terms micro (thin section or smaller scale), meso (hand specimen scale), and mega (outcrop or larger scale) can be used as a prefix Fabric: The relative orientation of the parts of a rock mass This is commonly used to refer to the crystallographic and/or shape orientation of mineral grains or groups of grains, but can also be used on a larger scale Preferred linear orientation of the parts is termed linear fabric, preferred planar orientation planar fabric and the lack of a preferred orientation is referred to as random fabric Augen: Distinctive large lenticular mineral grains or grain aggregates in a finer grained matrix of a rock, having the shape of an eye in cross section, around which the foliation of the matrix is wrapped, commonly forming symmetric or asymmetric trails Cleavage: The property of a rock to split along a regular set of parallel or sub parallel closely spaced surfaces More than one cleavage may be present in a rock Fault: A dislocation surface along which there has been displacement of the rock mass on one side relative to the one on the other side Foliation: Any repetitively occurring or penetrative planar feature in a rock body Examples include: layering preferred planar orientation of inequant mineral grains (planar fabric) preferred planar orientation of lenticular or elongate grain aggregates More than one kind of foliation with more than one orientation may be present in a rock Foliations may become curved or distorted The surfaces to which they are parallel are called s surfaces Where possible, the type of foliation should be indicated Fracture: A general term for any surface of break in a rock mass, whether or not it causes displacement, due to mechanical failure Fracture includes cracks, joints, and faults Joint: A single fracture in a rock with or without a small amount (