Foliation
Many metamorphic rocks are characterized by the presence of planar structures that repeat themselves parallel to one another within the rocks and are visible at the meso- to the microscale. These structures are termed ‘foliations’, from the Latin folium, ‘leaf’, a term describing any set of subparallel planar features occurring pervasively within a rock that are defined by objects, or fabric elements, like minerals or mineral aggregates. In this sense, foliation has a purely geometrical definition and the term comprises primary foliations, resulting from sedimentary and igneous processes such as bedding and lamination or flow banding, and secondary foliations, which form in rocks in diagenetic and metamorphic environment after their deposition or crystallization. The most important type of secondary foliations are tectonic foliations, which are produced by strain in response to tectonic forces during rock deformation. Another type of secondary foliation is represented by compaction or diagenetic foliations which form during the lithification of sediments into sedimentary rocks. This section focuses primarily on tectonic foliations, which are the primary target of investigation in structural geology.
What defines foliations?
Foliations are by definition the result of the presence of planar objects, known as fabric elements, that occur pervasively and repetitively within a rock. Fabric elements may consist of layers of different compositions, preferred orientation of platy or elongate minerals parallel to one another, alternations of bands with different grain sizes, flattened objects or mineral aggregates, or other similar features. Many examples are illustrated on the diagram on this page. The presence of foliations/oriented fabric elements, make the resulting rock anisotropic, i.e. characterized by different physical properties depending on the direction of examination, setting foliated rocks apart from unfoliated, isotropic rocks, in which minerals are randomly oriented and constant physical properties in all directions. One example of anisotropy in foliated rocks is the tendency to split preferentially along foliation planes, while being more resistant to fracturing on the direction perpendicular to foliation planes.
The presence of fabric elements sets foliations apart from sets of fractures, which can produce pervasive planar elements within a rock that are not, however, defined by actual objects (i.e. they are cracks crosscutting the rock).
Foliation, cleavage, and schistosity
The terms ‘foliation’, ‘cleavage’, and ‘schistosity’ have all been used to define sets of penetrative planar elements in deformed rocks. However, they are not synonyms and have been used differently over the years by various authors. In general, cleavage is the tendency of a rock to split preferentially along well-defined planes and the term was coined for very low-grade metamorphic rocks such as slates, where no fabric elements associated with cleavage surfaces is visible to the naked eye. In reality, cleavage planes are defined by alignments of tiny platy minerals and cleavage domains that are visible only under the microscope. A special type of cleavage is phyllitic cleavage, which is characterized by cleavage planes that are very reflective and show a lustrous sheen caused by the alignment of tiny phyllosilicates that are still too small to be seen as individual grains but large enough to reflect light (as in phyllites). Schistosity, on the other hand, indicates foliation planes defined by mineral grains and fabric elements that are visible to the naked eye and, as such, it is the type of foliation that occurs in medium- to high-grade metamorphic rocks. Some authors use the term ‘foliation’ as a synonym of schistosity, while others as an umbrella term comprising cleavage and schistosity. In this website, foliation is used to denote all types of cleavage and schistosity in general.
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