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Structure is the arrangement of the parts of a rock

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Structure is 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. The term fabric is a translation of Sander’s (1930) term Gefüge . Fabric was defined by Knopf & Ingerson (1938) as “the spatial data that governs the arrangement in space of the component elements that go to make up any sort of external form”. In current practice these elemental parts are only considered as contributing to a fabric if “they occur over and over again in a reproducible manner from one sample of a rock to another” (Hobbs et al., 1976). This means that although the fabric of a body may be considered at any scale the term is normally used at the crystallographic or mineral aggregate scale. Thus, for example, the preferred orientation of inequant mineral grains will produce planar or linear fabrics. The term texture is used in two ways. The commonest way is as a term for the spatial arrangement and relative size of mineral grains and their internal features (Spry, 1969). In this sense texture is synonymous with microstructure or at least certain aspects of microstructure. On the other hand, in material science and increasingly for some geologists (e.g. Barker, 1990; Vernon, 2004) texture means the presence of preferred orientation. In this sense texture is synonymous with microfabric. Given this dual use the SCMR recommends that only (micro)structure and (micro)fabric are used. If the term texture is used, its meaning should be made quite clear. www.bgs.ac.uk/scmr/home.html
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KHB Page 6 1/29/2007Paper_3.doc 4B Fault rocks While some fault rocks might be considered to fall outside the remit of metamorphic nomenclature, many undergo chemical as well as structural changes, and the deformation occurs within the P-T range of metamorphism. The definition of fault rocks is problematic and many of the definitions involve processes. Different minerals deform in different ways depending on the temperature conditions, this precludes definitions of fault rock terms that are mineralogically based. The definitions presented are systematic and general. As might be expected, mylonite proved difficult to define. Since its original definition by Lapworth (1885) there have been many nomenclature schemes proposed for mylonites and related rocks (e.g. Quensel, 1916; Knopf, 1931; Spry, 1969; Higgins, 1971; Sibson, 1977). More recently a Penrose conference on mylonites (Tullis et al., 1982) failed to arrive at an agreed definition of a mylonite, mainly because of the problem of knowing if, for example, plastic processes have been involved in the grain size reduction. Commonly detailed microstructural analysis of thin sections is required which makes it difficult to apply these definitions on a hand specimen scale. The definition given below is non-genetic from the point of view of the mechanism of deformation. The definition of mylonite also covers cohesive foliated cataclasites and this is in recognition of the fact that, in many cases, these
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