FABRICS I. FABRICS - TWO MAIN TYPES A. Foliation - any planar, or curviplanar, fabric within a rock (includes cleavages) 1. Designated "S" for surface may be: 2. Penetrative - discernable at scale of hand specimen 3. Semipenetrative - discernable at scale of outcrop 4. Nonpenetrative - not B. Lineation - linear penetrative fabric within a rock II. KINDS OF FOLIATION: A. Compositional banding 1. Bedding 2. Transposition of bedding 3. Gneissosity (e.g., in a gneiss) - forms due to metamorphic differentiation - segregation of minerals into alternating zones during dynamothermal metamorphism 4. Some phases are more soluble than others and their components will migrate while remaining materials will be enriched. B. Alignment of inequant mineral grains, usually phyllosilicates, but also with chain silicates, especially amphibole 1. Types of cleavages 2. Schistosity C. Inequant shaped grains that do not crystallize with inequant mineral habits but have been strained 1. Due to either a. Growth of grains under directed stress, * Metamorphic foliation b. Distortion of grains by post- crystallization strain * Mylonitic foliation III. CLEAVAGES - preferred directions of fracturing A. Axial plane cleavage - B. Cleavage fan - disposed symmetrically about the axial surface C. Domainal character - structural laminations 1. Cleavage domains 2. Microlithons IV. TYPES OF CLEAVAGE A. Continuous cleavage - domainal character is too fine to see without microscope 1. Slaty - commonly spaced cleavage defined by discrete cleavage domains (surfaces) of highly oriented phyllosilicates. form by: a. Growth of parallel phyllosilicates under directed stress b. Mechanical rotation of phyllosilicates c. Pressure solution processes 2. Schistosity and phyllitic cleavage - has microstructural evidence for extensive dynamothermal recrystallization and neomineralization a. Recrystallization far exceeds recovery 3. Mylonitic foliation - has microstructural evidence for dynamic recrystallization and recovery a. Recovery generally exceeds recrystallization B. Disjunctive cleavage- microlithons can be discerned with naked eye 1. Crenulation cleavage - cuts a host rock that already possesses a pre-existing continuous cleavage a. Typically the pre-existing cleavage is crenulated into microfolds two types: * Discrete crenulation cleavage - domains sharply truncate the continuous cleavage of the microlithons like tiny faults * Zonal crenulation cleavage - marked by wider cleavage domains that coincide with tight appressed limbs of microfolds in the pre-existing continuous cleavage preserved within microlithons May be symmetric or asymmetric 2. Spaced cleavage - consists of an array of parallel to anastomosing, stylolitic to smooth, fracture-like partings that are commonly occupied by clayey or carbonaceous matter a. Typically found in folded but unmetamorphosed sedimentary rocks b. Displays characteristic offset (separation) of bedding markers along cleavage c. Sutured, nonsutured, planar, wavy, or anastomosing V. KINDS OF LINEATION: A. Compositional lineation 1. Pencil gneiss B. Preferred orientation of grains 1. Dimensional preferred orientation - alignment of inequant shaped mineral grains, due to either a. Growth of inequant grains under directed stress * "Mneral lineation", especially chain silicates, like hornblende b. Distortion of pre-existing gains by strain * "Elongation" or "stretching lineation" 2. Crystallographic preferred orientation - alignment of crystallographic axes a. Lattice preferred orientation - LPO C. Intersection lineation - intersection of 2 planar surfaces D. Other types of lineation 1. Crenulation lineation - penetrative, small-scale (mm’s) fold hinge lines 2. Usually asymmetrical (s, z, m) 3. Slip lineations - includes grooves and slickenfibre lineations 4. Pressure shadows 5. Boudinage 6. Mullions