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Metamorphic Textural (Microstructural) Terms
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- 1PorphyroblastLarge grain growing during metamorphism in a finer-grained matrix. Minerals which generally occur as porphyroblasts include almandine garnet, staurolite, andalusite, kyanite. Rock may be porphyroblastic.
- 2Poikiloblast (Poikiloblastic)Grains, especially porphyroblasts, containing abundant small inclusions.
- 3PorphyroclastLarge relict grain, preserved in finer-grained matrix. Generally applied to grains in rock which has been strongly deformed.
- 4Megacryst (Megacrystic)Large grain, regardless of origin, of any shape in fine-grained matrix.
- 5SchistosityA type of cleavage produced by deformation and recrystallization resulting in a preferred orientation of inequant mineral grains.
- 6FoliationAny penetrative set of more or less parallel surfaces
- 7CleavageA regular set of roughly parallel closely spaced surfaces produced by deformation and along which the rock will preferentially split.
- 8Slaty cleavageA cleavage defined by the preferred orientation of phyllosilicates which are too fined grained to be seen by the naked eye.
- 9Crenulated cleavageA spaced cleavage resulting from small scale crenulations of a pre-existing cleavage. It is commonly associated with varying degrees of metamorphic mineral segregation.
- 10LineationA set of sub-parallel pervasive linear features due to:
a) preferred orientation of acicular minerals (mineral lineation)
b) Intersection of different foliations (intersection lineation)
c) elongation of normally equigranular minerals due to deformation (stretching lineation) - 11BlastoporphyriticAn inherited porphyritic microstructure derived from igneous rock where phenocrysts, even though replaced by metamorphic minerals, still show their characteristic habit.
- 12LepidoblasticAbundant platy mineral grains with a strong planar preferred shape orientation - defining a schistosity/schistose rock.
- 13NemoblasticAbundant acicular grains producing a lineation.
- 14GranoblasticA mosaic of equigranular, equidimensional, anhedral grains. Inequant grains, if present, are randomly orientated.
- 15Granoblastic polygonalWhen the boundaries are straight, around 120 between them
- 16DecussateInterlocking network of elongate of platy grains aligned in all directions and bounded by rational crystal faces.
- 17PseudomorphVolume for volume replacement of one phase of distinctive shape by another mineral or minerals.
- 18MatrixFine grained minerals surrounding porphyroblasts. Generally quartz, chlorite and micas are matrix minerals.
- 19Equilibrium textureTexture approaching a minimum energy configuration (by grain growth or grain boundary migration).
- 20Tectonic fabricPreferred orientation of inequant mineral grains (micas, amphiboles, etc)
Planar fabric only - S-tectonite (usually implies flattening strain)
Linear fabric only - L-tectonite (usually implies constrictional strain)
Linear and planar - L-S-tectonite (combined flattening and stretching) - 21Relict mineralMineral unstable in prevailing mineral assemblage, preserved by incomplete reaction.
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- (Keywords) Igneous and Metamorphic Fichas sobre Metamorphic Textural (Microstructural) Terms, creado por Stephen Booth el 23/04/2017.
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- Stephen Booth
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