Kaolinite subgroup
Kaolinite (triclinic) Al4[(OH)8/Si4O10]
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Colour: colourless Pleochroism: non pleochroic Habit: It usually occur as fine-grained aggregates in flour-like and clay-like masses. Masses are formed by very tiny plates (15-3µm) Relief: low positive Cleavage, fracture: perfect, one direction cleavage.
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Isotropic or anisotropic: anisotropic Extinction: inclined Interference colours: 1st order, grey
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Mica minerals
Muscovite (monoclinic) KAl2[(OH,F)2/AlSi3O10]

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Colour: colourless Pleochroism: non pleochroic Habit: platty Relief: positive Cleavage, fracture: perfect, one direction cleavage. Inclusions: occasionally Zircon inclusions
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Isotropic or anisotropic: anisotropic Extinction: nearly straight at 87º Interference colours: 2nd order Distinguished features: intergrowths of Muscovite and Biotite
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Sericite

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Is a fine grained Muscovite or Paragonite. Sericite is a common alteration of feldspars in areas that have been subjected to hydrothermal alteration.
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I
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Biotite (monoclinic) K(Mg,Fe2+,Fe3+,Al,Ti)3[(OH,F)2/Al(Si,Al)Si2O10]

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Colour: brown, green Pleochroism: strong pleochroism Habit: platty Relief: positive Cleavage, fracture: perfect, one direction cleavage. Inclusions: common Zircon inclusions Alternations: Chlorite minerals are common alternation of Biotite
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Isotropic or anisotropic: anisotropic Extinction: nearly straight at 87º Interference colours: 2nd/3rd order Distinguished features: intergrowths of Biotite and Muscovite
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Chlorite minerals (monoclinic) (Fe, Mg, Al.)6[(OH)2/(Si, Al.)4O10]

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- Clinochlore (Mg,Fe2+)5Al[(OH)8[AlSi3O10]
- Pennine (Mg,Al)6[(OH)2/AlSi3O10]
Colour: green Pleochroism: strong pleochroism Habit: platty Relief: positive Cleavage, fracture: perfect, one direction cleavage. Alternations: Chlorite minerals are common alternation of Biotite
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Isotropic or anisotropic: anisotropic Extinction: inclined Interference colours: subnormal (purple, brown)
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Hydromicas
Glauconite (K,Na,Ca)(Fe,Al,Mg,Fe)2[(OH)2/(Al,Si)4O10] x nH2O

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Colour: green Pleochroism: weak pleochroism Habit: fine grained aggregates, rounded pellets Relief: low positive (grains are flat) Cleavage, fracture: perfect, one direction cleavage.
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Isotropic or anisotropic: anisotropic Extinction: inclined Interference colours: 2nd order (blue, green)
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Last modified: Monday, 3 April 2017, 10:51 AM