Kaolinite subgroup

Kaolinite (triclinic) Al4[(OH)8/Si4O10]

Kaolinite 1P
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.
Kaolinite PX
Isotropic or anisotropic: anisotropic
Extinction: inclined
Interference colours: 1st order, grey

Mica minerals

Muscovite (monoclinic) KAl2[(OH,F)2/AlSi3O10]

Muscovite 1P
Colour: colourless
Pleochroism: non pleochroic
Habit: platty
Relief: positive
Cleavage, fracture: perfect, one direction cleavage.
Inclusions: occasionally Zircon inclusions
Muscovite PX
Isotropic or anisotropic: anisotropic
Extinction: nearly straight at 87º
Interference colours: 2nd order
Distinguished features: intergrowths of Muscovite and Biotite

Sericite

Sericite 1P
Is a fine grained Muscovite or Paragonite. Sericite is a common alteration of feldspars in areas that have been subjected to hydrothermal alteration.
Sericite PX
I

Biotite (monoclinic) K(Mg,Fe2+,Fe3+,Al,Ti)3[(OH,F)2/Al(Si,Al)Si2O10]

Biotite 1P
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
Biotite PX
Isotropic or anisotropic: anisotropic
Extinction: nearly straight at 87º
Interference colours: 2nd/3rd order
Distinguished features: intergrowths of Biotite and Muscovite

Chlorite minerals (monoclinic) (Fe, Mg, Al.)6[(OH)2/(Si, Al.)4O10]

Chlorite 1P
  • 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
Chlorite PX
Isotropic or anisotropic: anisotropic
Extinction: inclined
Interference colours: subnormal (purple, brown)

Hydromicas

Glauconite (K,Na,Ca)(Fe,Al,Mg,Fe)2[(OH)2/(Al,Si)4O10] x nH2O

Glauconite 1P
Colour: green
Pleochroism: weak pleochroism
Habit: fine grained aggregates, rounded pellets
Relief: low positive (grains are flat)
Cleavage, fracture: perfect, one direction cleavage.
Glauconite PX
Isotropic or anisotropic: anisotropic
Extinction: inclined
Interference colours: 2nd order (blue, green)
Last modified: Monday, 3 April 2017, 10:51 AM