Minerals of Mica Group

Mica is a group of transparent to translucent minerals, the members of which have similar chemical and physical properties. Mica is a member of Silicate minerals. These minerals are known as Micas. Micas are general found in thin slidal layers, which is why these are also known as ”Sheet Minerals”. There are more than thirty types of mica minerals, among which most famous are purple lepidolite, black biotite, brown phlogopite and clear muscovite. There are two major subgroups of micas based on their crystallographic design;

(1). Dioctahedral Micas:

Most important mica mineral of Dioctahedral Micas. Muscovite is known as common mica, potash mica, and isinglas. It is a major subgroup of the micas, where there are two octahedrally coordinated D cations per formula unit. AD2T4O10(OH,F)2, in which A is K or Na, D is Al, Mg, Li, Fe etc.; T is mostly Si +/- Al.

Muscovite (A member of Dioctahedral Group of Mica)

Properties of Muscovite:

  • Crystal Systems: Members of this Muscovite mica have monoclinic crystal systems.
  • Cleavage: All of the muscovite minerals have a plane perfect basal cleavage.
  • Hardness: Like all micas, Muscovites are soft minerals, and have a hardness of 2-2.5 on Moh’s scale.
  • Specific gravity: Specific gravity of these minerals is 2.76 -2.88.
  • Luster: Pearly or Silky
  • Colours: Colourless, pale, shade of green and brown, mostly transparent in thin sheets.
  • Habit: Usually they are found in tabular and foliated mass.

All Members of Muscovite:

  1. Brittle Mica : AB23(X, Si)4O10(O, F, OH)2
  2. Fluorluanshiweiite: KLiAl1.5(Si3.5Al0.5)O10F2
  3. Kreiterite: CsLi2Fe3+Si4O10F2
  4. Luanshiweiite: KLiAl1.5(Si3.5Al0.5)O10(OH)2
  5. Orlovite: KLi2Ti(Si4O10)OF
  6. UM1988-22-SiO:AlCaFFeHKLiMg: KLiMgAl2Si3O10F2
  7. UM2003-27-SiO:AlCsFFeHLi: CsFe2+3(Si3Al)O10(F,OH)2
  8. UM2003-30-SiO:AlFLiRb: RbLi2AlSi4O10(F,OH)2
  9. UM2003-31-SiO:AlHLiRb: RbFe2+3(Si3Al)O10(OH,F)2
  10. Voloshinite: Rb(LiAl1.51.5)(Al0.5Si3.5)O10F2
  11. Yangzhumingite : KMg2.5(Si4O10)F2

Occurrence of Muscovite Minerals: 

Muscovite is a characteristic mineral of granites and granite-pegmatite, pegmatites (large crystals of muscovite are found), gneisses, and schists, and as a contact metamorphic rock or as a secondary mineral resulting from the alteration of topaz, feldspar, kyanite, etc. It is characteristic of peraluminous rock, in which the content of aluminum is relatively high. In pegmatites, it is often found in immense sheets that are commercially valuable. Muscovite is in demand for the manufacture of fireproofing and insulating materials and to some extent as a lubricant.

Uses of Muscovite: 

Muscovite is widely used for insulation as a dielectric in electrical industry. Ground mica is used as a filler. Muscovite is also used for heat-resistant supports and windows.

(2). Trioctahedral Micas (Biotite Minerals):

A Trioctahedral type of micas (Biotite and same) is a major subgroup of the micas, in which there are three octahedrally coordinated D cations per formula unit. AD3T4O10(OH,F)2,
In the above formula, A is mostly K or Na, D is Al, Mg, Li, Fe etc.; and T is mostly Si +/- Al, etc

Properties of Biotite:

  • Crystal Systems: Members of biotite has a monoclinic crystal system.
  • Cleavage: Perfect basal.
  • Hardness: 2.5 -3 on Moh’s scale.
  • Sp.gr: 2.8-3.2
  • Lustre: Pearly or Spendent.
  • Colours: Dark green, brown to black.
  • Habit: Usually in irregular foliated masses.

Occurrence of Biotite: 

Biotite is a rock-forming mineral of Mica group found in a wide range of crystalline igneous rocks such as granite, diorite, gabbro, peridotite, and pegmatite. It also forms under metamorphic conditions when argillaceous rocks are exposed to heat and pressure to form schist and gneiss. Although biotite is not very resistant to weathering and transforms into clay minerals, it is sometimes found in sediments and sandstones.

Other Members of Trioctahedral Micas:

Uses of Biotite:

  • Biotite is commercially used on a very small scale.
  • Ground mica is used as a filler and extender in paints.
  • It is also used as an additive to drilling muds.
  • Ground mica is also used as an inert filler and mold-release agent in rubber products.
  • It is also used as a non-stick surface coating on asphalt shingles and rolled roofing.
  • It is also used in the potassium-argon and argon-argon methods of dating igneous rocks.

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