This document summarizes several types of phyllosilicate minerals in the mica group, including their chemical formulas, crystal structures, and common occurrences. Muscovite, biotite, and phlogopite are some of the most common micas. They have layered structures and perfect basal cleavage. Muscovite is widely used in electrical and insulation applications due to its properties. The document also describes lepidolite and its identification by lithium content and lilac color.

1. MICA GROUP 6/9/07

2. INTRODUCTION Minerals in this group show considerable variation in chemical and physical properties but all are characterized by a platy morphology and perfect basal cleavage, which is a consequence of their layered atomic structure. Phlogopite, muscovite and lepidolite are of consideral economic importance

3. Phyllosilicates (sheet silicate or layered silicate) A group of silicate minerals characterized by silicate (SiO4) tetrahedra which are linked together and arranged in the form of a flat sheet (the Greek phyllon means “leaf”). The group includes the mica and clay mineral groups. Phyllosilicates are generally flaky, soft and of low density. They form at relatively low temperature and may replace earlier minerals as a result of alteration They are major constituent of fine grained argillaceos rocks and of some metamorphic rocks.

4. Most common micas- muscovite, paragonite, glauconite, lepidolite, biotite and zinnwaldite Composition variable within mica group but the general formula : W (X,Y) 2-3 Z4 O10(OH,F)2. W= is generally K (Na in paragonite) X,Y= represent Al, Mg, Fe 2+, Fe 3+ and Li. Z= Si or Al

5. Chemical formulas of major micas Muscovite- K Al 2 (Al Si3 O 10) (OH)2 Paragonite- Na Al 2 (Al Si3 O 10) (OH)2 Phlogopite- K Mg 3 (Al Si3 O 10) (OH)2 Biotite – K (Mg, Fe)3 (Al Si3 O 10) (OH)2 Lepidolite- K Li2 Al (Si4 O10) (OH)2 A chemical feature which most micas have in common is their water content. Except for those with high fluorine content show approximately 4-5% H2 O.

6. Paragenesis Igneous rocks Muscovite- Granites, Granitic Pegmatites and aplites. Phlogopite- Peridotites Biotite- Gabbros, norites, diorites, granites, pegmatites Lepidolite and Zinnwaldite- Pegmatites and high- temperature veins

7. Metamorphic Rocks Muscovite, Paragonite and biotite: Phyllites, Schists and gneisses. Phlogopite: metamorphosed limestones and dolomite Sedimentary Rocks Muscovite and Paragonite- Detrital and authigenic sediments. Glauconite : greensands

8. MUSCOVITE (Monoclinic) One of the most common micas it can vary considerably in composition, as a result of atomic substitutions. Some Na always present replacing K Replacement is greater the higher temperature of formation. Sodium content of muscovite may thus be a useful geological thermometers.

9. Al replaced by Mg and Fe. Less commonly by Cr (giving a bright green colour, var. Fuchsite or mariposite) Diagnositic features: the perfect cleavage . Muscovite occurs in variety of associations. In igneous; its in granite and pegmatites. It is abundant in mineral in schists and gneisses of low and medium grade metamorphism Sericite is a variety of muscovite which formed from the alteration of feldspar.

10. uses The combination of perfect cleavage, flexibility , elasticity , low thermal conductivity ,and high dielectric strength makes muscovite a unique mineral and one which is essential in industry. Used in electrical industry for condensers, as insulating material between commutator segments and in heating elements.

11. These are phyllosilicates in which the Si : O ration is 4:10. General formula X 2 Y 4-6 Z 8 (OH,F) 4 X= Ca, Na or K ; Y= Al, Mg, or Fe(also Mn, Cr, Ti, Li); Z= Si or Al (also Fe 3+ and Ti) Micas are said to be dioctahedral when the number of Y ions is 4 (eg. muscovite, pargasite and glauconite) and Trioctahedral when it is 6 (eg. Phlogopite, Zinwaldite, Lepidolite)

12. Biotite & Phlogopite Monoclinic system Biotite K (Mg, Fe) 3 (Al Si 3 O 10 ) (OH) 2 and Phlogopite K Mg 3 (Al Si 3 O 10 ) (OH) 2 Phlogopite and Biotite are arbitary divisions of a single phase of variable composition.

13. Phlogopite is the pale coloured, low-iron part of the series. A marked correlation exists between composition and geological environment In igneous rocks the iron content of biotite increases with silica content of the rock, ultrabasic rocks containing phlogopite, granite and granite pegmatites containing iron-rich biotites. Phlogopite occurs in ultrabasic igneous rocks, in some marbles, and in magnesium-rich pegmatites.

14. Biotite occurs in greater variety of gelogical environments than any of the other micas. Formed under a wide range of temperature and pressure conditions, and it occurs abundantly in many regionally and contact metamorphosed sediments. Found in many igneous rocks but most commonly in the intermediate and acidic families. Biotite of volcanic rocks are in general poorer in Fe 2+ and richer in Fe 3+ and Ti than those in their intrusive equivalents.

15. Lepidolite Monoclinic Commonly pale lilac colour Can be distinguished from other micas by its lithium content (flame test) and by its lilac colour and common occurrence in complex granite pegmatites. Ore of lithium and also used as a raw material in glass and ceramics.