MICA GROUP

6/9/07

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

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.

• 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

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.

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

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

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.

• 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.

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.

• These are phyllosilicates in which the Si : O ration is 4:10.

• General formula X2 Y 4-6 Z8 (OH,F) 4

• X= Ca, Na or K ; Y= Al, Mg, or Fe(also Mn, Cr, Ti, Li); Z= Si or Al (also Fe3+ 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)

Biotite & Phlogopite

• Monoclinic system

• Biotite K (Mg, Fe)3 (Al Si3 O10) (OH) 2 and Phlogopite

• K Mg3 (Al Si3 O10) (OH) 2

• Phlogopite and Biotite are arbitary divisions of a single phase of variable composition.

• 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.

• 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 Fe2+ and richer in Fe3+ and Ti than those in their intrusive equivalents.

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.