Mica group

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.