Garnet Group of minerals and it classification

Garnet Group of minerals and it classification

1. INTRODUCTION
Garnet is a group of six minerals which are isomorphous in nature and
never occur pure as represented by their chemical composition.
It occurs especially
in some metamorphic
rock such as schist.
Garnet vary only slightly in physical properties and some of the minerals
may be so similar that they are indistinguishable from one another
without X-Ray
or other geochemical analysis.

2. CHEMICAL COMPoSITION
The garnet
has general
formula X3Y2Z,O12
The minerals of garnet group are characteristic of metamorphic rocks but
are also found in some igneous types and as detrital grains in sediments.
The garnets may
be divided into two series---Pyralspite (pyrope,
almandine, spessartine) and Ugrandite (uvarovite, grossular, andradite).

3. ATOMIC STRUCTURE
The unit cell of garnet contains eight
XYZ,O2 formula unit.
The structure consists ot alternating
ZO tetrahedra and YO,
octahedra
which share corners to form a three-
dimensional network.
Within this there are cavities that
Portion of the garnet structure projected down z showing the
Fig. 11
framework of
alternating tetrahedra and octahedra (shaded) and the
cight-fokd triangular dodecahedra (drawn as distorted cubes) coordinating
the X cations. Large open circles
represent O, smaller
opcn circles the Y cations, solid circles the Si ions, and the hatched ones the X cations
(aftcr Novak, G. A. & Gibbs, G. V, 1971, Amer. Min., 56, 791-825)
can be described as distorted cubes
of eight oxygens which contain the X
Ions.

4. MEMBERS OF GARNET GROUP OF MINERALS
Chemically Garnet fall into two main group:
I.
Pyralspite
2. Ugrandite
Pyralspite (Pyrope, almandine. spessartine) and Ugrandite (uvarovite. grossular, and radite).
Pyrope
=
Mg Al,Si,O12
Uvarovite = Ca,Cr,Si,O12
Almandine = FeAl,Si,O12 Grossular = Ca,Al,Si,O12
Spessartine = Mn,Al,SIl,O12 Andradite = Ca3FeSi,O12

5. CHEMISTRY OF GARNET
Garnets commonly
occur as well developed crystal.
The typical form of the crystal have 12 or 24 sides and are called dodecahedron and
trapezohedron respectively.
Crystal habits can be co-related with chemical composition i.e. dodecahedron are most
likely to be grossular rich, trapezohedron tend to be pyrope, almandine or spessartine
and combination of dodecahedron and trapezohedron
are generally andradite rich.

6. PARAGENESIS OF GARNET GROUP OF MINERALS
Pyrope
I. Found in Some ultrabasic rock such as Kimberlite and mica peridotite.
2 Essential constituent of Eclogite.
3. In eclogite facie, garnet crystallizes
under
pressure of 7-15 kb with temperature ranging 3.
from 700 to 1200 °C.
Almandine
1. Typically occurs in garnetifierous schist formed by regional metamorphism.
2
2. Also in some granulite facies of metamorphic rocks.

7. PARAGENESIS OF GARNET GROUP OF MINERALS
Spessartine
I. Common in
granitic pegmatites.
Associated with Skarn deposit
of metasomatic origin.
2
Grossular
I. Ocurs in thermal and regional metamorphic conditions.
2 Found as product of Ca-metasomatism.
Andradite
I. Associated with thermal
metamorphism of impure calcareous rocks.
Uvarovite
I. Rare Garnet mineral and mainly associated with chromite and
spessartine.

8. PHYSICAL PROPERTIES
Grossular
grossular
Plhysical Grossular
properties
garnet
Chemical CaAl,Si, O2
Composition
Crystal System Cubic
Form
Rhombohedron
Colour
Pink or yellowish green
Green Grossular Grossular
Streak Brown
Lustre Vitreous
Fracture Conchoida
Cleavage
None
Hardness 75
359
Sp. Gravity

9. PHYSICAL PROPERTIES
Almandine
Almandine
Almandine
Physical
Ppropertie
Chemical FeAl,S,O2
Compositon
Cubic
Cystal System
Combination of
Form
Rhombohedron
and
trapezohedron
Deep
red to brownish Almandine
Colour Almandine
black
White
Streak
Vitreous
Lustre
Subconchoidal
Fracture
None
Cezvage
Hardness
6.5-75
3.94.2
Sp. Gravity

10. PHYSICAL PROPERTIES
Pyrope in red colour ycope in ed colaur Pyrope Physical
properties
MeAlSiO2 Chemical
Composition
Cubic
Cystal System
Usually rounded or
angular fragments
Form
Pinkish red
Colour
White Pyrope
in red colour
Streak
Vitreous Lustre
Conchoidal
Fracture
None Cleavage
75 Hardness
3.7
Sp.Gravity