Garnets is a group of 6 members that are crystallizes in the cubic system with a profound rhobdodecahedral cleavage.
- It has a general formula X3Y2(SiO4)3
- Where X = Di-valent cation [Ca+2, Mg+2, Fe+2, Mn+2]
- Y = Tri-valent cation [Al+3, Fe+3, Cr+3, Ti+3]
- All garnets are neso/ionosilicates.
- All garnets are isomorphs means they have same morphology but different chemical composition.
Structure
Structurally they are neso solicates –
All Garnets are divided into two groups –
- Pyralspite Group
- Pyrope – [Mg3Al2(SiO4)3] , high temp. variety
- Almandine – [Fe3Al2(SiO4)3], high pressure variety
- Spessartite – [Mn3Al2(SiO4)3]
- Ugrandite Group
- Uvarovite – [Ca3Cr2(SiO4)3]
- Grossularite – [Ca3Al2(SiO4)3], contact metamorphism variety
- Andradite – [Ca3Fe2(SiO4)3]
Physical and Optical Properties
Physical propoerties
- Colour – green, red, black [depends on variety]
- Form – dodecahedral [cubic]
- Lusture – vitreous to sub-vitreous
- Cleavage – cleavage-less
- Hardness – 7
- Sp. Gravity – high
Optical properties
- colour – pale pink , green
- Paleochroism – non-pleochroic
- RI – very high
- Cross Nicol – isotropic , completely dark
Varities – There are many varities of the Garnet minerals –
- Melanite – Black Andradite
- Schorlomite – Titaniferous andradite
- Demontoid – Clear green andradite
- Topozolite – yellow andradite
- Hessonite / Cinnamon – ellow brownish grossulerite
Occurance –
- Garnets mainly develop in the metamorphic rocks, developed during the metamorphism.
- They can also be found in the igenous and metamorphic rocks.
Views This Month : 5201
Views This Year : 38311
Total views : 40910