Feldspar are the most common rock forming minerals of the Earth’s crust
- The general formula of feldspar is – X Al (Al,Si) Si2O8
- where X = K, Na, Ca or Ba
- Feldspar occurs in all environment and all three kind of rocks, igneous , sedimentary and metamorphic.
Structure
Feldspars are tectosilicates, where each silica tetrahedral share all corners with its four neighboring tetrahedra.
- Each tetrahedra contain Al+3 instead of Si+4
- The charge is balanced by incorporating Na+, Ca+2 or K+
- Resulted Si : O = 1 : 2
There minerological composition of feldspars can be expresssed in terms of three mineral triangle as :
- Orthoclase : KAlSi3O8
- Albite : NaAlSi3O8
- Anorthite : CaAl2Si2O8
Classification
Feldspar Group of minerlas divided into two groups :
- Alkali feldspar (K-Na bearing Feldspar)
- Plagioclase feldspar (Ca-Na bearing feldspar)
| Alkali feldspar : KAl Si3O8 | Plagioclase feldspar : All Triclinic Ab = NaAlSi3O8 , An = CaAl2Si2O8 |
| 1. Orthoclase : Monoclinic 2. Sanidine : Monoclinic (high temperature) 3. Microcline : Triclinic 4. Hyalophane : Between Orthoclase and Celsian : Monoclinic 5. Celsian : BaAl2Si2O8 : Triclinic | 1. Albite : Ab100An0 – Ab90An10 2. Oligoclase : Ab90An10 – Ab70An30 3. Andesine : Ab70An30 – Ab50An50 4. Labradorite : Ab50An50 – Ab30An70 5. Bytownite : Ab30An70 – Ab10An90 6. Anorthite : Ab10An90 – Ab0An100 |
Physical Properties
- Colour :
- Alkali feldspar : Flash red, Green
- Plagioclase : Grayish white
- Habit : Prismatic , Massive, Tabular
- Streak : White
- Lusture : Sub-viterous
- Hardness : 6
- Cleavage : 2 set
- Specific gr. : 2.5 – 2.6 g/cc
- Increase from Albite to Anorthite
Optical Properties
Under PPL
- Relief : Low
- Form : Subhedral
- Paleochroism : Nonpaleochroic
- Cleavage : present
- RI : Low
Under XPL
- Isotrophism : Anisotropic
- Optical Habit : +ve (plagioclase), -ve alkali feldspar
- Interference Colour : 1st and 2nd order gray, blue
- Birefringance : Low
- Extinction :
- Plagioclase : Inclined (12o-55o)
- Alkali feldspar : Inclined (5o-12o)
- 2V : Orthoclase (60o-65o), Microcline (77o-84o)
- Twinning : Polysynthetic , Lamellar
Varities :
- Amazonite : Deep green Microcline
- Sanidine : High temperature : K-feldspar
- Adularia : Low temperature Orthoclase
- Moonstone : Opalescent Adularia
- Murchisonite : Red feldspar with yellowish-goldenlusture
- Varities of Albite : Aventurine, Peristerite, Cleavendite (white albite), Pericline (white semiopaque albite)
Some advanced material
Occurrance :
- Found in all type of rocks , Igneous , Sedimentary and Metamorphic
- Feldspars are mined from granite blocks, Pegmatites and feldspar rich sands
Uses of feldspar :
- Used in manufacture of porcelain and pottery.
- Manufacture of opalscent glass, blinder for abrasives.
- Used in the facing of artificial buildings.
- Production of earthen ware, sanitary ware and enamelled brick.
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