and Quartz group Dr AVTejankar Professor amp Head Dept of Geology Deogiri College Aurangabad FELDSPAR GROUP Introducti o n This is the most important group of rock forming ID: 1021104
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1. Feldspar, Feldspathoid and Quartz groupDr. A.V.TejankarProfessor & HeadDept. of GeologyDeogiri College, Aurangabad
2. FELDSPAR GROUP
3. IntroductionThis is the most important group of rock forming silicates which occur in igneous, sedimentary and metamorphic rocks. They constitute about 2/3rd of the igneous rocks.
4. Atomic structureThe feldspars are tectosilicates with a structure of a framework silicate of (Si, Al) O4 are linked to one another by sharing oxygen ions in all directions. The resultant framework of tetrahedra has large interstices,occupied by the pottasium ions in the alkali feldspars.
5. TypesFeldspars are of two types :Alkali feldspar(microcline,orthoclase and sanidine)Plagioclasefeldspar(albite,oligoclase,ande sine,labradorite,bytownite and anorthite)
6. Chemical compositionThey are alumino silicates of potassium, sodium, calcium and barium, and may be regarded as isomorphous mixtures of the four end member given below, of which the first 3 are common while the fourth is rare.Orthoclase and microcline. (KAlSi3O8) 2)Albite. (NaAlSi3O8)Anorthite (CaAl2Si2O6)Celsian (BaAl2Si2O6)
7. Physical properties1)Colour-White,grey or flesh2)Form-Tabular3)Lustre-Vitreous4)Streak-White5)Cleavage-2 sets, but one is better developed.6)Fracture-Even to uneven7)Hardness-68)specific Gravity-2.57 to 2.7
8. Optical propertiesIn Plane Polarised lightColour-Colourless, turbidForm-irregularRelief-Low to moderate 4)Cleavage-AbsentIn Crossed NicolsAnisotropism-Anisotropic mineralTwinning-Plagioclase feldspar typically shows lamellar twinningInterference colour-The feldspars showgrey or white interference colours of loworder.
9. Varieties1)Sanidine-A high temperature pottasium feldspar2)Adularia-Low temperature orthoclase3)Moon stone-Opalescent adularia or albite4)Aventurine-Gem variety of albite5)Amazon stone-Bright green microcline
10. OccurrencePlagioclase are abundant in all types ofigneous rocks except the ultrabasic rocks.In basic and intermediate lavas, plagioclase phenocrysts, as well as small grains in the ground mass are abundant and exhibit zoning.Alkali feldspars are restricted to alkali and acid igneous such as syenites, granites, metamorphic gneisses and less frequently in sedimentary rocks like arkose.