![•The feldspar are the most important group of rock forming mineral
(light colour) constiting about 2/3rd
of the igneous rock and are used for
the classification of such rocks.
•They are alumina silicate of K,Na,Ca or Ba and may be consider as
isomorphous compounds of the four substance listed below, the first three
of these being of the greatest importance, while the fourth is rare.
These are
1.Orthoclase(or) potassium aluminium silicate K[AlSi2O8]
2. Albite (Ab) sodium aluminium silicate Na[AlSi2O8]
3. Anorthite(An) calcium aluminium silicate Ca[Al2Si2O8]
4. Celsian(Ce); Barium aluminium silicate;Ba[Na2/Al2Si2O8]](https://image.slidesharecdn.com/429165835-feldspar-sandeep-250217074601-b17e6b7e/85/Feldspar-Occurrence-and-importance-of-Feldspar-ppt-3-320.jpg)
Feldspar Occurrence and importance of Feldspar.ppt
- 2. 1. INTRODUCTION. 2. CHEMICAL COMPOSITION. 3. CLASSIFICATION. 4. ATOMIC STRUCTURE. 5. CHEMISTRY. 6. MODES OF OCCURRENCE. 7. DISTINGUISH FEATURES. 8. VARIETIES. 9. TEXTURAL OCCURRENCE. 10. USES.
- 3. •The feldspar are the most important group of rock forming mineral (light colour) constiting about 2/3rd of the igneous rock and are used for the classification of such rocks. •They are alumina silicate of K,Na,Ca or Ba and may be consider as isomorphous compounds of the four substance listed below, the first three of these being of the greatest importance, while the fourth is rare. These are 1.Orthoclase(or) potassium aluminium silicate K[AlSi2O8] 2. Albite (Ab) sodium aluminium silicate Na[AlSi2O8] 3. Anorthite(An) calcium aluminium silicate Ca[Al2Si2O8] 4. Celsian(Ce); Barium aluminium silicate;Ba[Na2/Al2Si2O8]
- 4. The general formula for the feldspar can be written as Al(Al,Si) Si2O8. Where, W = Na ,K,Ca,& Ba Here Si: Al ratio varying from 3:1-1:1 W
- 5. The feldspar group of mineral can be broadly classified into 3 isomorphous series •ALKALI FELDSPAR SERIES PLAGIOCLASE FELDSPAR SERIES (TRICLINIC SYSTEM) •BARIUM FELDSPARS
- 6. •ALKALI FELDSPHAR SERIES They are classified into four types on the basis of their optical properties. The alkali feldspars are as follows: •orthoclase (monoclinic),— KAlSi3 O8 •microcline (triclinic)— KAlSi3 O8 •anorthoclase (triclinic) — (Na,K)AlSi3 O8 Sanidine is stable at the highest temperatures, and microcline at the lowest.Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye. Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can only be seen using an electron microscope. •sanidine (monoclinic) —(K,Na)AlSi3 O8
- 7. Alkali Feldspar: Miscibility gap Because of dissimilarity in size between the Na+1 (0.95 Å) and K+1 (1.33 Å) complete solid solution occurs only at high temp. In orthoclase series the solid solution in more limited on extent and the phase separation occur triclinic member of the High Alibite series are called Anorthoclase.
- 8. PLAGIOCLASE FELDSPHAR SERIES The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses): All the above mineral crystallized in triclinic system
- 9. •The plagioclase feldspars are triclinic. •The plagioclase series follows (with percent anorthite in parentheses): Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling. •However, diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. • The immiscibility gaps in the plagioclase solid solution are complex compared to the gap in the alkali feldspars. •The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae.
- 10. •BARIUM FELDSPARS The barium feldspars are monoclinic and comprise the following: •celsian — BaAl2 Si2 O8 •hyalophane — (K,Na,Ba)(Al,Si)4 O8 Feldspars can form clay minerals through chemical weathering.
- 12. The number of feldspar group belongs to teckto silicate structure. The structure of feldspar is a continuous three dimensional network of SiO4 and AnO4 tetrahedra with the positive charge Na,K,Ba,Ca actuated in the interstices of negatively charged network. Hence, each Tetrahedron is linked by all four corners so that every oxygen atom is shared between two tetrahedron. This gives Si: O = 1:2, So it is derivative of SiO2 structures by in corporation of Al into the tetrahedron network when Si+4 is replaced by Al+3 . The most common Si : Al are 1:1 or 3:1,which correspond AlSi3O8 respectively.The cations that balances the negative charge on these formula units are ,with coordination no. 8 or greater. Hence the Tecktosilicates are Aluminosilicates of Na,K,Ca or Ba
- 13. •The alkali feldspar are essentially a series varying from Orthoclase to Albite but normally it also content same amount of Anorthite in solid solution . •The amount of Anorthite present in fairly small ,generally as less than 5%.So Feldspar can be consider as a three component system . •The compound beings Orthoclase (KAlSi3 O8 ), Albite (CaAl2 Si2 O8 ).The complexities are introduced by both solid solutions. •Relation that exits among these three components and the existence of polymorphic forms. •Isomorphic substitution of one kind of alkalic metal for another gives rise to the varieties of feldspars.
- 14. The Ba-feldsphar are very rare •At temp above 600 O C,complete solid solution exists between Anorthoclase and Albite •But there is no solid solution between orthoclase and anorthite. Hence feldspar intermediate these two do not exists. •The system albite and anorthite completely exists as solid solution of high temperature but there are complication at low temperature. -:ALTERATION:- •Orthoclase, Microcline and the series involving them are more susceptible to alteration than Plagioclase. •The alter to secondary mica (sericite), Keoline and cryptocrystalline silica. •The plagioclase in addition alter to calcite, epidote, zeolite etc.
- 15. ALKALI FELDSPHAR •ALKALI FELDSPHAR IN IGNEOUS ROCK Alkali feldspar are important constituent of acid and alkali igneous rock and presentably found in syenite, granite and their volcanic equivalent. They also occur in pegmatite and many igneous gneisses -: In plutonic rocks Orthoclase is essentially present where as in volcanic rocks sanidine is present. -: pure alkali feldspar is never separated its range of composition vary from Orthoclase -20 to orthoclase -47(Or20 - Or47 ) -: Alkali feldspar in acid volcanic rocks are greater in potash than feldspar of alkaline rocks.
- 16. •ALKALI FELDSPAR IN METAMORPHIC ROCKS “K-Feldspar is stable product of high grade of thermal metamorphism as well as in regional metamorphism”. It occurs in a variety of thermally metamorphosed sediments including shales, impure sand stones, Lime stones & Dolomites. It is a typical minor in the sillimanite zone of regional metamorphism. In Precambrian granites and gneissose,K-Feldsphar is Microcline. •ALKALI FELDSPAR IN SEDIMENTARY ROCKS K-Feldspar crystallized during the formation of sedimentary rocks at a temp. Prevailing at the earth’s surface and at a moderate depth in the crust. It is commonly in sandstones, silt stone, shales and sediments. -: Authogenic Feldspars (Feldspar which is formed in insitu) have always a small, well rounded and pertain detrital cones.
- 17. PLAGIOCLASE FELDSPAR AND THEIR OCCURRENCE •In igneous rocks: Most common in Basalt; which may occur as ground mass or phenocrystals. -: Zonning in feldsphar in a common feature. -: In plutonic rocks Anorthite is not at all a common minerals but it has been reported in “Tractolite”. -: In granite and granodiorite, Albite and Plagioclase are common minerals. In pegmatite Albite is common In Dunite Andesine is common In Gabbro Labrodorite is common In Anorthosite Anorthite is common.
- 18. •In Metamorphic rocks Albite is an essential constituent of metamorphic schist .In chlorite and Biotite Zones of regional Metamorphism,Albite is a common product .In Garnet and higher Zones Calcium content of plagioclase increases and in Kyanite zone it is andesine. •In sedimentary rocks Plagioclase is a common authegenic mineral forming contemporeously with sedimentary rocks as well as by replacement of detroidal precipited material.
- 19. Orthoclase may be distinguished from sanidine and Orthoclase by highest optic mass and presence of macro-perthite texture. Orthoclase has cleavage angle 900 while it is less than 900 in triclinic members. Alkali Feldspar distinguished from the members of plagioclase by absence of lamellar twinning.
- 20. 1. SANIDINE: - A high temperature potassium feldspar. It is glassy transparent tabular Orthoclase formed above 9000 c 2. ADULARIA:-Colorless crystalline orthoclase or low temperature orthoclase’s
- 21. 3. AMAZON STONE:-Bright green Microcline 4. AVENTURINE:- Albite showing reddish tone like reflection (Gem variety of albite)
- 22. -: PERTHITE:-Intergrowth between orthoclase/microcline and Albite or Oligoclase, but Albite patches found in Microcline masses. Thin section morphological view
- 23. -: GRAPHIC: Intergrowth between quartz and plagioclase Feldspar -: MYRMEKITE: Intergrowth between quartz and Orthoclase Feldspar
- 24. 1-Alkali- feldspar are used in manufacturing of porcelain,pottery, opalascent,glass,blonder of abbratives,wheels etc. 2-Amazone stone and precious labrodorite are used as precious stone. 3-Alkali-feldspar are also used as attrasives.