Feldspar
KAlSi3O8 - Orthoclase
KAlSi3O8 - Microcline
NaAlSi3O8 - CaAl2Si2O8 - Plagioclase
In the manufacture of high-class, colourless glass, feldspar should have a maximum of 0.1% Fe2O3 though upto 0.3% is permissible.
The History Says
The word comes from the German "feldt spat", meaning "field spar", "spar" meaning common clevable material - the material dredged up on farm lands during plowing. Most of them are not affected by acid (exception the Ca rich plagioclase - Anorthite). Orthoclase got is name from the Greek phrase meaning "straight fracture", Microcline from a Greek phrase meaning "small incline", and Plagioclase from the Greek phrase meaning "oblique fracture".
The Present Scenario
Feldspar is used as bonding agent along with magnesium oxide, magnesium chloride and other synthetic glue in the manufacture of abrasives, wheels, discs and other shapes.
FELDSPAR comprises a group of minerals containing potassium, sodium, calcium and aluminium silicates. They are the most common rock-forming minerals. The common feldspar is potassium feldspar, namely, orthoclase (K2O, Al2O3, 6SiO2). Sodium feldspar is albite (Na2O, Al2O3, 6SiO2) and calcium feldspar is anorthite (CaO, Al2O3, 2SiO2). A variety of crossed, hatched, twinned orthoclase (to be seen under the petrological microscope only) is called microcline. Sodium and calcium feldspars form an somorphous mixture known as plagioclase feldspars.
In between sodium and calcium, the other feldspars of the plagioclase series are oligoclase, andesine, labradorite and bytownite. They are composed of suitable proportions of sodium and calcium with an increasing percentage of calcium begining from mineral oligoclase to bytownite, turning completely into calcium feldspar (anorthite). A rock containing only plagioclase feldspars is called anorthosite.
The commercial feldspar is orthoclase. The potassium molecule is replaced by sodium to some extent and hence, orthoclase feldspar usually contains a small percentage of sodium. The composition range of the commercial feldspar varies within the limits of potash, soda and upto oligoclase.
Potash and soda feldspar occur as essential constituents of granite, syenite and gneisses. However, workable deposits are found in pegmatite veins consisting mainly of feldspar, quartz-feldspar veins and also occur with mica pegmatites. Feldspar is of widespread occurrence and is mined in almost all countries.
Mohs’ hardness: 6～6.5, specific weight 2.56～2.69g／cm3,the feldspar is an alumino-silicate of alkali metals or alkaline-earth metals. Its color is rosy, yellowy, or offwhite. Having a low melt point, wide fusion spectrum, whose liquid has a high viscosity, the feldspar can reduce the distortion of the flans and increase the eligibility of the firing, so it is used as the flux for the flans and the main component of the glaze.
In the glass industry, the feldspar can introduce alkali metal oxide and reduce the consumption of soda. The Al2O3 in it can reduce the crystallization inclination of the glass, and improve the viscosity and many other performances of the glass.
Kalium feldspar:
| Components |
SiO2 |
Al2O3 |
CaO |
Na2O |
Fe2O3 |
K2O |
MgO |
Content (%) |
56.20 |
23.20 |
0.05 |
5.1 |
0.09 |
12.75 |
0.62 |
The kalium feldspar（KAlSi3O8）belongs to the wry single crystals. Specific weight: 2.56-2.59g/cm3, melting point: 1150±20℃, having a long fusion phase and perfect fusion viscidity, it is widely used in industry for making the flans or glaze for ceramics, electrical ceramics, grind materials, and it can so be used to produce potasium fertilizes.
Natrium feldspar:
| Components |
SiO2 |
Al2O3 |
CaO |
Na2O |
Fe2O3 |
K2O |
MgO |
Content (%) |
64.80 |
18.99 |
0.80 |
11.40 |
0.09 |
0.62 |
0.60 |
Specific weight 2.6-2.65g/cm3, gray with yellow, the albite (NaAlSi3O8) belongs to the wry tricrystal. It is nice aid for the fusion of ceramics, because it can save the consumption of the fuel, enhance the mechanical strength of the flans and reduce the adsorption of water. So it is mainly used in the fields of ceramics, glass, and porcelain enamels.
