What is Feldspar?

Feldspar: A rock-forming mineral, industrially important in glass and ceramic industries, pottery and enamelware, soaps, abrasives, bond for abrasive wheels, cements and concretes, insulating compositions, fertilizer, poultry grit, tarred roofing materials, and as a sizing (or filler) in textiles and paper. Albite is feldspar mineral and is a sodium aluminum silicate. This form of feldspar is used as a glaze in ceramics.

Feldspar History & General Information

Feldspar is the mineral name given to a group of minerals distinguished by the presence of aluminum (Al) and the silicaion (SiO₄) in their chemistry. This group includes aluminum silicates of soda (sodium oxide), potassium (potassium oxide), or lime (calcium oxide). Feldspar is the single most abundant mineral group on Earth. Together, the varieties of feldspar account for one half of the Earth’s crust. The minerals included in this group are orthoclase, microcline, and the plagioclase feldspars. They form in a variety of thermal environments, during the crystallization of liquid rock (magma), by metamorphism of rocks deep in the earth, and in sedimentary processes.
Feldspars are relatively hard at 6 on Mohs' hardness scale. Feldspars are generally light-colored, including white, pink, tan, green, or gray. The color varies due to impurities within the crystal structure. Feldspar is the mineral that gives granite its pink, green or gray color.
When feldspar weathers from igneous or metamorphic rocks, it can accumulate as sand. It is, however, easily weathered, and eventually will break down into clay.
The name feldspar is a contraction of the longer name fieldspar, some early specimens were found in fields. The term spar is a generic term used by geologists to refer to any non-metallic mineral with a glassy (vitreous) luster that breaks on distinct flat surfaces (planes). The name was officially given its name by Johan Gottschalk Wallerius in 1747.
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".
Orthoclase - when transparent it is faceted into a gemstone. Clear or pale yellow in color it is a collectors item, and of little value to the jewelry industry as both beryl and even citrene are harder and more durable.
Microcline - variety amazonite, is sometimes cut into cabochons, and used in jewelry. The blue-green color is caused by a lead impurity. It is rarely used as more people are aware of turquoise and chrysocolla.
Moonstone - moonstone can be made up of any number of different feldspars including, orthoclase, plagioclase, albite, and microcline. They are all very similar when cut into cabochons, they can best be distinguished by their different densities. Moonstone tends to be silver, pale green, pale blue, or creamy colored. It is translucent and shows a blue-white sheen sometimes called "adularescence".
Plagioclase - there are two distinct varieties used in jewelry, the dark-blue-black Labradorite, and the orange-honey colored Sunstone.
• Labradorite gets its coloring from a "labradorescence", reflected light from a multitude of small parallel, plate-like structures, with minute inclusions of ilmenite, rutile, and magnetite. The color patterns are similar in that shown in nature with oil on water. It is rarely faceted, but often cut into attractive cabochons. Value is not really very high as it has not caught on as a popular material and is abundant.
• Sunstone is a species of plagioclase called "oligoclase" or known sometimes in the jewelry industry as "adventurine feldspar". It gets is shiller in the same way that labradorite does, but the background color is brown to orange, and the composition is full of small hematite crystals that give it additional sparkle.

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.

Feldspar Sources

Feldspar is mined from large granite bodies (called plutons by geologists), from pegmatite (formed when the last fluid stages of a crystallizing granite becomes concentrated in small liquid and vapor-rich pockets that allow the growth of extremely large crystals), and from sands composed mostly of feldspar.
Because feldspar is such a large component of the Earth’s crust, it is assumed that the supply of feldspar is more than adequate to meet demand for a very long time to come. It is so abundant that geologists and economists have not even compiled data on potential deposits of feldspar for future consumption. Present mines worldwide are adequately meeting the need for raw feldspar.

Feldsapr Specification

Feldspar Uses

This proportion of feldspar varies in different products like

• Wall tile -5%
• Floor tile -30%
• Statutory porcelain -50%
• Sanitary china and prodelain bodies -30%
In the glass industry also, potash feldspar is used in varying proportions. It may contain 10 to 15% of the batch. Feldspar, in this industry, is valued for two purposes, firstly, it acts as a flux and secondly, for alumina content. Alumina provides the resistance in glass to impact, bendings, and thermal shock.
In ceramic bodies, potash feldspar is preferred although soda feldspar works as a good flux and can be satisfactorily used in developing ceramic bodies. Potash feldspar has p.c.e. value 10, while soda feldspar has 8-9. Anorthite (the calcium feldspar), though it contains twice as much alumina as soda and potash feldspars and also contains lime which is an important ingredient of glass, is not preferred in the glass industry because it is more refractory (p.c.e. 12). Glass industry prefers to use mainly orthoclase (potash feldspar) because it tends the melt to clear glass while albite (soda feldspar) tends the melt to a translucent glass.
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. In the preparation of glazes the fine powder of feldspar is mixed with silica powder and a thin slurry is made. The fired up goods are dipped into the slurry and fired again. This imparts glaze to the surface. Some other ingredients are also added to the slurry to impart glaze and lustre. Each pottery factory has its own technique of preparing slurry which is regarded as a trade secret.
Feldspar is used to make dinnerware and bathroom and building tiles. In ceramics and glass production, feldspar is used as a flux. A flux is a material that lowers the melting temperature of another material, in this case, glass.
Substitutes and Alternative Sources
Feldspar can be replaced by other minerals and mineral mixtures of similar physical properties. Minerals that could be used to replace feldspar include pyrophyllite, clays, talc, and feldspar-silica (quartz) mixtures. The abundance of feldspar will make these substitutions unnecessary for the foreseeable future.
Green Feldspar (variety Amazonite) with quartz and albite.
From Pikes Peak region, Colorado

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