Feldspars are one of the most abundant among rock forming minerals in the earth's crust. It comprises group of minerals consisting of aluminium silicate with potassium, sodium and calcium as mixture. It may also contain small amount of lithium, rubidium and barium as isomorphous admixtures of alkalies.
Feldspar is traditionally used as a source for alumina and alkali in ceramics & glass applications which account for 85-90% of total consumption of the mineral. It plays an important role as fluxing agents in ceramics and glass applications, and are also used as functional fillers in the paint, plastic, rubber and adhesive industries. Few firms in India are utilising feldspar as a bonding agent in the manufacture of bonded abrasive like wheels and discs of garnet, corrundum, emery etc. It is also used in the manufacture of artificial teeth, fertiliser and white cement. Certain varities of feldspars are used as semi precious stone e.g. moonstone which is orthocase colourless feldspars with white or bluish tints.
Ceramics
In ceramics applications, feldspars are used as fluxing agent, softening melting and wetting other batch constituents. Surface tension pulls the remaining solid particles together, giving a densification of the ceramic body with rising temperature the alkalis become more active and first dissolve the clay particles and then the free silica. The flux controls the degree of vitrification of the ceramic body during firing. As different ceramic bodies require different degrees of vitrification, the amount of flux varies accordingly. Potassium feldspar (K-spar) has technical advantages over sodium feldspar (Na-spar), as it forms a melt of high viscosity that decreases only slightly with increased temperature, thereby making ceramics stronger and more refractory K-spars are mainly used in electrical porcelain applications and porcelain enamels. Na-spars are more often used in ceramics because they are cheaper. After clay, feldspar is the biggest ingredient in the raw material batch for ceramic bodies. Typical feldspar contents are <25% in earthernware, 25-35% in sanitary ware, 15-30% in white ware, 10-55% in floor and wall tiles and 30-55% in electrical porcelain.
Glass
The alkali content in feldspar acts as a flux, lowering the glass batch melting temperature and thus cutting production cost. But feldspar are primarity added to glass batches for their alumina content. Alumina acts as a matrix former by reducing the tendency for glass to devitrify, and as a stabiliser, which improves the chemical durability of glass and its physical resistance to scratching, bending, breaking and thermal shock. Typical feldspar contents of glass range from 0.05% for flat glass and textile-grade fibreglass, 8% for container glass, 11 % for some speciality glass application, and up to 18% for insulation fibreglass.
Refractory
In the refractory industry, feldspar is used as one of the batch constituents in the manufacture of acid proof refractory bricks. It is mixed with fireclay to make the acid proof refractory bricks.
Abrasives
Fine grained feldspar is used in abrasive industry as mild abrasive material for incorporation in scouring powders. Although its hardness is around 6, because of its semiconchoidal fracture, it is used as a mild abrasive. In scouring powder, feldspar is preferred to silica sand on considered reasons of better health safety as its silica content is combined and not free. Ground feldspar is used as non-skid dusting agent for city floors. It is also used as a bond in bonded abrasives. Vitrified wheels with a clay-feldspar bond are preferred because of their high rigidity and dimensional stability.
Electrode
In welding electrode industry, feldspar is used as a flux. Its fluxing properties make it an ideal ingredient in batch mixes for coating the electrodes. Feldspar, as a flux, is generally incorporated into the dry blend mix for coating electrodes used in the manual metal arc process. Feldspar incorporated into the mix carries out two important functions namely (i) It acts as an arc stabiliser, & (ii) helps in weld pool protection. In the case of weld pool protection, slag formers such as felspar form a molten/solid barrier to protect the weld pool and new weld metal. Feldspar can form between 5 and 10% of the total dey blend mix. In India, mostly potassium felspar is used though in other countries both potassium and sodium bearing feldspar are used. There is an increasing trend for using high potassium bearing feldspar because they give a smoother arc which is important in certain applications. Each type of feldspar gives a unique slag viscosity, but finally it is the manufacture's experience, rather than chemical understanding, that dictates' which felspar affects the viscosity in a particular way.
Fillers & extenders
Physical properties, such as good dispersibilty, chemical inertness and stable pH. low free silica content and brightness of 89-95% lend fine ground feldspathic materials excellent filler qualities. However, in this field of application, feldspar complete with other, cheepoo and geographically more converient minerals, including baryter, calcium carbonate kaolin, mica and tale. As aresult a this, feldspathic materials are often considered the second choice in filler & extender applications. On the whole, fillers still represent a small market for feldsparthic minerals, but demand in the paint, plastic and rubber industries is increasing because in paint application that are subject to acidic environment (e.g.out door paints, anti corrosive paint, grout & plasters)
Plagioclase Feldspar
These feldspars are also called soda-lime feldspar. The end members of the plagioclase feldspars are albite (NaAl Si3O8) and anorthite (Ca AI2Si2O8) The albite and anorthite molecules are completely miseible and together form an isomorphous series from pure albite at one end to oligoclase, andesine, laboradonite, bytownite and to pure calcium silicate, anorthite at the other end. The plagioclase feldspars usually occur in almost all igneous rocks. The calcium rich anorthite occurs in basic rocks such as gabbros basalts, where as soda rich plagioclase feldspars are found in more acidic igneous rocks such as granites, diorites and quartz¬porphyrie.
Orthoclase Feldspar
Orthoclase is a very common mineral. These form a high temperature continuous series of solid solutions K (AI Si3O8) - Na (AI Si3O8) which gradually change after a process of slow cooling into potassium rich orthoclase and microcline and sodium-rich components (anorthoclase). The orthoclase feldspars occurs in acidic rocks particularly granite and granite pegmatities, where as microcline is more common in intrusive acid rocks such as granites, granodiorites and syenites. It is also the principal mineral of pegmatites.
Hyalophanes
The hyalophanes are rare orthoclase feldspars containing small quantities of barium. They form an isomorphous series of K (Al Si3O8) and Ba (A12 Si2O8). Among the three feldspars,orthoclase are important from commercial point of view. Most of today's marketed feldspars are mined from pegmatities or feldpathic sand deposits and coarse grained granites.
