Potash spar

FELDSPAR. The most common mineral in crystalline rocks. Hardness 6.0-6.5 Mohs. Usually occurs as small grains intimately associated with other minerals, but commercial deposits are obtained from pegmatites. Feldspars form a group of which the principal types are potash spar (ortho-clase, microcline), soda spar (alhite), lime spar (anorthite), and lime-soda spar (ohgo-clase, andesine, labradorite and bytownite). They are aluminum sdicates of potassium, sodium and calcium. 

The softening range of the body decreases, and refractoriness increases with increased KjO, at the expense of NaO and CaO, in the feldspar used. With semi vitreous bodies which do not contain any CaO, soda feldspar compositions mature slightly sooner than potash feldspar. When hme is added the situation is reversed. The reason lime causes a sharp increase in the fusibUity of potash feldspar but not of soda spar. The glassy content of the body produced with soda feldspar is less viscous than that produced by potash spar, and thus the soda spar bodies deform more readily. 

Unfortunately, there is no pronounced eutectic in the flint-feldspar system. However, approximately 20% quartz can be added to a pure potash feldspar and 10% to a pure soda spar without materially raising the melting points. Feldspars of different potash-soda ratios are sometimes interchangeably reduced. The development of leucite crystals in melting, which accounts for the high viscosity of molten feldspar, depends upon the amount of potash spar present. Translucency is best with potash spar bodies thermal expansion is highest for bodies with high soda spar content. 

The role of feldspar in glazes is similar to that in bodies. It again is used for its fluxing action and should be ground very finely, preferably to 200 mesh, for more uniform and thorough reaction with other ingredients. Both potash and soda feldspars are used in glazes. The potash spar is desirable because... 

The ease with which feldspar enters into solution during smelting depends upon its fineness 120-140 mesh material is common in the enamel industry. Potash spar almost always is used. Soda spars frequently cause fishscaling in enamels that are entirely satisfactory when made with potash. Such impurities as garnet, hornblende, tourmaline and biotite mica cannot be tolerated. They maintain their identity throughout smelting, rise to the surface of the milled enamel and appear as hlack or hrown specks in the finished product. Muscovite mica, a potassium-aluminum sUicate, on the other hand, readily enters into solution and, therefore, is not particularly harmful. 

Potash spar - Potash spar is principally orthoclase or microcline and contains at least 10% K2O. 

Feldspathic sand - This is natural silica sand containing 10-35% feldspar, usually potash spar or soda spar. This sand is described, in decreasing feldspar content, as river sand, dune sand, and beach sand. 

This alkali can only be obtained from either of the above minerals, which are silicates. One part of the mineral in fine powder is mixed with two of fluor-spar, and the mixture heated with sulphuric acid, until the whole of the silica is dissipated. There then remains a mixture of sulphates of alumina, lime, and lithia, and, in the case of lepidolite or spodumene, potash. By boiling with carbonate of ammonia in excess, the alumina and lime are precipitated, and the filtered liquid is evaporated to dryness, and ignited to expel the sulphate of ammonia. The residue is sulphate of lithra, or sulphates of lithia and potash. In the latter case, by the cautious addition of chloride of barium, the sulphuric acid is separated as sulphate of baryta, and the lithia and potash converted into chlorides. These being dried, are digested in absolute alcohol, which dissolves the chloride of lithium. The lithia is now free from other bases to obtain it in the separate state, the chloride is converted into sulphate, by being boiled with oil of vitriol, and the solution of the sulphate decomposed by the exact equivalent of barytic water, by which the sulphuric acid is precipitated, while the free lithia is dissolved, and the solution, if evaporated, leaves hydrate of lithia, LO, HO. 

When mineral ehameleon (or better, permanganate of potash) is acted on by snlphnric acid and fluoride of calcium (fluor spar) with the aid of heat, a greenish-yellow gas or vapour is disengaged, which, with moist air, instantly becomes red. It acts powerfully on glass. Its composition seems to correspond to that of the preceding compound, and its production is to be explained in the same way, substituting fluoride of calcium for chloride of sodium. (Wohler.)... 

Klaproth found that precipitated alumina is soluble in caustic potash(i789). He proved by analysis that anhydrite (then called muriacite) is calcium sulphate free from water and that calcite and aragonite are two different crystalline forms of calcium carbonate and confirmed that bitter spar (dolomite) is a compound of calcium and magnesium carbonates. 

None of the minerals in the feldspar group is found pure. Potash feldspars always contain some alhite (soda spar) and soda feldspars always contain some anorthite (hme spar). Feldspar is found in practically all igneous rocks throughout the United States and Canada. Chief commercial sources are in North Carolina, South Dakota and Georgia. European deposits are located in Norway,... 

Free quartz acts as a diluent in feldspar and decreases the fluxing power. In unbumed ceramic bodies, feldspar acts as an antiplastic the same as sand does. Soda feldspar and quartz mixtures deform much more rapidly after deformation begins than do the potash feldspars or any of their mixtures with quartz. High-soda content in general indicates low deformation temperature. The fusion point of a feldspar depends upon the alkalies present, and becomes lower as soda content increases and potassium oxide content decreases. Some spars fuse as low as cone 4, others as high as cone 10, but the average is cone 8-9. 

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