Potash feldspar

Imai, H. (1986) Formation of potash feldspar in the propylites in the epithermal gold-silver mining areas (Preliminary report). Mining Geology, 36, 231-238 (in Japanese). 

Suspended solid surfaces (particles or colloids) in waters play a prominent role in controlling the concentration of dissolved trace elements. Most of these elements are eliminated by sedimentation after incorporation on to or into particles, generally by complexation with the surface sites. The most common inorganic particles and colloids are non-clay silicates (quartz, potash feldspar, plagioclase, opaline silica (diatoms)) clays (illite, smectite) carbonates (calcite, dolomite) Fe-Mn oxides (goethite, magnetite) phosphates (apatite) sulfides (mackinawite). Particles and colloids in a water body may be classified as a function of their origin ... 

The first minerals to crystallise, at highest temperatures and pressures, are poor in silicon (<50% Si02) and are rich in calcium, iron and magnesium and they form minerals, such as olivine and the pyroxenes, which make up the basic or mafic rocks. The acidic or silicic rocks are the last to form, are rich in silicon (>65% Si02) and are dominated by quartz, the potash feldspars and muscovite. 

Mica, biotite, potash, feldspars, silimanite, zircon, graphite, iron carbide, lead sulfate... 

As previously discussed, both potassium tetrasilicate and potassium disilicate melt congruently at 770 2°C and 1045 2°C, respectively. Both compounds crystallize readily from appropriate melts when no potash feldspar crystals are present. In the region of the boundary curve between the tetrasilicate field and potash feldspar field, it is possible to follow the metastable extension of the potassium tetrasilicate liquidus surface beneath the stable potash feldspar field. Near the boundary curve between the disilicate field and the potash feldspar field, a metastable extension of the disilicate liquidus surface beneath the stable potash feldspar field is also possible providing once again that no potash feldspar crystals are present. 

The ternary compound, potash feldspar, has an incongruent melting point at 1150 + 20°C. In all compositions that lie in this field, considerable difficulty is experienced in obtaining potash feldspar crystals. Even after several months of crystallization at temperatures about 50 to 75°C below liquidus temperature, Schairer and Bowen [75] observed only a few percent of very small feldspar laths in the glasses. For those extremely viscous melts that also lie on or very near the leucite-silica line, Schairer and Bowen never crystallized feldspar. 

The effect of the addition of potassia on the refractoriness of Al203-Si02 compositions can be seen from the phase diagram in Fig. 6. All compositions in the corundum-mullite-leucite area are completely solid at temperatures below 1315 + 10°C. In the mullite-potash feldspar-leucite area, all compositions are completely solid at temperatures below 1140 20°C. For those in the mullite-potash feldspar-silica area, only at temperatures below the ternary eutectic at 985 + 20°C are the compositions totally solid. 

Feldspars are common aluminosilicate minerals that are present in many different igneous rocks including granites and rhyolites [11], When exposed, these rocks are susceptible to physical and chemical attack. Water, along with the sun, plant roots, and other forces physically attack rock formations causing crevice formation and fracture [3], Water also attacks rocks chemically. Over time, anhydrous aluminosilicate compounds such as those present in igneous minerals react with water to form hydrated species [20], The classic chemical reaction for clay formation involves the decomposition of potash feldspar due to the action of water-containing dissolved C02 to form kaolinite (insoluble) and soluble ionic species (Reaction 1) [14],... 

Evans BW, Guidotti CV (1966) The sillimanite-potash feldspar isograd in western Maine, U.S.A. Contrib Mineral Petrol 12 25-62... 

Plagioclase feldspar (Na-Ca aluminosilicate), other than albite, stains red potash feldspar (microcline or orthoclase) stains yellow, and quartz and glassy slag are unstained. Calcite takes on various shades of pink and dolomite may be deep red. The particles are normally examined in immersion mounts, utilizing liquids of known index of refraction and a cover glass. 

Photograph 10-11 Raw feed thin section of 45- to 75-pm material stained with a sodium cobaltinitrite-barium chloride-amaranth solution, revealing potash feldspars (yellow) and calcite (pink to red). (S A6879)... 

A coarse graphic intergrowth of quartz and potash feldspar of small optic angle that appears to be sanidine is typical. Quartz is also found in graphic intergrowth with more sodic extensions of plagioclase. Pegmatite differentiates are common in all thick basic sheets and those of the Ferrar Dolerites display the usual features of such bodies, as described by Walker (1953). 

In granodiorite, the plagioclase is oligoclase or andesine and is at least double the amount of potash feldspar present, the latter forming 8 to 20% of the rock. The plagioclases are nearly always euhedral (minerals completely bounded by crystal faces), as may be biotite and hornblende. These minerals are set in a quartz-potash feldspar matrix. 

K20 AI2O3 6 S1O2 2.56 — Orthoclase microcline potash feldspar... 

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. 

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. 

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