Feldspar definitions and example sentences on Wordnik

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Definitions (7)

American Heritage Dictionary (1)

noun Any of a group of abundant rock-forming minerals occurring principally in igneous, plutonic, and some metamorphic rocks, and consisting of silicates of aluminum with potassium, sodium, calcium, and, rarely, barium. About 60 percent of the earth's outer crust is composed of feldspar.

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In mineralogy, one of a very common group of closely related minerals, all silicates of aluminium, together with either calcium, sodium, potassium, or in one case barium. They crystallize in the monoclinic or triclinic system with closely similar angles. The prismatic angle is not far from 120°, and they have two easy cleavages which make an angle of 90°, or nearly 90°, with each other. Their specific gravity lies between 2.6 and 2.8, and their hardness between 6 and 7. In color they vary from clear and glassy to white, grayish, and light shades of yellow, red, or green, rarely darker green to black. They occur in distinct crystals, also in massive forms varying in structure from coarsely cleavable to granular-crystalline, compact, and hornstone-like. They form an essential constituent of many of the common crystalline rocks, as granite, gneiss, syenite, diorite, most kinds of basalt, andesite, trachyte, etc. The monoclinic feldspars are orthoclase and hyalophane. The former is a potash feldspar (see orthoclase), and is the commonest of the group; the latter is a baryta feldspar, and is a rare species. Closely related to orthoclase is the triclinic microcline (which see), having the same composition, but varying slightly in form. Besides these there are the triclinic (lime-soda) feldspars, called in general plagioclase, because of the oblique angle between their two cleavages, and forming a series varying progressively in composition, form, optical characters, and specific gravity from the lime feldspar anorthite to the sodium feldspar albite; the intermediate species are considered as isomorphous compounds of these two extremes in varying proportions. Those ordinarily recognized are, named in order, labradorite, andesin, and oligoclase, the last approaching most closely to albite. The increase in soda in the members of the series is accompanied by an increase of silica, the species being increasingly acidic in the order named: thus, anorthite contains 43 per cent, of silica, and albite 69 per cent. The specific gravity diminishes in the series from anorthite (2.75) to albite (2.61). Certain triclinic feldspars containing considerable potash and with an angle of cleavage varying but little from 90° are sometimes grouped under the name anorthoclase. Common feldspar, or orthoclase (and microcline), is much used in the manufacture of porcelain; some kinds are employed for ornaments, as aventurin feldspar or sunstone, also moonstone (an opalescent variety of orthoclase), albite or oligoclase, and, most of all, the species labradorite, beautiful for its play of colors Also felspar.
Blue feldspar. Same as lazulite.
Glassy feldspar. See orthoclase.

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Potash is also obtainable from feldspar, but I do not know whether any plant has been established for its production from this rock. — My Four Years in Germany
Not only could she cook and sew hut she could read and write – skills that were scarce among the early settlers John built a home for his family, using feldspar, a stone that sparkled in the sunlight. — THE CAT WHO WENT BANANAS
Potassium feldspar (affectionately called “K-spar” by a couple of generations of students), for example, has the formula KAlSi3O8. — Magazine - Analog Science Fiction And Fact - 2007 - Issue 03 - March (v1.0) [lit]
That's as hard as the mineral feldspar, which is a major component of granite. — FSF,October2005
Geologically it should be 70 percent feldspar, the rest being a mix of quartz, magnetite, mica or clay and calcite. — Anchorage Daily News - Alaska News

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