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MICROCLINE
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MICROCLINE , a See also:rock-forming See also:mineral belonging to the feldspar See also:group (see See also:FELSPAR). Like See also:orthoclase it is a potash-feldspar with the See also:formula KAlSi308, but differs from this in crystallizing in the anorthic See also:system. The name (from See also:Greek See also:Awl's, small, and ,XLeav, to incline) was given by A. Breithaupt in 183o, and has reference to the fact that the See also:angle (8g° 30' ) between the two perfect cleavages differs but little from a right angle: the See also:species was, however, first definitely established by A. See also:Des Cloizeaux in 1876. The crystals and cleavage masses are very like orthoclase in See also:appearance, and the hardness (6) and specific gravity (2.56) are the same for the two minerals; there are, however, important See also:differences in the twinning and in the See also:optical characters. In addition to being twinned according to the same See also:laws as orthoclase, microcline is repeatedly twinned according to the See also:albite-See also:law and the pericline-law, producing a very characteristic grating or See also:cross-hatched structure which is especially prominent when thin sections of the mineral are examined in polarized See also:light. This lamellar structure is often on a very See also:minute See also:scale, sometimes so minute as to be almost indistinguishable: it has therefore been suggested that orthoclase is really a microcline in which the twin-lamellae are ultra-microscopic. In a See also:section parallel to the basal See also:plane c (001) of a microcline crystal the lamellae do not extinguish optically parallel to the edge be as in orthoclase, but at an angle of 15° 3o'; further, the obtuse See also:bisectrix of the optic axes in microcline is inclined to the normal of the plane b (or()) at an angle of 15° 26' . See also:Green microcline is distinctly pleochroic.
Microcline occurs, usually with orthoclase, as a constituent of pegmatites, granites and gneisses; it is rare in porphyries and is not known in volcanic rocks. A beautiful crystallized variety of a See also:bright See also:verdigris-green See also:colour is known as See also:amazon-See also: Chesterlite is a variety occurring as crystals on See also:dolomite in See also:Chester See also:county, See also:Pennsylvania. Closely allied to microcline is the anorthic soda-potash-feldspar known as anorthoclase or natron-microcline. Here See also:sodium pre-dominates over See also:potassium and a little See also:calcium is also often See also:present, the formula being (Na, K) AlSi3O8. It resembles microcline in having a cleavage angle of very nearly 9o° and in the cross-hatched structure, the latter being usually very minute and giving rise to a mottled extinction. It is the characteristic feldspar of volcanic rocks which are See also:rich in soda, and is typically See also:developed in the lavas of the See also:island of See also:Pantelleria near See also:Sicily and those of See also:Kilimanjaro and See also:Mount See also:Kenya in See also:East See also:Africa: the rhomb-shaped porphyritic feldspars of the " rhomb-See also:porphyry " of See also:southern See also:Norway also belong here. (L. J. Additional information and CommentsThere are no comments yet for this article.
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