Triclinic Section – Microcline Composition & Structure
Microcline
Composition. Like orthoclase, KAlSi3O8 = Silica 64.7, alumina 18.4, potash 16.9.
Crystallization. Triclinic. Axial lengths and angles only slightly different from those of orthoclase. Ordinarily the crystals of the two species cannot be told apart except by very accurate measurements or a microscopical examination. Microcline crystals are usually twinned according to the same laws as orthoclase. Also microscopically twinned according to the albite and pericline laws, characteristic of the triclinic feldspars. A thin section of microcline under the microscope in polarized light usually shows a characteristic grating structure, caused by the crossing at nearly right angles of the twin lamella formed according to these triclinic twinning laws. Orthoclase, being monoclinic, could not show such twinning.
Structure. In cleavage masses or in crystals.
Physical Properties. cleavable parallel to base and brachypinacoid, with angle of 89° 30’ (orthoclase would have 90°). H. = 6-6.5. G. = 2.54 – 2.57. Vitreous luster. Color white to pale yellow. Also sometimes green (Amazon stone) or red. Transparent to translucent.
Tests. Same as for orthoclase. The two species only to be distinguished from each other by careful examination (see above).
Occurrence. Same as for orthoclase. Much that passes as ortho-clase in reality is microcline. Occurs with a green color in the Ural Mts. And at Pike’s Peack, Colorado, and is known as Amazon stone.
Name. microcline is derived from two Greek words meaning little and inclined referring to the slight variation of the cleavage angle from 90°
Use. Same as for orthoclase. Amazon stone is at times polished and used as an ornamental material.
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