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Encyclopedia Britannica



MICROCLINE

This article appears in Volume V18, Page 381 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: MIC-MOL
MICROCLINE , a
rock
 -forming
mineral
  belonging to the feldspar group (see FELSPAR). Like orthoclase it is a potash-feldspar with the
formula
  KAlSi308, but differs from this in crystallizing in the anorthic system. The name (from Greek Awl's, small, and ,XLeav, to incline) was given by A. Breithaupt in 183o, and has reference to the fact that the angle (8g 30' ) between the two perfect cleavages differs but little from a right angle: the species was, however, first definitely established by A. Des Cloizeaux in 1876. The crystals and cleavage masses are very like orthoclase in
appearance
 , and the hardness (6) and specific gravity (2.56) are the same for the two minerals; there are, however, important differences in the twinning and in the optical characters. In addition to being twinned according to the same laws as orthoclase, microcline is repeatedly twinned according to the albite-law and the pericline-law, producing a very characteristic grating or cross-hatched structure which is especially prominent when thin sections of the
mineral
  are examined in polarized light. This lamellar structure is often on a very minute 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 section parallel to the basal 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 bisectrix of the optic axes in microcline is inclined to the normal of the plane b (or()) at an angle of 15 26' . 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
bright
 
verdigris
 -green colour is known as amazon-stone (q.v.). Chesterlite is a variety occurring as crystals on dolomite in Chester county,
Pennsylvania
 .
Closely allied to microcline is the anorthic soda-potash-feldspar known as anorthoclase or natron-microcline. Here sodium pre-dominates over potassium and a little calcium is also often 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 rich in soda, and is typically developed in the lavas of the island of Pantelleria near Sicily and those of Kilimanjaro and
Mount
  Kenya in
East
  Africa: the rhomb-shaped porphyritic feldspars of the " rhomb-
porphyry
  " of southern Norway also belong here.
(L. J. S.)


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