1911 Encyclopædia Britannica/Microcline

by Leonard James Spencer

MICROCLINE, a rock-forming mineral belonging to the feldspar group (see Felspar). Like orthoclase it is a potash-feldspar with the formula KAlSi₃O₈, but differs from this in crystallizing in the anorthic system. The name (from Greek μικρός, small, and κλίνειν, to incline) was given by A. Breithaupt in 1830, and has reference to the fact that the angle (89° 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 bc as in orthoclase, but at an angle of 15° 30′; further, the obtuse bisectrix of the optic axes in microcline is inclined to the normal of the plane b (010) 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 predominates 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 90° 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.)