orthoclase (medium temperature polymorph) monoclinic Colorless, white, gray, flesh pink, simple twinning sometimes visible (Carlsbad), hardness 6-8 3. microcline (low temperature polymorph) triclinic White to pale yellow, common in coarse cleavable masses, short prismatic crystals commonly twinned (Carlsbad), hardness 6-7 Orthoclase and microcline are very difficult to tell apart in hand specimen. Cell Parameters: a = 8.6Å, b = 13.03Å, c = 7.17Å. Orthoclase does not show the lamellar twinning that is common in microcline and is occassionally present as striations on cleavage surfaces. Microcline is the stable form below about 400 C. Orthoclase and sanidine are stable above 500 C and 900 C, respectively. Other than that, enviroment of formation is the only other hand sample clue to distinguish orthoclase from sanidine or anorthoclase. It is a type of potassium feldspar, also known as K-feldspar.The gem known as moonstone (see below) is largely composed of orthoclase. Microcline (KAlSi 3 O 8) is an important igneous rock-forming tectosilicate mineral.It is a potassium-rich alkali feldspar.Microcline typically contains minor amounts of sodium.It is common in granite and pegmatites.Microcline forms during slow cooling of orthoclase; it is more stable at lower temperatures than orthoclase. As microcline tends to be more resistant to weathering than orthoclase and plagioclase (Folk, 1974), the general paucity of microcline suggests that pegmatite was not especially abundant in the source area, even though they are locally abundant within the Aishihik batholith and younger plutons of the Long Lake plutonic suite, to the west of the Whitehorse trough (Johnston et al., 1996). This mineral is often called potassium feldspar or K-feldspar because potassium always exceeds sodium in its formula. Orthoclase, or orthoclase feldspar (endmember formula K Al Si 3 O 8), is an important tectosilicate mineral which forms igneous rock.The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Well, Craig just blew me out of the water. But I’ll add a couple of things that may be helpful. Sanidine and anorthoclase usually have a flattened crystal habit. Although petrologists had used twinning as a method of distinguishing orthoclase from microcline, Wright and Stewart (1958) used the difference in certain d values to calculate a degree of disorder of a sample and thus have a quantitative method to identify the structural state of a potassium feldspar. Sanidnine: Crystal System: Monoclinic . Potassium feldspar comes in three different crystal structures that depend on the temperature it formed at. Craig just blew me out of the water orthoclase does not show the lamellar twinning that is common in and! 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