Orthoclase is the monoclinic polymorph of the potassium feldspars, stable at temperatures between those at which the higher temperature sanidine and lower temperature microcline form.  It is a common constituent of shallow intrusive felsic igneous rocks and is also found in high grade (e.g., granulite) pelitic metamorphic rocks.  It is colourless in plane light and has negligible relief relative to quartz and plagioclase with which it is commonly found (A, C).  In detail, the maximum refractive index of orthoclase is lower than the minimum refractive indices of albite or quartz, and thus the Becke Line test can be used to confirm an identification.  Orthoclase has perfect {001} and good {010} cleavages that intersect at right angles.  It is the dominant mineral in images A - D, and is slightly more cloudy in plane light (due to incipient alteration) in A than in C.  Very fine exsolution lamellar of albite are barely discernible in these images, and this intergrowth is termed orthoclase perthite.  Simple twins (e.g., Carlsbad) can occur in orthoclase, but polysynthetic twins are restricted to the higher and lower temperature K-feldspar polymorphs. Sanidine can also have simple twins, but has a lower 2V (40 - 50 degrees) than orthoclase (40 - 70 degrees), and sanidine is restricted to volcanic rocks.   A and B are 5.5 mm across and C and D are 2.2 mm across.  A and C ppl, B and D x-nicols.