22 Photos
Microcline
Chemical Formula	KAlSi3O8
Species	Silicates
Crystal System	Triclinic
Mohs Scale	6
Specific Gravity	2.55-2.63
Color	White, grey, greyish yellow, yellowish, tan, salmon-pink, bluish green, green.
Streak	White
Luster	Vitreous
Refractive Index	n = 1.514 - 1.529 n = 1.518 - 1.533 n = 1.521 - 1.539
Diaphaneity	Transparent, Translucent
Cleavage	Perfecton , good on
Fracture	Irregular/Uneven
Crystal Habit：Prismatic crystals (to 50 m and 13,500 t), granular, massive. Commonly forms perthitic intergrowths.
Geological Setting：Plutonic felsic rocks, pegmatites, high grade metamorphic veins, hydrothermal veins

Microcline (KAlSi₃O₈) 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. Sanidine is a polymorph of alkali feldspar stable at yet higher temperature. Microcline may be clear, white, pale-yellow, brick-red, or green; it is generally characterized by cross-hatch twinning that forms as a result of the transformation of monoclinic orthoclase into triclinic microcline.

Microcline may be chemically the same as monoclinic orthoclase, but because it belongs to the triclinic crystal system, the prism angle is slightly less than right angles; hence the name "microcline" from the Greek "small slope." It is a fully ordered triclinic modification of potassium feldspar and is dimorphous with orthoclase. Microcline is identical to orthoclase in many physical properties; it can be distinguished by x-ray or optical examination; viewed under a polarizing microscope, microcline exhibits a minute multiple twinning which forms a grating-like structure that is unmistakable.

Perthite is either microcline or orthoclase with thin lamellae of exsolved albite.

Amazon stone, or amazonite, is a beautiful green variety of microcline. It is not found anywhere in the Amazon basin, however. Spanish explorers who named it apparently confused it with another green mineral from that region.

The largest documented single crystals of microcline were found in Devils Hole Beryl Mine, Colorado, US and measured ~50x36x14 m. This could be one of the largest crystals of any material found so far.