Microcline

Microcline
General
Category	Tectosilicate
Formula (repeating unit)	KAlSi3O8
IMA symbol	Mcc[1]
Space group	P1 (no. 2)
Identification
Color	White, grey, greyish yellow, yellowish, tan, salmon-pink, bluish green, green.
Crystal habit	Can be anhedral or euhedral. Grains are commonly elongate with a tabular appearance. May contain lamellae which formed from exsolved albite.
Twinning	Typically displays albite twinning and pericline twinning. This combination leads to a grid pattern, hence microcline displays gridiron twinning. Can also display carlsbad twinning, simple twins, or lack twinning altogether. Lamellae in microcline are discontinuous and "pinch and swell".
Cleavage	Has perfect cleavage parallel to {001} and good cleavage on {010}. Cleavages intersect at 90°41'. It can be difficult to see cleavage in thin section due to microcline's low relief.
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	6–6.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent, translucent
Specific gravity	2.5–2.6
Optical properties	Biaxial negative
Refractive index	nα = 1.514 – 1.529 nβ = 1.518 – 1.533 nγ = 1.521 – 1.539
Birefringence	Up to first order white (roughly 0.007)
Pleochroism	N/A
2V angle	65–88°
Extinction	Inclined extinction to cleavage
Diagnostic features	Gridiron twinning distinguishes microcline from other feldspars. Distinguishable from plagioclase because the lamellae in plagioclase are continuous and do not "pinch and swell."
Alters to	Commonly alters to sericite or clay.
Relief	Low negative relief
Optical sign	Biaxial negative
Color in PPL	Colorless

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.

The chemical compound name is potassium aluminium silicate, and it is known as E number reference E555.