Microcline

Microcline
General
CategoryTectosilicate
Formula
(repeating unit)
KAlSi3O8
IMA symbolMcc[1]
Space groupP1 (no. 2)
Identification
ColorWhite, grey, greyish yellow, yellowish, tan, salmon-pink, bluish green, green.
Crystal habitCan be anhedral or euhedral. Grains are commonly elongate with a tabular appearance. May contain lamellae which formed from exsolved albite.
TwinningTypically 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".
Photomicrograph of thin section of microcline showing crosshatched crystal twinning (in cross polarized light)
CleavageHas 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.
FractureUneven
TenacityBrittle
Mohs scale hardness6–6.5
LusterVitreous
StreakWhite
DiaphaneityTransparent, translucent
Specific gravity2.5–2.6
Optical propertiesBiaxial negative
Refractive indexnα = 1.514 – 1.529 nβ = 1.518 – 1.533 nγ = 1.521 – 1.539
BirefringenceUp to first order white (roughly 0.007)
PleochroismN/A
2V angle65–88°
ExtinctionInclined extinction to cleavage
Diagnostic featuresGridiron twinning distinguishes microcline from other feldspars. Distinguishable from plagioclase because the lamellae in plagioclase are continuous and do not "pinch and swell."
Alters toCommonly alters to sericite or clay.
ReliefLow negative relief
Optical signBiaxial negative
Color in PPLColorless

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.

  1. ^ L.N. Warr (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.