Smith College - Geology 222b - Petrology
Petrographic Data

Formula K(AlSi3O8) .
Crystal System Triclinic Beta = 115.9   Alpha = 90.6   Gamma = 87.7
Crystal Habit Massive, tabular or short prismatic crystals Forms largest known crystals.
Cleavage (001) perfect 
(010) good at an angle of 90.6 degrees
Color/Pleochroism White to pale yellow
Colorless in thin section
No pleochroism.
Optic Sign Biaxial (-)  
2V 65-88  
Optic Orientation Y^c~18 
O.A.P. nearly parallel with (001)
Refractive Indices
alpha = 
beta = 
gamma = 
Max Birefringence 0.005-0.008 1st order white
Elongation Elongate parallel to c or a axis
Tabular parallel to (010)
Extinction Inclined Measurement difficult due to ubiquitous twinning 
Dispersion r > v Optic axis dispersion is weak.
Streak White
Mohs Hardness 6-6.5
Specific Gravity Measured: 2.54 - 2.57 g/cm3, Calculated: 2.56 g/cm3
Distinguishing Features Commonly cross-hatched twin pattern (tartan/gridiron) formed by intersecting albite and perthite twins. It may be untwinned or have simple twins: commonly Carlsbad, Baveno and Maneback twins.  Exsolved albite may form lamellae, irregular ribbons and blebs to form perthite.

Microcline is found in granite, granodiorite, pegmatite, syenite and related plutonic igneous rocks that cooled slowly at considerable depth. Due to the time needed to order the placement of Al atoms in the crystal structure, microcline is not found in volcanic rock, but is commonly found in high-grade metamorphic rocks. It can also be found in immature clastic sediments due to its stability in weathering environments. It is commonly associated with quartz and muscovite.

Editors Susan Vincent (00), Michael Bernstein (AC-08), Alianora Walker (11), Emily Carbone (12).

Photomicrograph of microcline in plane polarized light. Click on thumbnail for rollover image in crossed polarized light (sample J-18).
Photomicrograph of microcline in crossed polarized light showing tartan twinning. 

Photomicrograph of microcline in Williamsburg Granodiorite collected off of Massachusetts Rt-9 (sample SCW-1). Click on thumbnail for rollover image in plane polarized and crossed polarized light showing microcline's tartan twinning.

Photomicrograph of microcline in Concord Granite (sample WS-12). Click on thumbnail for rollover image in plane polarized and crossed polarized light.

Photomicrograph of microcline and plagioclase (left to right). Click on thumbnail for rollover image in plane polarized light and crossed polarized light, showing the contrast between microcline's tartan twinning and plagioclase's albite twinning (sample W-11).
Photomicrograph or microcline in Granitoid Gneiss from Fitzwilliam, NH (sample J-18). Click on thumbnail for rollover image in plane polarized and cross polarized light.
WWW Images

U.C.L.A. Petrographic Workshop

Brock University: Greg Finn's Mineral Database

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