Microcline |
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Property |
Value |
Comments |
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
Z^b~18 X^a~18 O.A.P. nearly parallel with (001) |
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Refractive Indices
alpha = beta = gamma = |
1.514-1.526
1.518-1.530 1.521-1.533 |
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Max Birefringence | 0.005-0.008 | 1st order white |
Elongation | Elongate parallel to c or a axis
Tabular parallel to (010) |
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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. | |
Occurrence | 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. |
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Editors | Susan Vincent (00), Michael Bernstein (AC-08), Alianora Walker (11), Emily Carbone (12). |