Origin of Name
From the Greek word micros (small) and klinein (to lean), referring to this mineral’s cleavage angles being close to 90°.
Chemical Formula - KAlSi₃O₈
Hand Specimen Identification
Hardness, luster, cleavage, habit, and association help identify microcline, but it is easily confused with other feldspars, especially orthoclase, its polymorph (see orthoclase). Distinctive “microcline” twinning or a bluish green color are diagnostic but may not be present or visible. X-ray or Optical measurements are needed for certain identification.
Physical Properties
| Hardness | 6 |
| Specific Gravity | 2.56 |
| luster | Pearly, Vitreous |
| Color | colourless |
| streak | white |
| Transparency | Translucent |
| Cleavage | Perfect (001), Good (010) |
Optical Properties
Microcline's "tartan plaid" twinning (a combination of albite and pericline twinning) is its most diagnostic characteristic in thin section. Plagioclase may show two sets of twins at about 90°, but in plagioclase the twins have sharp parallel boundaries, while in microcline they pinch and swell. Biaxial (—), alfa = 1.518, Beta = 1.524, Gama = 1.528, Delta = 0.010, 2V = 77° — 84°.
Crystallography
Triclinic, a = 8.58, b = 12.96, c = 7.21, alfa = 89.7°, Beta = 115.97°, gama = 90.87°, Z = 4; space group P1; point group 1.
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Habit
Prismatic, stubby to elongate crystals are typical. It is also common as cleavable masses or irregular grains. Twins, both contact and penetration, may be present. Combinations of polysynthetic albite and pericline twinning result in the “tartan plaid” appearance, normally only visible under a microscope.
Structure and Composition
The structure of microcline is similar to orthoclase and sanidine, but the SiO4 and AlO4 tetrahedra are more regularly ordered, leading to less symmetry. Ordering decreases with increasing temperature of formation: Low microcline has complete tetrahedral ordering. Intermediate and high microcline are less well ordered. A solvus limits solid solutions between microcline and albite, NaAlSi3O8 at low temperatures.
Occurrence and Associations
Microcline is common in many kinds of silicic igneous rocks, sediments such as arkoses, and a variety of metamorphic rocks. It easily forms from sanidine or orthoclase as rocks cool from high temperatures. Quartz and micas are typically associated minerals.
Varieties
When colored a deep green, microcline is given the name amazonite shown in below figure
Related Minerals
Related minerals include the other KAISi3O8 polymorphs, orthoclase, microcline, adularia, and perthite (see orthoclase), and the plagioclase feldspar series.