A cluster of microcline twinned crystals (10,0 x 9,5 cm) By Tom Epaminondas (mineral collector) and Eurico Zimbres (FGEL/UERJ) (Own work) [Public domain], via Wikimedia Commons
Microcline is a variety of feldspar. The only microcline you are likely to encounter is amazonite.
Highest values go to color, transparency, size.
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| Data | Value |
|---|---|
| Name | Microcline |
| Is a Variety of | Feldspar |
| Varieties | Amazonite |
| Colors | White, green, blue, pink, orange |
| Crystallography | Triclinic |
| Refractive Index | 1.514-1.539 |
| Luster | Vitreous. |
| Polish Luster | Greasy to vitreous. |
| Fracture Luster | Pearly to vitreous. |
| Hardness | 6-6.5 |
| Wearability | Poor |
| Fracture | Uneven to splintery |
| Specific Gravity | 2.54-2.58 |
| Birefringence | 0.007-0.010 |
| Cleavage | Perfect and easy in two directions |
| Stone Sizes | Amazonite cabochons up to almost any size are available; the material is usually sold by the pound to hobbyists. The same applies to perthite. The Amelia material has fine color and translucency, but perfect cleavage adds fragility. |
| Heat Sensitivity | Yes |
| Luminescence | Yellow-green in LW; inert SW; green in X-ray |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, UV-Long, X-ray Colors |
| Enhancements | Wax impregnation, hides fractures, common, moderately stable, detect with hot point. Irradiation, produces blue amazonite from white microcline, rare, stable, undetectable. |
| Typical Treatments | Fracture/Cavity Filling, Infusion/Impregnation, Irradiation |
| Transparency | Semitranslucent to opaque. |
| UV Long | Inert to weak yellowish green |
| UV Short | Inert |
| Absorption Spectrum | None distinct. |
| Phenomena | Aventurescence in amazonite. |
| Identifying Characteristics | Mottled coloring with grid like patterns |
| Formula |
KAlSi3O8 (Note, this is the same formula as orthoclase, but a different crystal system.) |
| Pleochroism |
None. |
| Optics |
α = 1.514-1.529; β = 1.518-1.533; γ = 1.521-1.539. Biaxial (-); 2V= 66-103°. |
| Optic Sign | Biaxial - |
Comments
Microcline crystals, associated with smoky quartz, are popular mineral specimens from the Pike’s Peak area of Colorado. The color of amazonite ranges from pale green to dark green and blue-green. Pinkish orthoclase is also often present as an intergrowth.
Name
Microcline is from Greek words meaning small and inclined because the cleavage is close to but not quite 90°.
FELDSPAR: Microcline-amazonite, Amelia Court House Virginia, (1 inch); Canada (~2 inches across). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Occurrence
Microcline occurs in acidic alkali-rich plutonic rocks; also in rocks such as pegmatites, granites, syenites, and schists.
Amazonite
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