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Formula | (Mg,Fe2+)2(Mg,Fe2+)5Si8O22(OH)2 | |
Crystal System | Monoclinic | Point Group: 2/m |
Cleavage | Perfect on {110} | Intersects at 56 and 124° |
Twinning | Simple or multiple | Parallel to {100}, prevalence of fine polysynthetic twinning |
Streak | White | Brittle |
Luster | Vitreous, silky | |
Hardness | 5-6 | |
Crystal Habit | Bladed, columnar, or fibrous crystals and granular aggregates | Grow up to 20 cm, elongate parallel to c-axis. Forms fibers, plates, and needles. |
Color | Colorless to pale green or pale brown in thin section |
Darker colors in thin section result from higher Fe content. May be translucent to non-transparent |
Pleochroism | X = colorless Y = colorless Z = pale green |
Weak pleochroism, found primarily in samples with higher Fe content |
Optic Sign | Biaxial (+) | |
2V | Measured: 70° - 90° |
2V varies sytematically with Mg content. |
Optic Orientation | X^a= -9° to -3° Y=b Z^c= -21° to -16° |
The Z^c extinction angle varies fairly systematically with composition. |
Refractive Indices |
alpha = 1.633-1.663 beta = 1.638-1.677 gamma = 1.655-1.697 |
Indicies of refraction decrease systematically with Mg content |
Maximum Birefringence | 0.020-0.037 | Birefringesnce decreases systematcally with Mg content |
Elongation | Length slow | |
Extinction | Inclined | |
Dispersion | r < v | Weak |
Mineral Group | Amphibole | |
Distinguishing Features | Hand Sample: Distinguished as an amphibole by habit and cleavage. Brown Varieties closly resemble orthoamphibole and green varieties closely resemble the calcic clinoampiboles - optical tests are required for definite identification. Thin section: Cummingtonite is differentiated by its inclined extinction, higher birefringence, high relief, and prevalance of fine polysynthetic twinning. |
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Occurrence | Medium-grade regionally metamorphosed mafic and ultramafic rocks; characteristic of metamorphosed iron formations. Cummingtonite is a late-stage mineral in some intermediate volcanic rocks, primarily gabbros and norites. Rarely found in silicic volcanic rocks. Associated minerals: tremolite, gedrite, garnet, magnetite, chlorite, plagioclase, hornblend, actinolite, quartz, and biotite. |
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Editors | Alyssa Doody (06), Monica Rolls (13) | |
Resources | -Handbook of Mineralogy, 2001, Mineralogical Society of America, cummingtonite.pdf |