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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 |
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Photomicrograph of cummingtonite in plain light, click to view larger image. 10x magnification. Note light brown pleochorism. |
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Photomicrograph of cummingtonite in plain light, click to view larger image. 10x magnification. |
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Photomicrograph of cummingtonite in plain light, click to view larger image. 10x magnification. |
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Photomicrograph of cummingtonite in plane light. Mag. 10x. |
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Photomicrograph of cummingtonite in plane light. Click to view larger image. Mag. 5x. |