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| Formula | (Mg,Fe)7Si8O22(OH)2 | |
| Crystal System | Monoclinic | |
| Crystal Habit | Fibrous, lamellar and radiated | |
| Cleavage | Perfect {110} | Intersects at 56 and 124°; Brittle |
| Form and Twinning | Columnar, bladed, or acicular and elongate parallel to the c axis. Silky & Fibrous | |
| Streak | White | |
| Luster | Vitreous | |
| Color/Pleochroism | Various shades of light brown, colorless to pale green. Tanslucent; will transmit light on thin edges. X = colorless, pale yellow Y = pale yellow, pale brown Z = pale green, pale brown |
Color in thin section is darker with with higher Fe content. Weak pleochroism in colored varities. |
| Optic Sign | Biaxial (+) | |
| 2V | 65 - 98° |
2V varies sytematically with Mg content. |
| Optic Orientation | Xa= +2 to -9° Y=b Zc=+10 to +21 |
Trace of cleavage and crystal length (=c axis) are length slow. The Zc extinction angle varies fairly systematically with composition. |
| Refractive Indices
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alpha = 1.630-1.696 beta = 1.638-1.709 gamma = 1.655-1.729 |
Indicies of refraction decrease systematically with Mg content |
| Max Birefringence | 0.020-0.045 | Birefringesnce decreases systematcally with Mg content |
| Extinction | Inclined | |
| Dispersion | r < v | Weak |
| Distinguishing Features | Hand Sample: Distinguished as a amphibole by habit and cleavage. Brown Varieties closly resemble orthoamphibole. Green varieties closely resemble the calcic clinoampiboles. Thin section/Grain Mount: Very similar to orthoamphibole and tremolite/actinolite iin thin section. Cummingtonite is differentiated by its inclined extinction, higher birefringence and prevalance of fine polysynthetic twinning. |
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| Occurrence | Occurs in metamorphosed mafic and ultramafic rocks, sometimes with an orthoamphibole and in granitic gneiss, hornfels, and granulite. Occasionally found in intermediate volcanic rocks, diorite, gabbro, norite and in skarns. Associated minerals: orthoamphibole, cordierite, garnet, plagioclase, hornblend, and biotite. |
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| Editors | Alyssa Doody (06). Data Source: Introduction to Mineralogy, W. Nesse (2000) p. 282-283. |
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Photomicrograph of cummingtonite in plane light. Mag. 10x. |
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Photomicrograph of cummingtonite under crossed polarized light. Mag. 10x |
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Photomicrograph of cummingtonite in plane light. Mag. 5x. |
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Photomicrograph of cummingtonite under crossed polarized light. Mag. 5x |