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Revision as of 19:01, 6 January 2014 by 76.6.77.177 (talk) (→Chemistry)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)| Cummingtonite | |
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| General | |
| Category | Inosilicate |
| Formula (repeating unit) | (Mg,Fe)7Si8O22(OH)2 |
| Strunz classification | 09.DE.05 |
| Crystal system | Monoclinic |
| Space group | Monoclinic 2/m |
| Unit cell | a = 9.53 Å, b = 18.23 Å, c = 5.32 Å; β = 101.97°; Z = 2 |
| Identification | |
| Color | Dark green, brown, gray, beige; colorless to pale green in thin section |
| Crystal habit | Rarely as distinct crystals. Columnar to fibrous and granular |
| Twinning | Simple and lamellar - common |
| Cleavage | Good on {110} intersecting at 54 and 126° |
| Fracture | Splintery |
| Tenacity | Brittle |
| Mohs scale hardness | 5 - 6 |
| Luster | Vitreous to silky |
| Diaphaneity | Translucent, will transmit light on thin edges. |
| Specific gravity | 3.1 - 3.6 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.639 - 1.671, nβ = 1.647 - 1.689, nγ = 1.664 - 1.708 |
| Birefringence | δ = 0.025 - 0.037 |
| Pleochroism | With increasing iron content, weak; X = Y = colorless; Z = pale green |
| 2V angle | Measured: 65° to 90°, Calculated: 70° to 90° |
| Diagnostic features | Characterized by light brown color and needlelike, often radiating habit. difficult to distinguish from anthophyllite or gedrite without optical and/or X-ray tests. |
| References | |
Cummingtonite is a metamorphic amphibole with the chemical composition (Mg,Fe)7Si8O22(OH)2, magnesium iron silicate hydroxide.
Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common form of magnesium-rich amphibole, the latter being metastable.
Cummingtonite shares few compositional similarities with alkali amphiboles such as arfvedsonite, glaucophane-riebeckite. There is little solubility between these minerals due to different crystal habit and inability of substitution between alkali elements and ferro-magnesian elements within the amphibole structure.
Name and discovery
Cummingtonite was first discovered in 1824 near the town of Cummington, Massachusetts, and it is from this town that the mineral takes its name. It is also found in Sweden, South Africa, Scotland, and New Zealand.
Chemistry
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Occurrence
Cummingtonite is commonly found in metamorphosed magnesium-rich rocks and occurs in amphibolites. Usually it coexists with hornblende or actinolite, magnesium clinochlore chlorite, talc, serpentine-antigorite minerals or metamorphic pyroxene. Magnesium-rich cummingtonite can also coexist with anthophyllite.
Cummingtonite has also been found in some felsic volcanic rocks such as dacites. Manganese rich species can be found in metamorphosed Mn-rich rock units. The grunerite end member is characteristic of the metamorphosed iron formations of the Lake Superior region and the Labrador Trough. With prograde metamorphism cummingtonite and grunerite morph to members of the olivine and pyroxene series.
References
- ^ http://rruff.geo.arizona.edu/doclib/hom/cummingtonite.pdf Handbook of Mineralogy, 2001, Mineral Data Pub.
- ^ http://www.mindat.org/min-1170.html Mindat data
- http://webmineral.com/data/Cummingtonite.shtml Webmineral data
- Mindat w/ location data
- Deer W.A., Howie R.A., & Zussman J. An Introduction to the Rock Forming Minerals, 2nd Edition, 1997, pp. 229–247.
- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7
- Klein, Cornelius., 2002, The Manual of Mineral Science, 22nd ed., John Wiley & Sons, Inc. ISBN 0-471-25177-1
- Mineral Galleries