Cummingtonite
Cummingtonite (/ˈkʌmɪŋtənaɪt/ KUM-ing-tə-nyte) is a metamorphic amphibole with the chemical composition (Mg,Fe2+
)
2(Mg,Fe2+
)
5Si
8O
22(OH)
2, magnesium iron silicate hydroxide.
| Cummingtonite | |
|---|---|
| General | |
| Category | Inosilicate |
| Formula (repeating unit) | (Mg,Fe2+ ) 2(Mg,Fe2+ ) 5Si 8O 22(OH) 2 |
| IMA symbol | Cum |
| Strunz classification | 9.DE.05 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) H-M symbol: (2/m) |
| Space group | Monoclinic Space group: C2/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 | |
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.
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