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Property |
Value |
Comments |
| Formula | Na(Mg,Fe)4Al2(AlSi7O22(OH)2 |
Gedrite is in a solid solution series with anthophyllite, and it tends to be more Fe- and Al- rich than the Mg- rich anthophyllite. It is an amphibole. |
| Crystal System | Orthorhombic | 2/m2/m2/m |
| Crystal Habit | Columnar, bladed, acicular | May have simple and lamellar twins on {100} |
| Cleavage | Perfect {210}; {100}, {010} | Cleavages intersecting at 56° and 124° distinguishes it as an amphibole. |
| Specific Gravity | 2.85 – 3.57 | |
| Hardness | 5.5-6 | |
| Color/Pleochroism | Colorless to pale yellow/brown in thin section. Has weak pleochroism with Z ≥ Y ≥ X: X = colorless, pale yellow, pale grayish brown, yellowish green Y = colorless, pale yellow, pale grayish brown, yellowish green Z = pale grayish brown, purplish brown, grayish green |
In hand sample: brown, gray, grayish brown, yellowish brown, brownish green, or white. Has a gray streak. |
| Optic Sign | Biaxial (+) or (-) | Gedrite is more commonly positive, and this sample is. Anthophyllite is more commonly negative. |
| 2Vγ | 72° to 111°. | |
| Optic Orientation | X = a Y = b Z = c O.A.P. = (010) |
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| Refractive Indices alpha = beta = gamma = delta = |
1.596 - 1.694 1.605 - 1.710 1.615 - 1.722 0.013 - 0.028 |
Gedrite typically has higher refractive indices than anthophyllite |
| Extinction | Parallel | Distinguishes gedrite and anthophyllite from other amphiboles. |
| Dispersion | r > v or r < v | r > v if positive, r < v if negative |
| Distinguishing Features | See the comments on cleavage, extinction, optic sign, and refractive indices. | |
| Occurrence | Found in metamorphic rocks only. | |
| Sources | Deer, W.A., Howie, R.A., and Zussman, J. (1963) Rock Forming Minerals Vol. 2: Chain Silicates, Longmans, Green and Co Ltd., London, 390 p. Nesse, W.D. (2012) Introduction to Mineralogy, Oxford University Press, New York, 480 p. |
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| Editors | Leah Tallent ('19) | |
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Photomicrograph of gedrite thin section in plain polarized light. Roll over image with the mouse to view it in cross polarized light. |
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Photomicrograph of gedrite thin section in plain polarized light. Roll over image with the mouse to view pleochroism. |
WWW Images |
Union College [under amphiboles]
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