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| Formula | (Fe,Mg)3Si4O10(OH)2 |
| | Optic class & sign | Biaxial negative |
| | Optical orientation | X near c, Y ≈ a, Z ≈ b |
| | Optical plane | Orthogonal to (010) |
| | Relief | Moderate-positive |
| | Refractive indices | nx = 1.580 -1.592
|
|
ny = 1.612 -1.622
|
|
nz = 1.615 -1.632
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|
| - |
| | Birefringence (max.) | 0.035 - 0.040 |
| | | - |
| | Optic Angle
| 2Vx
= Small - |
| | 2Vz
|
| | Sign of elongation | Length-slow, l(+) |
| | Interference figure | |
| | Colour / pleochroism | Colourless or pale yellow to pale green; pleochroism X > Y = Z |
| | Zoning | |
|
|
| Form | Habit | Tabular, acicular, fibrous |
| | | Surface | |
| | Cleavage | {001} |
| | Twinning | |
| | Extinction | With respect to cleavage, straight to very small inclined in sections orthogonal to c |
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|
| Reaction textures | |
| | Alteration / decomposition | |
|
|
| Occurence | Ign | |
| | | Met | Low-grade metamorphic ironstones |
| | | Sed | |
| | | Hyd | |
| | | Other | |
|
|
| Distinctive properties | High birefringence, extinction, restricted occurrence |
| | Additional comments | |
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