|
| Formula | Al(OH)3 |
| | Optic class & sign | Biaxial positive |
| | Optical orientation | X = b, Y near a, Z near c |
| | Optical plane | Orthogonal to (010) |
| | Relief | Low- to moderate-positive |
| | Refractive indices | nx = 1.568 -1.580
|
|
ny = 1.568 -1.580
|
|
nz = 1.587 -1.600
|
|
| - |
| | Birefringence (max.) | 0.019 - 0.020 |
| | | - |
| | Optic Angle
| 2Vx
|
| | 2Vz
= 0 - 40° |
| | Sign of elongation | Length-fast, l (-) |
| | Interference figure | |
| | Colour / pleochroism | Colourless to pale brown |
| | Zoning | |
|
|
| Form | Habit | Tabular, pseudohexagonal outline |
| | | Surface | |
| | Cleavage | {001} |
| | Twinning | {001} common |
| | Extinction | c∧Z = 21° |
|
|
| Reaction textures | Product of hydration of feldspars and other aluminous minerals |
| | Alteration / decomposition | Kaolinite |
|
|
| Occurence | Ign | |
| | | Met | |
| | | Sed | Bauxite, laterite, soils; less common as cement in sand or sandstone |
| | | Hyd | Veins |
| | | Other | |
|
|
| Distinctive properties | Birefringence, relief, optic sign; environment of occurrence |
| | Additional comments | Other Al(OH)3 polymorphs: norstrandite (nx = 1.580, ny = 1.583, nz = 1.602, 2Vz = 24°), doyleite (nx = 1.545, ny = 1.553, nz = 1.566, 2Vz = 77°) |
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