|
| Formula | CaAl2(Al2Si2O10(OH)2 |
| | Optic class & sign | Biaxial negative |
| | Optical orientation | X near c, Y near a, Z = b |
| | Optical plane | Orthogonal to (010), close to (100) |
| | Relief | Moderate-positive |
| | Refractive indices | nx = 1.630 -1.638
|
|
ny = 1.642 -1.648
|
|
nz = 1.644 -1.650
|
|
| n decreases with increasing substitution of Na for Ca |
| | Birefringence (max.) | 0.010 - 0.014 |
| | | Δn increases with increasing substitution of Na for Ca |
| | Optic Angle
| 2Vx
= 26 - 67° |
| | 2Vz
|
| | Sign of elongation | Length-slow, l (+) |
| | Interference figure | Acute bisectrix figure in basal sections, with 1st order isochromes |
| | Colour / pleochroism | Colourless |
| | Zoning | |
|
|
| Form | Habit | Platy, flaky, granular aggregates |
| | | Surface | Subhedral, anhedral |
| | Cleavage | {001} perfect |
| | Twinning | Twinning on {001} may occur |
| | Extinction | Inclined in (010) sections with max angles 8-9° to cleavage traces |
|
|
| Reaction textures | |
| | Alteration / decomposition | Clay minerals |
|
|
| Occurence | Ign | |
| | | Met | Emery deposits, aluminous phyllites and schists |
| | | Sed | Bauxites |
| | | Hyd | Carbonate veins |
| | | Other | |
|
|
| Distinctive properties | Habit, single perfect cleavage, low Δn. Shares properties with other colourless sheet silicates, including “bird’s-eye-maple” structure, but has low Δn. |
| | Additional comments | Intergrowths with muscovite or paragonite may occur |
|
|
Images 
