|
| Formula | BaAl2Si2O8 - (K,Na,Ba)(Al,Si)2Si2O8 |
| | Optic class & sign | Biaxial positive or negative |
| | Optical orientation | Cls: X near c, Y = b, Z near a; Hph: X near a, Y∧c 25-65°, Z = b |
| | Optical plane | Cls (010); Hph orthogonal to (010), near (001) |
| | Relief | Low-negative to moderate-positive |
| | Refractive indices | nx = 1.520 -1.587
|
|
ny = 1.524 -1.593
|
|
nz = 1.526 -1.600
|
|
| n increases with Ba content |
| | Birefringence (max.) | 0.006 - 0.013 |
| | | - |
| | Optic Angle
| 2Vx
|
| | 2Vz
|
| | Sign of elongation | Cls: Length-slow, l(+), Hph: length-fast, l(-) |
| | Interference figure | Broad diffuse isogyres on white background; Cls: large 2V; Hph: large to intermediate 2V |
| | Colour / pleochroism | Colourless |
| | Zoning | |
|
|
| Form | Habit | Short-prismatic, granular, acicular |
| | | Surface | |
| | Cleavage | (001), (010) at 90° |
| | Twinning | Carlsbad, Manebach, Baveno contact twins; simple twins only |
| | Extinction | |
|
|
| Reaction textures | |
| | Alteration / decomposition | |
|
|
| Occurence | Ign | Pegmatites, silica-undersaturated volcanics |
| | | Met | Mn-Ba-rich rocks, granitic gneiss, metamorphosed black shales |
| | | Sed | |
| | | Hyd | Baryte-carbonate veins, other hydrothermal veins in association with Mn minerals |
| | | Other | |
|
|
| Distinctive properties | Relief (relatively high compared to K-Na-rich feldspars), Δn, twinning. Hph: optically difficult to distinguish from orthoclase or untwinned plagioclase |
| | Additional comments | Cls: positive optic sign, Hph: negative optic sign |
|
|
Images 
