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| Formula | Ca2Al2(Al,Mg,Fe2+,Fe3+)(SiO4)(Si2O6)(OH,O)2(OH)2 |
| | Optic class & sign | Biaxial positive or negative |
| | Optical orientation | a near X, b = Y, c near Z; other orientation like b = Z does also occur |
| | Optical plane | (010) |
| | Relief | High |
| | Refractive indices | nx = 1.665 -1.710
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ny = 1.670 -1.720
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nz = 1.683 -1.726
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| n increases with Fe2+ and Fe3+; Cr-, Mn- and Fe-rich varieties may have higher refractive indices than those listed above, up to nx = 1776, ny = 1.814, nz = 1.836 |
| | Birefringence (max.) | 0.010 - 0.020 |
| | | Δn increases with Fe2+ and Fe3+; anomalous interference colours due to strong dispersion; birefringence zoning may occur; Cr-, Mn- and Fe-rich varieties may have higher Δn, up to 0.044 |
| | Optic Angle
| 2Vx
= 20 - 90° |
| | 2Vz
= 90 - 7° |
| | Sign of elongation | Length-slow, l (+) and length-fast, l (-), as long axis is b = Y |
| | Interference figure | 2V extremely variable, strong optic axis dispersion |
| | Colour / pleochroism | Nearly colourless (Mg-rich) to shades of green, yellow, brown, then strongly pleochroic Y > Z ≥ X; X = colourless, pale greenish-yellow to blue-green, , Y = light green, green, blue-green, brownish yellow, Z = colourless, yellow, yellow-brown; colour zoning may be present |
| | Zoning | |
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| Form | Habit | Acicular, fibrous, bladed, columnar; radiating parallel and random aggregates |
| | | Surface | Subhedral to euhedral |
| | Cleavage | {001} distinct, {100} fair |
| | Twinning | {001} and {100}, creating four-fold sectors in appropriate sections |
| | Extinction | Straight with respect to prism faces and cleavages |
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| Reaction textures | |
| | Alteration / decomposition | Relatively stable at low temperatures; sub-greenschist alteration product of mafic minerals |
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| Occurence | Ign | |
| | | Met | Ca-rich mafic rocks at sub-greenschist facies conditions (zeolite, pumpellyite-prehnite and pumpellyite-actinolite facies), and blueschist-facies conditions; also in low-temperature silicate-bearing carbonate rocks |
| | | Sed | May be part of heavy-mineral fraction of sands |
| | | Hyd | Low-temperature veins, amygdales, and other cavities; altered mafic rocks |
| | | Other | |
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| Distinctive properties | High relief, colour and pleochroism, anomalous birefringence, mineral association |
| | Additional comments | |
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