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| Formula | CaAl2[Si2O7](OH)2 ∙ H2O |
| | Optic class & sign | Biaxial positive |
| | Optical orientation | a = X, b = Y, c = Z |
| | Optical plane | (010) |
| | Relief | High |
| | Refractive indices | nx = 1.663 -1.665
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ny = 1.672 -1.675
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nz = 1.682 -1.686
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| - |
| | Birefringence (max.) | 0.019 - 0.021 |
| | | - |
| | Optic Angle
| 2Vx
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| | 2Vz
= 76 - 87° |
| | Sign of elongation | Length fast, l (-), in sections at a high angle to {001}; crystals elongate in c are length-slow, l (+) |
| | Interference figure | Basal sections yield acute bisectrix figures |
| | Colour / pleochroism | Colourless |
| | Zoning | |
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| Form | Habit | Tabular on {001}, showing rhomb-shaped sections ⊥ c and elongate-rectangular sections at a high angle to c; less commonly elongate-prismatic along c |
| | | Surface | Subhedral to euhedral, also granular-anhedral aggregates |
| | Cleavage | {001} and {010} perfect, orthogonal to each other |
| | Twinning | Simple or multiple twins on {110} |
| | Extinction | Straight to {001} and {100} faces and to {001} and {010} cleavages in sections parallel c |
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| Reaction textures | Replacement of plagioclase under high-pressure conditions; retrograde pseudomorphic replacement of lawsonite |
| | Alteration / decomposition | Relatively stable at low temperatures |
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| Occurence | Ign | |
| | | Met | High-pressure, low-temperature rocks (P > 3 kbar) of the lawsonite-albite-chlorite and lawsonite-blueschist facies; most common in, but not restricted to, rocks of mafic composition |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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| Distinctive properties | High relief, low birefringence; characteristic crystal form, restricted occurrence in rocks from high P/T gradients |
| | Additional comments | |
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