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| Formula | (Mg,Fe2+,Fe3+,Al)8(Al,Si)6O20 |
| | Optic class & sign | Biaxial positive or negative |
| | Optical orientation | a Ʌ X = 40-50°, b = Y, c near Z |
| | Optical plane | (010) |
| | Relief | High |
| | Refractive indices | nx = 1.701 -1.731
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ny = 1.703 -1.741
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nz = 1.705 -1.745
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| n increases with increasing Fe |
| | Birefringence (max.) | 0.005 - 0.012 |
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| | Optic Angle
| 2Vx
= 47 - 90° |
| | 2Vz
= 90 - 66° |
| | Sign of elongation | Length-slow, l (+) |
| | Interference figure | Most sapphirine is optically negative. Few isogyres of 1st order colours. Strong optic axis dispersion. |
| | Colour / pleochroism | Colourless to shades of blue, also green
Distinctly pleochroic with Z > Y > X. Near-endmember Mg-sapphirine may be entirely colourless.
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| | Zoning | |
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| Form | Habit | Tabular, elongate |
| | | Surface | Anhedral to euhedral |
| | Cleavage | Poor, commonly not observed |
| | Twinning | Various types of twins reported; multiple twinning is suspected to be rather sub-parallel growth of multiple crystals. |
| | Extinction | Inclined to crystal faces in random sections; straight to the main prism {010} faces in sections ∥ b |
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| Reaction textures | Breakdown product of Al-silicates |
| | Alteration / decomposition | Relatively stable |
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| Occurence | Ign | |
| | | Met | High-grade aluminous-magnesian rocks; in association with quartz indicative of ultrahigh-T granulite facies |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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| Distinctive properties | High n, blue colour, low Δn, lack of a distinct cleavage |
| | Additional comments | |
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