|
| Formula | Ca5Si2O7(CO3)2 |
| | Optic class & sign | Biaxial positive |
| | Optical orientation | X near c, Y = b, Z at a high angle to a |
| | Optical plane | (010) |
| | Relief | Moderate-positive |
| | Refractive indices | nx = 1.605 -1.617
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|
ny = 1.626 -1.635
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|
nz = 1.651 -1.652
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|
| |
| | Birefringence (max.) | 0.035 - 0.046 |
| | | |
| | Optic Angle
| 2Vx
|
| | 2Vz
= 85 - 89° |
| | Sign of elongation | Length-slow, l (+), or length-fast, l (-), in sections subparallel b, depending on the precise orientation of the section |
| | Interference figure | |
| | Colour / pleochroism | Colourless |
| | Zoning | - |
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|
| Form | Habit | Granular, platy |
| | | Surface | Subhedral to anhedral |
| | Cleavage | {201} |
| | Twinning | {100} simple or lamellar; may or may not be present |
| | Extinction | Z∧{201} cleavage trace = 12° |
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|
| Reaction textures | |
| | Alteration / decomposition | |
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|
| Occurence | Ign | |
| | | Met | Siliceous calcareous rocks and calcsilicate rocks in high-temperature contact-metamorphic zones |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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|
| Distinctive properties | Moderate relief, high Δn, inclined extinction, cleavage; occurrence restricted to high-T contact zones |
| | Additional comments | There is very little information in the literature on natural crystal habits of tilleyite. Hence, the crystal diagrams are purely schematic. |
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