|
| Formula | Ca3Si2O7 |
| | Optic class & sign | Biaxial positive |
| | Optical orientation | X near a, Y = b, Z near c |
| | Optical plane | (010) |
| | Relief | Moderate-positive |
| | Refractive indices | nx = 1.640 -1.643
|
|
ny = 1.643 -1.646
|
|
nz = 1.650 -1.652
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|
|
| | Birefringence (max.) | 0.008 - 0.010 |
| | | |
| | Optic Angle
| 2Vx
|
| | 2Vz
= 62 - 69° |
| | Sign of elongation | In tabular crystals, length-slow, l (+), in sections subparallel to c, and length-fast, l(-), in sections near-orthogonal to c |
| | Interference figure | |
| | Colour / pleochroism | Colourless |
| | Zoning | - |
|
|
| Form | Habit | Tabular, granular, prismatic |
| | | Surface | Subhedral to anhedral |
| | Cleavage | None |
| | Twinning | None |
| | Extinction | |
|
|
| Reaction textures | Replacement by larnite as temperature increases. Retrograde replacement by kilchoanite (see also below) |
| | Alteration / decomposition | |
|
|
| 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, low Δn, lack of lamellar twinning and cleavage; occurrence restricted to high-T contact zones |
| | Additional comments | Kilchoanite is an orthorhombic polymorph with nx = 1.647, ny = 1.648, nz = 1.650 and 2Vx = 60°; may show anomalous interference colours.
There is very little information in the literature on natural crystal habits of rankinite. Hence, the crystal diagrams are purely schematic.
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