|
| Formula | Ca3Mg(SiO4)2 |
| | Optic class & sign | Biaxial positive |
| | Optical orientation | X near c, Y = b, Z near a |
| | Optical plane | (010) |
| | Relief | High |
| | Refractive indices | nx = 1.702 -1.707
|
|
ny = 1.710 -1.712
|
|
nz = 1.723 -1.725
|
|
| |
| | Birefringence (max.) | 0.018 - 0.021 |
| | | |
| | Optic Angle
| 2Vx
|
| | 2Vz
= 66 - 76° |
| | Sign of elongation | |
| | Interference figure | |
| | Colour / pleochroism | Colourless |
| | Zoning | - |
|
|
| Form | Habit | Granular |
| | | Surface | Anhedral |
| | Cleavage | Poor |
| | Twinning | Lamellar on {100} common, also on [611} and
[611}
|
| | Extinction | X∧c = 13° |
|
|
| Reaction textures | |
| | Alteration / decomposition | |
|
|
| Occurence | Ign | |
| | | Met | Siliceous dolomitic rocks in high-temperature contact-metamorphic zones |
| | | Sed | |
| | | Hyd | |
| | | Other | |
|
|
| Distinctive properties | High relief, moderate Δn (lower than larnite), lamellar twins; restriction to high-T contact zones |
| | Additional comments | Δn values as low as 0.010 have been reported in the literature, but would need to be confirmed. Optical data summarised here are based on Phemister et al. (1942), Neuvonen (1952), Nawaz & Preston (1974), as in Deer et al. (Vol 1B; 1997).
There is very little information in the literature on natural crystal habits of merwinite. Hence, the crystal diagrams are purely schematic. |
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