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| Formula | MgCO3 |
| | Optic class & sign | Uniaxial negative |
| | Relief | Low negative to high positive; sections with c at a low angle to the thin section plane show a marked change of relief over a 90° rotation; pronounced chagrin in high-relief position (= polarizer parallel to long diagonal of rhomb). |
| | Refractive indices | no = 1.700
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ne = 1.509
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| n and Δn data for endmember composition; n increases with substitution of Fe or Mn for Mg; n decreases with substitution of Ca for Mg |
| | Birefringence (max.) | 0.191 |
| | | Δn increases with substitution of Fe or Mn for Mg; Δn decreases with substitution of Ca for Mg |
| | Sign of elongation | Hardly applicable due to very high Δn |
| | Interference figure | Well-defined isogyre cross, isochromes over multiple colour orders |
| | Colour / pleochroism | Colourless |
| | Zoning | |
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| Form | Habit | Granular, also fibrous aggregates |
| | | Surface | Anhedral, rarely euhedral |
| | Cleavage | 3 sets, rhombohedral {1011}, perfect |
| | Twinning | Generally rare; deformation twins on {0001} |
| | Extinction | Symmetrical to cleavage traces in sections subparallel to c. Mottled extinction due to preparation-related, miniscule surface breakouts at cleavage intersections |
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| Reaction textures | |
| | Alteration / decomposition | Soluble |
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| Occurence | Ign | |
| | | Met | Aggregates and disseminated grains in serpentinite and other Mg-rich rocks; metasomatic zones between calcareous rocks and intrusions |
| | | Sed | Evaporitic deposits; interaction of Mg-rich brines with carbonate sediments; magnesite deposits from near-surface alteration of Mg-rich ultramafic rocks, such as serpentinites, peridotites, pyroxenites, dunites |
| | | Hyd | Veins in Mg-rich rocks, amygdales in mafic volcanics |
| | | Other | |
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| Distinctive properties | High-order white interference colours, rhombohedral cleavage and relief change between nO and nE (as for all other rhombohedral carbonates). Lacks the lamellar twins that are characteristic for calcite and dolomite; occurrence restricted to Mg-rich rocks. |
| | Additional comments | n-Δn chart: Cal - calcite, Dol - dolomite, Kut - kutnahorite, Mgs - magnesite, Rds - rhodochrosite, Sid - siderite, Smt - smithsonite |
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