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| Formula | Ca(Mg,Fe,Mn)(CO3)2 |
| | Optic class & sign | Uniaxial negative |
| | Relief | Low-negative to high; sections with c close to the thin section plane show a marked change of relief over a 90° rotation; strong chagrin in high-relief position (= polarizer parallel to long diagonal of rhomb) |
| | Refractive indices | no = 1.690 -1.750
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ne = 1.510 -1.548
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| n increases with increasing Fe (and Mn) |
| | Birefringence (max.) | 0.180 - 0.202 |
| | | Δn increases with increasing Fe (and Mn); strong dispersion |
| | Sign of elongation | Hardly applicable due to very high Δn |
| | Interference figure | Well-defined isogyre cross, isochromes over multiple colour orders |
| | Colour / pleochroism | Colourless; staining through secondary Fe oxidation |
| | Zoning | - |
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| Form | Habit | Columnar, tabular, granular |
| | | Surface | Anhedral to euhedral, whereby faces may be curved |
| | Cleavage | 3 sets of rhombohedral cleavage {1011} |
| | Twinning | Simple on {0001}, {10Simple on {0001}, {1010}, {1120} |
| | Extinction | Symmetrical to cleavage traces and rhombohedral twin planes in sections subparallel to c. Mottled extinction due to preparation-related, abundant miniscule surface breakouts at cleavage intersections. |
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| Reaction textures | |
| | Alteration / decomposition | Fe oxides and hydroxides |
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| Occurence | Ign | Carbonatite, lamprophyre, dolerite |
| | | Met | Low-grade Fe-rich rocks |
| | | Sed | Argillaceous sediments and sedimentary rocks and sandstones, diagenetic and authigenic |
| | | Hyd | Veins and alteration zones |
| | | Other | |
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| Distinctive properties | High-order white interference colours, rhombohedral cleavage, relief change between nO and nE (as for all other rhombohedral carbonates). Distinction of ankerite from other carbonates in thin section by staining is possible. |
| | Additional comments | Ankerite mainly represents a solid solution between dolomite and the theoretical endmember Ferro-dolomite, CaFe(CO3)2. The latter is not found to exist in nature. Natural compositions range from CaMg0.8Fe0.2(CO3)2 to CaMg0.3Fe0.7(CO3)2.
n-Δn chart: Cal - calcite, Dol - dolomite, Kut - kutnahorite, Mgs - magnesite, Rds - rhodochrosite, Sid - siderite, Smt - smithsonite. The range of refractive indices and of Δn for ankerite is not indicated separately on the chart, but extends from just above dolomite towards siderite. |
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