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| Formula | FeCO3 |
| | Optic class & sign | Uniaxial negative |
| | Relief | Moderate positive to very 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 = 1.875
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ne = 1.633
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| n and Δn data for endmember composition; n decreases with substitution of Mn, Mg or Ca for Fe |
| | Birefringence (max.) | 0.242 |
| | | Δn decreases with substitution of Mn, Mg or Ca for Fe |
| | Sign of elongation | Hardly applicable due to very high Δn |
| | Interference figure | Well-defined isogyre cross, isochromes over multiple colour orders |
| | Colour / pleochroism | Colourless, yellow, yellow-brown; pleochroic if coloured O>E |
| | Zoning | |
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| Form | Habit | Granular, radiating fibres |
| | | Surface | Anhedral; also euhedral rhombohedrons, faces may be curved |
| | Cleavage | 3 sets, rhombohedral {1011}, perfect |
| | Twinning | Lamellar twins on {0112}, 3 sets at 75° may be present; simple contact twins on {0001} are rare |
| | Extinction | Symmetrical to cleavage traces and rhombohedral twin planes 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; Fe-oxides and Fe-hydroxides; pseudomorphs of alteration products after siderite are common |
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| Occurence | Ign | Carbonatites, rare in pegmatites |
| | | Met | Sideritic metasediments, meta-ironstones, metasomatic in carbonate rocks |
| | | Sed | Clay ironstones, siderite cherts, siderite mudstones and limestones; concretions |
| | | Hyd | Gangue mineral in veins; fissures and cavities in basaltic or andesitic rocks |
| | | 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). Relief is distinctly higher than that of other rhombohedral carbonates, and there is no negative relief in any position. Characteristic alteration. |
| | Additional comments | n-Δn chart: Cal - calcite, Dol - dolomite, Kut - kutnahorite, Mgs - magnesite, Rds - rhodochrosite, Sid - siderite, Smt - smithsonite |
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