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| Formula | CaCO3
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| | Optic class & sign | Uniaxial negative |
| | Relief | Low negative to moderate positive; 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 = 1.658
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ne = 1.486
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| n and Δn data for endmember composition; n increases with substitution of Ca by Mg, Mn or Fe |
| | Birefringence (max.) | 0.172 |
| | | Δn increases with substitution of Ca by Mg, Mn or Fe |
| | Sign of elongation | Hardly applicable due to very high Δn |
| | Interference figure | Well-defined isogyre cross, isochromes over multiple colour orders; metamorphic calcite may be anomalously biaxial up to 15° |
| | Colour / pleochroism | Colourless |
| | Zoning | |
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| Form | Habit | Commonly granular; fibrous in fibre veins |
| | | Surface | Anhedral in calcite aggregates, isolated grains in a matrix may be euhdral; euhedral crystals in cavities |
| | Cleavage | 3 sets, rhombohedral {1011}, perfect |
| | Twinning | Lamellar on {0112}, 3 sets at 75°, very common, also as deformation twins; simple contact twins on {0001}; lamellar twins may be introduced artificially during cutting, grinding or crushing |
| | 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 | Highly soluble; stable form of CaCO3 at surface conditions |
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| Occurence | Ign | Carbonatites; late-magmatic impregnations and precipitates, including amygdales of mafic volcanics |
| | | Met | Pure and impure calcareous metasediments (marble, calcareous schists, calcsilicate rocks), metamorphosed ultramafic rocks (e.g., ophicarbonate rocks) |
| | | Sed | Chemical precipitate or remains of shell- or skeleton-forming organisms (including transformation of original aragonite during diagenesis); main mineral in limestone, chalk, travertine; clastic component and/or secondary cementing mineral in calcareous sa |
| | | Hyd | Common hydrothermal mineral; veins, cavities; gangue mineral in hydrothermal deposits |
| | | Other | |
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| Distinctive properties | High-order white interference colours, rhombohedral cleavage lamellar twins, and relief change between nO and nE (as for all other rhombohedral carbonates).
Routine optical distinction of the two most common carbonates, calcite and dolomite: Select twinned grains with a distinct change of relief when turning the stage; then select position of max. relief (nO parallel to lower polarizer); the orientation of the twins with respect to O and E directions can now be checked (cf. crystal drawings). In calcite, the twin lamellae are at a high angle to c (i.e., in thin section the angle to ne' is larger than 55°). Quantitative distinction of different carbonates in a rock by staining or XRD techniques. |
| | Additional comments | Details of crystallographic orientation in rock samples may be examined optically in ultrathin (10 micron) sections.
n-Δn chart: Cal - calcite, Dol - dolomite, Kut - kutnahorite, Mgs - magnesite, Rds - rhodochrosite, Sid - siderite, Smt - smithsonite |
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