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| Formula | (K,Na)(Mg,Fe2+)2(Al,Fe3+)3(Si,Al)12O30 |
| | Optic class & sign | Uniaxial positive or negative |
| | Relief | Low-positive |
| | Refractive indices | no = 1.539 -1.547
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ne = 1.545 -1.551
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| n increases with increasing Fe content |
| | Birefringence (max.) | 0.002 - 0.008 |
| | | Δn increases with decreasing Fe content |
| | Sign of elongation | Length-slow, l(+) and length-fast, l(-), depending on crystal form (elongate vs. tabular) |
| | Interference figure | |
| | Colour / pleochroism | O: pale blue or pale pink, pale yellow-brown; E: colourless;
dichroism O > E (if optically positive)
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| | Zoning | |
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| Form | Habit | Elongate-prismatic, tabular |
| | | Surface | |
| | Cleavage | Indistinct |
| | Twinning | Rare |
| | Extinction | |
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| Reaction textures | |
| | Alteration / decomposition | |
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| Occurence | Ign | Cavities and groundmass of rhyolites, dacites, andesites |
| | | Met | Granulites (specifically ultrahigh-temperature rocks); high-temperature contact aureoles |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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| Distinctive properties | Low relief, colour and dichroism, low birefringence; distinction from cordierite also by XRD |
| | Additional comments | Commonly optically positive. Anomalously biaxial, with 2V up to 15°. |
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