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| Formula | (Na,K,Ca)(AlSi)4O8 |
| | Optic class & sign | Biaxial negative |
| | Optical orientation | a near X, b near Z, c near Y |
| | Optical plane | Close to (001) |
| | Relief | Low negative |
| | Refractive indices | nx = 1.519 -1.529
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ny = 1.524 -1.534
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nz = 1.527 -1.536
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| n increases with increasing Ab component |
| | Birefringence (max.) | 0.005 - 0.008 |
| | | Δn increases with increasing Ab component. Crystals commonly unzoned. |
| | Optic Angle
| 2Vx
= 42 - 52° |
| | 2Vz
|
| | Sign of elongation | Length-fast, l (-) for crystals elongate in a |
| | Interference figure | Broad diffuse isogyes on a white low-Δn background. Optic axis figures are commonly impractical due to intense twinning. |
| | Colour / pleochroism | Colourless |
| | Zoning | |
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| Form | Habit | Elongate-prismatic, elongate ∥ c |
| | | Surface | Euhedral to anhedral |
| | Cleavage | {001} perfect, {010} distinct, intersecting at 92° |
| | Twinning | Combined {010} albite and pericline twinning subparallel to {001} resulting in cross-hatched pattern |
| | Extinction | Inclined; aɅX = 10° |
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| Reaction textures | |
| | Alteration / decomposition | Kaolinite, illite, sericite |
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| Occurence | Ign | Na-rich, felsic volcanic and sub-volcanic rocks (alkalic rhyolite, trachyte, phonolite) |
| | | Met | |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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| Distinctive properties | Low n, low Δn, characteristic twinning, occurrence. Twin lamellae are very narrow compared to microcline |
| | Additional comments | |
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