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| Formula | (Na,Ca0.5)8-10Al8+xSi12-xO40 ∙ 12H2O; x = 0-2
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| | Optic class & sign | Uniaxial positive or negative |
| | Relief | Moderate- to low-negative |
| | Refractive indices | no = 1.480 -1.506
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ne = 1.493 -1.508
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| - |
| | Birefringence (max.) | 0.002 - 0.013 |
| | | - |
| | Sign of elongation | Length-slow, l (+); if Ca-rich also l (-) |
| | Interference figure | Commonly poor figures due to fibrous habit and low Δn |
| | Colour / pleochroism | Colourless |
| | Zoning | |
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| Form | Habit | Acicular, radiating aggregates |
| | | Surface | |
| | Cleavage | {110}, 2 sets |
| | Twinning | |
| | Extinction | Straight to traces of prism faces and cleavage in prismatic sections; symmetrical to traces of {110} faces and cleavage in sections ⊥c |
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| Reaction textures | |
| | Alteration / decomposition | Thomsonite; gonnardite is an alteration product of nepheline |
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| Occurence | Ign | |
| | | Met | |
| | | Sed | |
| | | Hyd | Cavities in basalt and leucite tephrite; altered skarn |
| | | Other | |
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| Distinctive properties | |
| | Additional comments | Also described as orthorhombic, with 2V = 50°.
n-Δn chart: Edi – edingtonite, Gon – gonnardite, Lmt – laumontite, Mes – mesolite, Mor – mordenite, Ntr – natrolite, Sco – scolecite, Thm - thomsonite |
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