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| Formula | Na2Al2Si3O10 ∙ 2H2O
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| | Optic class & sign | Biaxial positive |
| | Optical orientation | X = a, Y = b, Z = c |
| | Optical plane | (010) |
| | Relief | Moderate-negative |
| | Refractive indices | nx = 1.473 -1.483
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ny = 1.476 -1.486
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nz = 1.485 -1.496
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| n decreases with Ca substitution for Na |
| | Birefringence (max.) | 0.012 - 0.013 |
| | | - |
| | Optic Angle
| 2Vx
|
| | 2Vz
= 58 - 73° |
| | Sign of elongation | Length-slow, l (+) |
| | Interference figure | Acute bisectrix in sections orthogonal to c; optic axis figure may be difficult to obtain from fibrous material |
| | Colour / pleochroism | Colourless |
| | Zoning | |
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| Form | Habit | Acicular, fibrous, radial aggregates |
| | | Surface | Euhedral in open cavities, subhedral in in intergrown aggregates |
| | Cleavage | 2 sets {110}, parting on {010} |
| | Twinning | Rare |
| | Extinction | Straight to traces of prism faces and {110} cleavage in prismatic sections; symmetrical to traces of {110} faces and cleavage in sections ⊥c |
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| Reaction textures | |
| | Alteration / decomposition | Relatively stable under surface conditions; natrolite is an alteration product of nepheline, other feldspathoids and alkali feldspar |
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| Occurence | Ign | |
| | | Met | |
| | | Sed | |
| | | Hyd | Cavities (vesicles, amygdales, fractures) in mafic volcanic rocks, hydrothermally altered rocks originally containing feldspathoids, and/or or alkali feldspar |
| | | Other | |
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| Distinctive properties | |
| | Additional comments | n-Δn chart: Edi – edingtonite, Gon – gonnardite, Lmt – laumontite, Mes – mesolite, Mor – mordenite, Ntr – natrolite, Sco – scolecite, Thm - thomsonite |
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