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| Formula | Ca4Al8Si16O48 ∙ 18H2O
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| | Optic class & sign | Biaxial negative |
| | Optical orientation | X near a, b = Y, Z near c |
| | Optical plane | (010) |
| | Relief | Low-negative |
| | Refractive indices | nx = 1.502 -1.514
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ny = 1.512 -1.522
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nz = 1.514 -1.525
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| n decreases with substitution of NaSi for CaAl |
| | Birefringence (max.) | 0.010 - 0.012 |
| | | - |
| | Optic Angle
| 2Vx
= 23 - 47° |
| | 2Vz
|
| | Sign of elongation | Length-slow, l (+) |
| | Interference figure | |
| | Colour / pleochroism | Colourless |
| | Zoning | |
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| Form | Habit | Fibrous to columnar, radiating aggregates |
| | | Surface | Euhedral to subhedral |
| | Cleavage | 2 sets {110}, single set {010} |
| | Twinning | Simple twins on {100} common |
| | Extinction | Inclined; symmetrical to traces of {110} faces and cleavage in sections ⊥c |
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| Reaction textures | |
| | Alteration / decomposition | Relatively stable under surface conditions; dehydration to leonhardite |
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| Occurence | Ign | |
| | | Met | Very-low-grade metabasites; laumontite is indicative for zeolite-facies P-T conditions above c. 170°C and 1 kbar |
| | | Sed | |
| | | Hyd | Cavities (vesicles, amygdales, fractures) in a variety of mafic to felsic igneous rocks |
| | | 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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