Laumontite n. : A mineral, of a white color and vitreous luster. It is a hydrous silicate of alumina and lime. Exposed to the air, it loses water, becomes opaque, and crumbles.

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Taxonomy
Laumontite, free encyclopedia, calcium, mineral, prismatic crystals, aluminium silicate, zeolites, search, molecular formula, substitute, monoclinic, space group, C2, diamond-shaped, cross-section, termination, Impurities, System of Mineralogy.

Defination from the web:
laumontite goes back to the early days of mineralogy. ... Laumontite easily dehydrates when stored in a low humidity environment. ... The dehydrated laumontite is very friable, often falling into a powder at the slightest touch. It is a common mineral, found worldwide. ...