Laumontite is a mineral, one of the zeolite group. Its molecular formula is Ca(AlSi2O6)2 · 4H2O, a hydrated calcium-aluminium silicate. Potassium or sodium may substitute for the calcium but only in very small amounts.
It is monoclinic, space group C2/m. It forms prismatic crystals with a diamond-shaped cross-section and an angled termination. When pure, the color is colorless or white. Impurities may color it orange, brownish, gray, yellowish, pink, or reddish. It has perfect cleavage on [010] and [110] and its fracture is conchoidal. It is very brittle. The Mohs scale hardness is 3.5-4. It has a vitreous luster and a white streak.It is found in hydrothermal deposits left in calcareous rocks, often formed as a result of secondary mineralization. Host rock types include basalt, andesite, metamorphic rocks and granites. It forms at a temperature of about 100 °C (212 °F), and becomes unstable above about 150 °C (302 °F), and so its presence in sedimentary rocks indicates that these have experienced intermediate diagenesis.
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