Laumontite

Laumontite is a mineral, one of the zeolite group. Its molecular formula is CaAl2Si4O12·4H2O, 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. The identification of laumontite goes back to the early days of mineralogy. It was first named lomonite by R.