Hydromagnesite. From the mineral collection of Brigham Young University Department of Geology, Provo, Utah, Mineral Specimens 653. Courtesy of the U.S. Geological Survey Denver Library Photographic Collection. (Photo by Andrew Silver.)

Formula: Mg5(CO3)4(OH)2.4H2O Monoclinic


Hydromagnesite forms white chalky masses or, occasionally sprays of radiating acicular to bladed crystals due to the alteration and weathering of magnesium-rich rocks.

CRAWFORD COUNTY: Hydromagnesite is a major component of moon-milk covering the roof of the Bridgeport Cave (SE NE NE sec. 2 T.6N. R.6W.) near Prairie du Chien (Boyd, 1965, Cronin, 1970). The white plastic moonmilk is a bacterial precipitate and covers the cave roof to a depth of up to 1 cm. over an area of approximately 12 meters by 8 meters.

ONEIDA COUNTY: Hydromagnesite occurs in calc-silicate rocks at the Lynn massive sulfide deposit where it is associated with tremolite, diopside and epidote (Kennedy et al., 1991).