WGNHS
Wisconsin Geological and Natural History Survey

Hydromagnesite

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Close-up of white chalky hydromagnesite
Hydromagnesite. From the mineral collection of Brigham Young University Department of Geology, Provo, Utah, Mineral Specimens 653. (Image courtesy U.S. Geological Survey Denver Library Photographic Collection; image by Andrew Silver.)

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

Description:

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 moon-milk 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).