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 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).