Hydromagnesite is a hydrated magnesium carbonate mineral with the formula Mg5(CO3)4(OH)2·4H2O.
It generally occurs associated with the weathering products of magnesium containing minerals such as serpentine or brucite. It occurs as incrustations and vein or fracture fillings in ultramafic rocks. It occurs in hydrothermally and marble as speleothems ", deposited from water that has seeped through magnesium rich rocks. It is the most common cave carbonate after calcite over a temperature range of approximately 220 °C to 550 °C, releasing water and carbon dioxide leaving a magnesium oxide residue.
It was first described in 1836 for an occurrence in Hoboken, New Jersey
Stromatolites greater than 9) freshwater lake (Salda Gölü) in southern Turkey and cyanobacteria
Microbial deposition of hydromagnesite is also reported from playas in British Columbia The hydromagnesite-magnesite playas near Atlin, British Columbia are some of the most studied deposits of hydromagnesite. These deposits have been characterized in the context of a biogeochemical model for CO2 sequestration.
One of the largest deposits of hydromagnesite exists in Greece. It consists of a natural mixture with huntite. Local people have used the white mineral as a source of material for whitewashing buildings for centuries. In the mid 20th century the minerals, ground to a fine powder, found use as a filler for rubber shoe soles. The locals used the granite mills designed for grinding wheat. Commercial exploitation of the minerals began in the late 70s and early 80s with the mineral being exported worldwide. The Greek deposit is still operated commercially, although the world's largest commercially operated reserves are in Turkey.