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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 and serpentinites. It occurs in hydrothermally altered dolomite and marble. It commonly appears in caves as speleothems and "moonmilk", deposited from water that has seeped through magnesium rich rocks. It is the most common cave carbonate after calcite and aragonite.[1] It thermally decomposes, over a temperature range of approximately 220°C to 750°C, releasing water and carbon dioxide leaving a magnesium oxide residue.

It was first described in 1836 for an occurrence in Hoboken, Hudson County, New Jersey.[2]

Stromatolites in an alkaline (pH greater than 9) freshwater lake (Salda Gölü) in southern Turkey are made of hydromagnesite precipitated by diatoms and cyanobacteria.[4] Microbial deposition of hydromagnesite is also reported from playas in British Columbia.[5]


Its most common industrial use is as a fire retardant additive for polymers. Hydromagnesite decomposes endothermically, giving off water and carbon dioxide, leaving a magnesium oxide residue. The initial decomposition begins at about 220 °C making it ideal for use as a filler in polymers and giving it certain advantages over the most commonly used fire retardant, aluminium hydroxide.[6]


1. ^ a b http://rruff.geo.arizona.edu/doclib/hom/hydromagnesite.pdf Handbook of Mineralogy
2. ^ a b http://www.webmineral.com/data/Hydromagnesite.shtml Webmineral data
3. ^ http://www.mindat.org/show.php?id=1979&ld=1#themap Mindat
4. ^ Braithwaite, C. and Veysel Zedef, Living hydromagnesite stromatolites from Turkey, Sedimentary Geology, Volume 106, Issues 3-4, November 1996, Page 309, DOI: 10.1016/S0037-0738(96)00073-5
5. ^ R. W. Renaut, Recent Mamgnesite-Hydromagnesite sedimesntation in Playa Basins of the Caribou Plateau, http://www.em.gov.bc.ca/DL/GSBPubs/GeoFldWk/1990/279-288-renaut.pdf British Columbia Geologic Survey
6. ^ Rothon. R., Particulate-Filled Polymer Composites, 2nd Edition, 2003

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