hydromagnesite

{{Infobox mineral | name = Hydromagnesite | category = Carbonate mineral | boxwidth = | boxbgcolor = | image = Hydromagnesite balloon.jpg | imagesize = | caption = Hydromagnesite balloon in Jewel Cave | formula = | IMAsymbol = Hmgs | molweight = 467.64 g/mol | strunz = 5.DA.05 | dana = 16b.07.01.01 | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P21/c | color = Colorless, white | colour = | habit = Acicular and as encrustations; pseudo-orthorhombic | twinning = Polysynthetic lamellar on {100} | cleavage = {010} Perfect, {100} Distinct | fracture = Uneven | tenacity = Brittle | mohs = 3.5 | luster = Vitreous, silky, pearly, earthy | streak = White | diaphaneity = Transparent to translucent | gravity = 2.16–2.2 | density = | polish = | opticalprop = Biaxial (+) | refractive = nα = 1.523 nβ = 1.527 nγ = 1.545 | birefringence = δ = 0.022 | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= Fluorescent, short UV=green, long UV=bluish white. | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = }} Hydromagnesite is a hydrated magnesium carbonate mineral with the formula .

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, and occurs in hydrothermally altered dolomite and marble. Hydromagnesite 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. The mineral thermally decomposes, over a temperature range of approximately 220 °C to 550 °C, releasing water and carbon dioxide leaving a magnesium oxide residue.