Hydromagnesite
A hydrated magnesium carbonate forming white crusts where serpentine and brucite weather — the familiar white coating on serpentinite.
Intro adapted from Wikipedia (CC BY-SA).
Hydromagnesite is an IMA-recognized mineral species in the carbonate class, with the chemical formula Mg₅(CO₃)₄(OH)₂·4H₂O.
It crystallizes in the monoclinic crystal system (space group P2_1/c).
Key properties: Mohs hardness 3.5; specific gravity 2.24–2.25; Vitreous; silky to pearly to dull in aggregates luster; refractive index 1.532; optically biaxial.
IMA symbol Hmgs, grandfathered status — known before the IMA’s founding, named in 1828.
Incrusting and filling fractures in altered ultramafic rocks and serpentinites; in low-temperature hydrothermally altered dolomitic xenoliths and marble; common in caves as a component of “moonmilk” and speleothems.
It is found associated with Calcite, dolomite, aragonite, brucite, magnesite, artinite, pyroaurite, periclase, “opal”, chromite.
In allusion to its content of water and similarity to magnesite.