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Formula: Mg5(CO3)4(OH)2.4H2O
Hydrated carbonate containing hydroxyl
Crystal System: Monoclinic
Specific gravity: 2.24 to 2.25 measured, 2.25 calculated
Hardness: 3½
Streak: White
Colour: Colourless to white, colourless in transmitted light
Solubility: Effervesces in acids
Environments
Hydromagnesite is a low-temperature hydrothermal mineral occurring in veinlets in serpentinite
and altered magnesium-rich igneous rocks and as an alteration product of brucite in
periclase marble; it is also common in caves.
Associated minerals include calcite, dolomite,
aragonite, brucite,
magnesite,
artinite, pyroaurite,
periclase, opal and
chromite
(HOM, Mindat, Dana).
Localities
At Gulsen, Kraubath an der Mur, Leoben District, Styria, Austria, hydromagnesite occurs as an alteration product of
brucite in
marble-serpentinite
(Dana).
At the Brownley Hill mine, Nenthead, Alston Moor, Eden, Cumbria, England, UK, At Brownley Hill hydromagnesite spherules to 0.4 mm are found
encrusting alstonite crystals in cavities within a sulphide-rich vein. Hydromagnesite is the last
to crystallise in a complex series of carbonates here
(Minrec 31.3.246).
At the type locality, Castle Point, Hoboken, Hudson county, New Jersey, USA, hydromagnesite is associated with
dolomite, brucite,
artinite and aragonite
(Mindat).
At Staten Island (Richmond county), New York City, New York, USA, hydromagnesite is common at artinite
occurrences, where it occurs as rosettes of crystals to 1 mm filling seams in serpentinite
(R&M 84.3.231).
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