Formula: Mg5(CO3)4(OH)2.4H2O
Hydrated carbonate containing hydroxyl
Specific gravity: 2.236
Hardness: 3½
Streak: White
Colour: Colourless to white, colourless in transmitted light
Solubility: Effervesces in acids

Hydrothermal 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).


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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