Formula: CuMg(CO3)(OH)2
Anhydrous carbonate containing hydroxyl, rosasite group
Specific gravity: 3.02 to 3.22
Hardness: 2½
Colour: Light blue-green, Cu-rich cores slightly darker blue, Mg-rich rims very pale blue-green to nearly white
Solubility: Slowly but completely soluble with effervescence in cold dilute mineral acids

Metamorphic environments
Hydrothermal environments

Mcguinnessite is a rare secondary mineral, typically formed in serpentinised peridotite (HOM).


At the Nakauri mine, Shinshiro city, Aichi Prefecture, Japan, mcguinnessite is associated with glaukosphaerite, gaspéite, and jamborite (HOM).

At the type locality, the Copper King Mine, Red Mountain, Red Mountain Mining District, Mendocino county, California, USA, mcguinnessite occurs sparsely as spherules, 0.1 to 2mm in diameter, on joint and vein surfaces, with goethite, malachite, azurite, chrysocolla, copper sulphides, chlorite and vuagnatite in rodingitised metamorphically-altered gabbro dikes in a serpentinised peridotite. Most commonly, mcguinnessite is found on thin crusts of hydrous iron oxides on weathered joint surfaces. However, it may also be found either directly on fresh host rock surfaces or on vuagnatite crystals. In a few places a thin crust of malachite covers mcguinnessite as the last-formed mineral. Mcguinnessite has also been found coated with, and possibly replaced by, chrysocolla. (Minrec 12.143-147).

At Cedar Hill Quarry, Fulton Township, State Line Chromite Mining District, Lancaster county, Pennsylvania, USA, mcguinnessite is associated with coalingite and pokrovskite (HOM).

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