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Formula: CuMg(CO3)(OH)2
Anhydrous carbonate containing hydroxyl, rosasite group
Specific gravity: 3.02 to 3.22
Hardness: 2½
Streak:
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
Environments
Metamorphic environments
Hydrothermal environments
Mcguinnessite is a rare secondary mineral,
typically formed in serpentinised
peridotite
(HOM).
Localities
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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