Formula: Mn2+9(AsO4)2(AsO3)(OH)9.2H2O
Compound arsenate, hematolite group
Specific gravity: 3.57 to 3.79
Hardness: 4½
Colour: Colourless (crystal centres), red to brown (exteriors), blackish; colourless to pale brown or reddish in transmitted light.
Solubility: Readily soluble in acids. Oxidises readily upon exposure to air.

Hydrothermal environments

Synadelphite is a rare low-temperature hydrothermal mineral.

At Långban, Värmland, Sweden, synadelphite is associated with manganoan biotite, allactite and hausmannite.

At the type locality, the Moss mine, Värmland, Sweden, synadelphite cuts hausmannite ores in dolomitic marble, associated with pyrochroite, manganite, jacobsite, allactite and calcite.

At Sterling Hill, New Jersey, USA, synadelphite is associated with kraisslite and allactite.

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