Brushite

Brushite is a mineral, formula CaHPO₄·2H₂O. It is believed to be precursor of apatite and is found in guano-rich caves; formed by the interaction of guano with calcite and clay at a low pH.

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The other source of brushite is from calcium phosphate kidney stones, the original precipitating material is usually brushite. 9% of supragingival calculus consist of brushite

A 300 year sealed-in-lead coffin burial caused the skeleton of a man to turn into brushite crystals. ^[1]