Anhydrous phosphate containing hydroxyl, forms a series with frondelite.
Specific gravity: 3.45 to 3.6
Hardness: 3½ to 4½
Streak: Grey-green, pale brown
Colour: Dark green, olive-green, brown, yellow-brown, greenish black, black; commonly exhibits colour banding; may be bronze-brown or reddish brown when oxidized (Mindat).
Solubility: Soluble in hydrochloric acid, but not in nitric or sulphuric acid (Mindat, Dana).
Rockbridgeite is a secondary mineral found as alteration product of
triphylite or other
manganese phosphates in granite pegmatites, associated with
strunzite and goethite (Dana, HOM, Mindat).
At the Moculta phosphate deposit and Tom's quarry, Mount Lofty Ranges, South Australia, rockbridgeite is a rare mineral associated with dufrénite (AJM 17.1.25).
At the type locality, the Midvale Mine, Rockbridge county, Virginia, USA, rockbridgeite occurs in a granite pegmatite (Mindat).
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