Phosphosiderite , Rockbridgeite Locality: Indian Mountain, Cherokee Co., Alabama Size: miniature, 5.2 x 4.5 x 3.2 cm Rockbridgeite over Phosphosiderite (?) A rare example of the classic phosphates from this locality, which are seldom seen as anything but smudges. Yet, this piece is actually quite rich and attractive! — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Rockbridgeite
A green-black iron phosphate namesake of its own supergroup, from Rockbridge County, Virginia.
Intro adapted from Wikipedia (CC BY-SA).
Rockbridgeite is an IMA-recognized mineral species in the phosphate class, with the chemical formula (Fe²⁺₀.₅Fe³⁺₀.₅)₂Fe³⁺₃(PO₄)₃(OH)₅.
It crystallizes in the orthorhombic crystal system (space group Bbmm). It is a member of the Rockbridgeite structural group.
Key properties: Mohs hardness 3.5–4.5; specific gravity 3.45; Vitreous to dull luster; optically biaxial.
IMA symbol Rkb, grandfathered status — known before the IMA’s founding, named in 1949.
Found in “limonite” iron ore deposits; an alteration product of primary phosphates in complex granite pegmatites.
It is found associated with Triphylite, huréaulite, barbosalite, roscherite, “limonite”.
The name refers to the occurrence in Rockbridge Co., Virginia, USA, taken as the type locality.