Brown single crystal aggregate of the extremely rare K-Mn-Nb phosphate mineral johnwalkite from the type and only known locality worldwide in South Dakota (Champion Mine, Keystone, Pennington County, South Dakota, United States of America). Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Johnwalkite
Johnwalkite is an IMA-recognized mineral species in the phosphate class, with the chemical formula K(Mn²⁺,Fe³⁺)₂(Nb,Ta)O₂(PO₄)₂·2(H₂O,OH).
It crystallizes in the orthorhombic crystal system (space group Pb2_1m).
Key properties: Medium greenish yellow streak; Vitreous luster; optically biaxial.
IMA symbol Jwk, IMA status: approved, named in 1985.
A rare secondary mineral derived from the dissolution of triphylite-lithiophilite and columbite-tantalite in a complex granite pegmatite.
It is found associated with Fluorapatite, childrenite-eosphorite, rockbridgeite-frondelite, hureaulite, fairfieldite, ludlamite, jahnsite-whiteite, goethite, manganese oxides, quartz.
The mineral was named to honor Richard JOHNson (1936-1998) and Frank WALKup (1943-1993), mineral preparators, National Museum of Natural History, Washington, D.C., USA.