What is Johnbaumite?
Johnbaumite is a rare calcium arsenate mineral of the apatite supergroup. Its ideal end-member formula is Ca5(AsO4)3OH, although natural material can contain limited phosphate, fluorine, chlorine and lead substitutions. It was named for mineralogist John Leach Baum.
Mineralogy and chemistry
Johnbaumite is the hydroxyl-dominant arsenate apatite, related to svabite (fluorine-dominant) and turneaureite (chlorine-dominant). Chemical analysis matters because apatite-group compositions form substitutional series and visually similar material cannot be identified by colour alone.
Crystal structure
Johnbaumite is hexagonal, space group P63/m. The structure is built from columns of face-sharing calcium polyhedra linked by arsenate tetrahedra, with channels that host the apatite-group anions and cations. The representative model shown here is based on COD 9013633, a structure determination from the Harstigen mine, Värmland, Sweden.
Appearance and properties
Reported colours include white, colourless, yellow and green. It commonly occurs as prismatic, granular or massive material with vitreous to pearly lustre. Hardness is about 4.5–5 on the Mohs scale and specific gravity is approximately 3.6–3.8.
Formation and localities
Johnbaumite occurs in oxidised arsenic-bearing and phosphate-rich mineral environments. The Harstigen mine in Värmland, Sweden is the type locality for the modern structure study; additional occurrences are documented in arsenate-bearing districts including Germany, the United States and Mexico.
Collecting and care
Johnbaumite is a collector and reference mineral rather than a jewellery material. Because it contains arsenate, keep specimens labelled, avoid creating dust by cutting or grinding, wash hands after handling and keep it away from children, food and wellness use.
