The crystal structure of johnbaumite, ideally Ca5(AsO4)3OH, was determined and refined using crystals from the Harstigen mine, Värmland, Sweden, on the basis of X-ray diffraction data. The structure was refined to R1 = 0.038 in space group P63/m, with unit-cell parameters a = 9.7242(2), c = 6.9657(9) Å, and V = 570.43(2) Å3. The chemical composition of the sample, determined by electron-microprobe analysis, is (Ca5.02Pb0.03)∑5.05[As2.91P0.04Si0.02]∑2.97O12(OH0.97Cl0.03).

Johnbaumite belongs to the calcium arsenate apatites, together with svabite, Ca5(AsO4)3F, and turneaureite, Ca5(AsO4)3Cl. Johnbaumite is topologically similar to the other members of the apatite supergroup: columns of face-sharing M1 polyhedra run along c and are connected through TO4 tetrahedra, giving rise to a zeolite-like microporous framework, with channels hosting M2 cations and X anions.

This is the first published single-crystal structure study of johnbaumite and the first report of this calcium arsenate apatite from the Harstigen mine. Actually, johnbaumite was identified at this locality at the end of the 19th Century but it was described as svabite.

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