Guérinite Ca6(HAsO4)3(AsO4)2∙10.5H2O – a revision of the mineral formula

Abstract A single crystal of guérinite from the Plaka Mine, Lavrion, Greece, was examined by X-ray diffraction. The mineral crystallizes in the monoclinic space group P 2 1 / n with unit cell parameters a = 17.628 (2), b = 6.728 (1), c = 23.384 (3) Å, β = 90.68 (1)°, V = 2773.4 (6) Å 3 . The originally reported mineral formula Ca 5 (HAsO 4 ) 2 (AsO 4 ) 2 ∙9H 2 O, Z = 5 should be revised to Ca 6 (HAsO 4 ) 3 (AsO 4 ) 2 ∙10.5H 2 O, Z = 4. Accordingly, guérinite is no longer considered a polymorph to ferrarisite. The structure is characterized by complex layers consisting of corner- and edge sharing of CaO n ( n = 7, 8) polyhedra and arsenate groups. The connection between these units is achieved exclusively via hydrogen bonding.