Manganrockbridgeite, Mn2+2Fe3+3(PO4)3(OH)4(H2O), a new member of the rockbridgeite group, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria

Abstract

Abstract. Manganrockbridgeite, Mn22+Fe33+(PO4)3(OH)4(H2O), is a new member of the rockbridgeite group, from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria. It occurs in association with frondelite, kenngottite, hureaulite and hematite. It forms compact intergrowths and clusters of shiny greenish black blades up to 200 µm long and 20 µm wide but only a few micrometres thick. The crystals are elongated on [100] and flattened on {001}, with perfect cleavage parallel to {001}. Individual thin blades are green in transmitted light and red under crossed polars. The calculated density is 3.40 g cm−3. Manganrockbridgeite is biaxial (+/-), with α= 1.795(5), β= 1.805(calc), γ=1.815(5) (white light) and 2V(meas.) = 90(2)∘. The empirical formula from electron microprobe analyses, Mössbauer spectroscopy and crystal structure refinement is (Mn1.072+Fe0.692+Fe0.163+)Σ1.92(Fe3+)2.88(PO4)3(OH)3.64(H2O)1.44. Manganrockbridgeite has monoclinic symmetry with space group P21/m and unit-cell parameters a=5.198(2), b=16.944(5), c=7.451(3) Å, β=110.170(9)∘, V=616.0(4) Å3 and Z=2. The crystal structure was refined using both laboratory and synchrotron single-crystal diffraction data. Whereas other rockbridgeite-group minerals have orthorhombic symmetry with a statistical distribution of 50 % Fe3+ / 50 % vacancies in M3-site octahedra forming face-shared chains along the 5.2 Å axis, monoclinic manganrockbridgeite has full ordering of Fe3+ and vacancies in alternate M3 sites along the 5.2 Å axis.