Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Anatoly V. Kasatkin, Igor V. Pekov, Radek Škoda, Nikita V. Chukanov, Fabrizio Nestola, Atali A. Agakhanov, Aleksey M. Kuznetsov, Natalia N. Koshlyakova, Jakub Plášil, Sergey N. Britvin

Fluorpyromorphite, Pb₅(PO₄)₃F, a new apatite-group mineral from Sukhovyaz Mountain, Southern Urals, and Tolbachik volcano, Kamchatka

Journal of Geosciences, volume 68 (2023), issue 1, 81 - 93

DOI: http://doi.org/10.3190/jgeosci.368

Fluorpyromorphite, ideally Pb₅(PO₄)₃F, a new apatite-group member, an F-dominant analog of pyromorphite and hydroxylpyromorphite. It is a supergene mineral found at two localities: Sukhovyaz Mountain, Ufaley District, Southern Urals (holotype) and Mountain 1004, Tolbachik volcano, Kamchatka (co-type), both in Russia. At Sukhovyaz, fluorpyromorphite forms anhedral grains up to 0.2 mm across (usually much smaller), filling cavities in quartz and sometimes partially replacing fluorapatite. Associated supergene minerals include pyromorphite, hydroxylpyromorphite, fluorphosphohedyphane, mimetite, and nickeltsumcorite. At Tolbachik, fluorpyromorphite occurs in the oxidation zone of paleo-fumarolic deposits in close association with pyromorphite, fluorphosphohedyphane, wulfenite, cerussite, munakataite, vanadinite, chrysocolla, and opal. It forms crude long-prismatic to acicular crystals up to 0.1 mm long and up to 5 μm thick combined in bunches and spherulites up to 0.2 mm. Fluorpyromorphite is colorless (Sukhovyaz) or yellow (Tolbachik), translucent to transparent and has a vitreous luster. It is brittle, with an uneven fracture and poor cleavage on (001). The calculated density values are 7.382 (holotype) and 6.831 (cotype) g/cm³. Fluorpyromorphite is optically uniaxial (-). In reflected light, it is light-grey, weakly anisotropic. The reflectance values (R_min/R_max, %) are: 15.8/16.6 (470 nm), 16.2/17.2 (546 nm), 15.9/16.9 (589 nm), 15.4/16.2 (650 nm). The chemical composition is (electron microprobe, wt. %; holotype/co-type): CaO 0.10/3.16, SrO 0.17/0.00, PbO 83.51/77.39, P₂O₅ 16.13/16.35, CrO₃ 0.00/0.49, SeO₃ 0.00/0.98, F 1.00/1.35, Cl 0.29/0.40, H₂O_calc 0.13/0.00, -O = (F,Cl) -0.49/-0.66, total 100.84/99.46. The empirical formulae based on 13 anions (O + F + Cl + OH) pfu are Pb_4.95Ca_0.02Sr_0.02P_3.00O₁₂F_0.70(OH)_0.19Cl_0.11 (holotype) and Pb_4.26Ca_0.69P_2.83Se⁶⁺_0.09Cr⁶⁺_0.06O_11.99F_0.87Cl_0.14 (co-type). Fluorpyromorphite is hexagonal, space group P6₃/m, unit-cell parameters (from powder X-ray diffraction data; holotype / co-type) are: a = 9.779(5) / 9.732(1), c = 7.241(9) / 7.242(1) Å, V = 599.6(7) / 594.0(2) ų, and Z = 2. The crystal structure was refined using the Rietveld method to R_p= 0.1764 (holotype). Fluorpyromorphite is isostructural with other members of the apatite group, a subdivision of the apatite supergroup.