Goryainovite, Ca₂PO₄Cl, a new mineral from the Stora Sahavaara iron ore deposit (Norrbotten, Sweden)

Abstract

Goryainovite, Ca₂PO₄Cl, is a new halophosphate, the chlorine analogue of ‘spodiosite’, Ca₂PO₄F. It is orthorhombic, Pbcm, a = 6.215(2), b = 7.011(2), c = 10.788(3) Å, V = 470.0(8) ų, Z = 4 (from powder X-ray diffraction data). The mineral is found in a magnetite-serpentine rock of the Stora Sahavaara iron ore deposit (67.408°N 23.297°E) where it forms small (up to 20 μm in diameter) rounded inclusions in magnetite, in close association with hydroxylapatite–chlorapatite, pyrrhotite, pyrite, chalcopyrite, valleriite and thorianite. Goryainovite is a transparent, colourless mineral with a vitreous lustre and a white streak. Cleavage is not observed, and the fracture is conchoidal. The Mohs hardness is c. 4. In transmitted light, the mineral is colourless, biaxial (–), β ≈ 1.66 (for λ = 589 nm). D_calc = 2.98 g·cm⁻³. The mean chemical composition specified with electron microprobe is (wt.%): P₂O₅ 33.19, Cl 16.96, CaO 53.25, O = Cl –3.83, total 99.57. The empirical formula calculated on the basis of 3 cations per molecule is Ca_2.01[P_0.99O_3.98]Cl_1.01. The simplified formula is Ca₂PO₄Cl. The strongest X-ray powder diffraction lines [d in Å, (I), (hkl)] are 2.845(90)(113), 2.746(100)(211), 2.333(25)(114), 2.028(15)(132), 1.9569 (30)(115), 1.8370 (20)(025). The Raman spectrum of goryainovite includes 10 bands of -group vibrations. Goryainovite represents probably an early-formed phosphate of the magnetite-serpentine rock and crystallized together with magnetite. When chlorine fugacity decreased, almost all goryainovite was transformed into chlorapatite–hydroxylapatite, and only grains isolated in magnetite remained unaltered. The mineral is named in honour of Prof. Pavel M. Goryainov (b. 1937) for his contribution to the knowledge of the geology and petrology of banded iron formation of the north-eastern part of the Fennoscandian Shield.

Keywords:

• Goryainovite
• new mineral
• halophosphate
• Stora Sahavaara iron ore deposit
• Sweden

Acknowledgements

We are grateful to Dr V. Benes from Northland Resources Inc, for materials and helpful discussions. We are also grateful to N.L. Mikhailova and V.J. Kuznetsov (Kola Science Centre, Russian Academy of Sciences), who helped us with the DSC and XRD analyses of synthetic Sr₂PO₄Cl. Raman and XRD analyses of goryainovite were carried out using resources of the X-ray Diffraction and Geo Environmental centres of St. Petersburg State University. The detailed comments by E. Jonsson, M. Ripa and an anonymous reviewer helped to significantly improve this paper.