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TASSIEITE, (Na,)Ca₂(Mg,Fe²⁺,Fe³⁺)₂(Fe³⁺,Mg)₂(Fe²⁺,Mg)₂(PO₄)₆·2H₂O, A NEW HYDROTHERMAL WICKSITE-GROUP MINERAL IN FLUORAPATITE NODULES FROM GRANULITE-FACIES PARAGNEISS IN THE LARSEMANN HILLS, PRYDZ BAY, EAST ANTARCTICA

Edward S. Grew^1,, Thomas Armbruster², Olaf Medenbach³, Martin G. Yates⁴ and Christopher J. Carson⁵

¹ Department of Earth Sciences, University of Maine, 5790 Bryand Research Center, Orono, Maine 04469–5790, USA
² Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, Freiestrasse 3, CH–3012 Bern, Switzerland
³ Institut für Geowissenschaften/Mineralogie, Ruhr-Universität Bochum, D–44780 Bochum, Germany
⁴ Department of Earth Sciences, University of Maine, 5790 Bryand Research Center, Orono, Maine 04469–5790, USA
⁵ Research School of Earth Science, Australian National University, Canberra ACT 0200, Australia

E-mail address: esgrew{at}maine.edu

Tassieite (IMA 2005–051), with an end-member formula NaCa₂(Mg₂)(Fe³⁺Mg) ₂(Fe²⁺)₂(PO₄)₆·2H₂O, is a newly recognized Mg-dominant phosphate of the wicksite group. A representative composition derived with an electron microprobe is SiO₂ 0.01, P₂O₅ 44.54, SO₃ 0.06, MgO 10.95, MnO 0.38, FeO 25.40 (meas.), FeO 14.93 (calc.), Fe₂O₃ 11.63 (calc.), Na₂O 1.96, CaO 11.56, SrO 0.02, Y₂O₃ 0.26, Ce₂O₃ 0.08, Yb₂O₃ 0.13, UO₂ 0.04, F 0.04, H₂O 3.78 (calc.), sum 100.34 wt% (excluding F), which gives Na_0.60Ca_1.96Mg_2.59Mn_0.05Fe²⁺_1.98Fe³⁺_1.39Y_0.02Yb_0.01S_0.01P_5.98O₂₄·2H₂O for 14 cations excluding Na, and 24 O; the Fe²⁺: Fe³⁺ ratio is calculated from stoichiometry, and the H₂O, from ideal content. Overall, the analyses of all grains gave Na in the range 0.46–0.97 atoms per formula unit, and X_Mg = Mg/(Mg + Fe²⁺) (atom ratio) is in the range 0.45–0.77 (for tassieite: X_Mg > 0.5). Single-crystal X-ray diffraction gives an orthorhombic symmetry, Pbca, a 12.4595(7), b 11.5955(16), c 12.7504(7) Å, V 1842.1(3) Å₃, calculated density 3.45 g/cm³, Z = 4. The mineral is isostructural with wicksite, but with the M1 site dominated by Mg. Mg is the dominant divalent octahedrally coordinated cation in the structure, which is our rationale for recognizing tassieite as a distinct species. Indexed lines in the powder pattern [d in Å(I)(hkl)] are 6.40(5)(002), 3.497(40)(302), 3.000(80)(114), 2.895(80)(040), 2.735(100)(420,412), 2.545(10)(224) and 2.091(30)(106). The mineral is optically biaxial +, 1.712(2), ß 1.713(2), 1.722(2) (589 nm), 2V (meas.) 46(1)°, 2V (calc.) 37°. Pleochroism: X dark blue, Y blue, Z light brown; absorption: X > Y >> Z. Tassieite occurs in bands of secondary fluorapatite or in pseudomorphs of stornesite-(Y) within a fluorapatite nodule in a paragneiss specimen from between Johnston Fjord and Tassie Tarn (whence the name), Stornes Peninsula, Larsemann Hills, in Antarctica. Associated minerals are stornesite-(Y), wagnerite, xenotime-(Y), monazite-(Ce), pyrite, mélonjosephite and several unidentified Ca ± Na – Mg – Fe phosphates. Larger grains of tassieite (0.5–1 mm) show crystal faces and cleavage traces, but most grains (up to 0.3 mm) are platy and anhedral or irregular in outline. Tassieite seems to have formed hydrothermally from the alteration of stornesite-(Y) and wagnerite.

Keywords: phosphate, new mineral species, electron microprobe, crystal structure, Larsemann Hills, Antarctica.