Tassieite (IMA 2005–051), with an end-member formula NaCa 2 (Mg 2 )(Fe 3+ Mg) ∑ 2 (Fe 2+ ) 2 (PO 4 ) 6 ·2H 2 O, is a newly recognized Mg-dominant phosphate of the wicksite group. A representative composition derived with an electron microprobe is SiO 2 0.01, P 2 O 5 44.54, SO 3 0.06, MgO 10.95, MnO 0.38, FeO 25.40 (meas.), FeO 14.93 (calc.), Fe 2 O 3 11.63 (calc.), Na 2 O 1.96, CaO 11.56, SrO 0.02, Y 2 O 3 0.26, Ce 2 O 3 0.08, Yb 2 O 3 0.13, UO 2 0.04, F 0.04, H 2 O 3.78 (calc.), sum 100.34 wt% (excluding F), which gives Na 0.60 Ca 1.96 Mg 2.59 Mn 0.05 Fe 2+ 1.98 Fe 3+ 1.39 Y 0.02 Yb 0.01 S 0.01 P 5.98 O 24 ·2H 2 O for 14 cations excluding Na, and 24 O; the Fe 2+ : Fe 3+ ratio is calculated from stoichiometry, and the H 2 O, 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 + Fe 2+ ) (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) A, V 1842.1(3) A 3 , calculated density 3.45 g/cm 3 , Z = 4. The mineral is isostructural with wicksite, but with the M 1 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 A( 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 +, α 1.712(2), β 1.713(2), γ 1.722(2) (589 nm), 2 V (meas.) 46(1)°, 2 V (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, melonjosephite 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.