Hatertite, ideally Na2(Ca, Na)(Fe3+, Cu)2(AsO4)3, was found in a fumarole of the North Breach of the Great fissure Tolbachik volcano eruption (1975–1976), Kamchatka Peninsula, Russia. The mineral occurs as individual, prismatic and tabular, honey-yellow crystals up to 0.3 mm across. It has a vitreous luster and yellow streak. Hatertite is monoclinic, C2/c, a = 12.590(2), b = 12.993(3), c =6.700(2) Å, β =113.72(2)°, V = 1003.4(3) Å3, Z =4, Dcalc = 4.06 g/cm3. The eight strongest lines of the powder X-ray diffraction pattern are [dobs in Å (I) (hkl)]: 6.493(25)(020); 3.628 (25)(1̄31); 3.204(39)( 1̄12,131); 3.065(18)(002); 2.976(28)(3̄12, 2̄22); 2.830(100)(240), 2.632(36)(1̄32); 1.647(19)(2̄04,640). Hatertite is optically positive, α = 1.820(3), β = 1.825(3), γ = 1.833(3), 2Vmeas. = 60(10)°, 2Vcalc. = 77°. The orientation is Y = b. The chemical composition determined by the electron-microprobe analysis is as follows (wt.%): Na2O 8.49, K2O 2.41, MnO 1.64, CaO 7.06, Fe2O3 11.15, ZnO 2.05, CuO 8.10, Al2O3 2.22, As2O5 55.67, total 98.79. The empirical formula (based on 12 O apfu) is (Na0.47K0.32)(Na0.84Ca0.16) (Ca0.62Na0.19Zn0.16Mn0.14) (Fe3+0.44Cu0.32Al0.13Na0.11)2(As1.01O4)3. A general crystal chemical formula for hatertite should be written as NaNa(Ca1−xM+x)(Fe3+1+xM2+1−x)(AsO4)3, where 0.5 > x >0, M+ is an unspecified monovalent cation, and M2+ is an unspecified divalent cation. The crystal structure was solved by direct methods and refined to an agreement index R1 = 0.028 on the basis of 751 independent observed reflections. Hatertite is a new arsenate member of the alluaudite group. Its structure is based upon chains of edge-sharing octahedra running along [−101] and linked by T(2)O4 tetrahedra into layers parallel to (010). The layers are further interlinked through T(1)O4 tetrahedra to form a three-dimensional octahedral-tetrahedral framework with the A(1) and A(2)’ sites in the interstices. The mineral was named in honor of Prof. Frédéric Hatert (b. 1973), University of Liège, Belgium, for his contributions to the mineralogy and crystal chemistry of alluaudite-group minerals.

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