Hylbrownite, Na3MgP3O10·12H2O, a new triphosphate mineral from the Dome Rock Mine, South Australia: description and crystal structure

Hylbrownite, ideally Na3MgP₃O10·12H₂O, the second known triphosphate mineral, is a new mineral species from the Dome Rock mine, Boolcoomatta Reserve, Olary Province, South Australia, Australia. The mineral forms aggregates and sprays of crystals up to 0.5 mm across with individual crystals up to 0.12 mm in length and 0.02 mm in width. Crystals are thin prismatic to acicular in habit and are elongate along [001]. Forms observed are {010}, {100}, {001}, {210} and {201}. Crystals are colourless to white, possess a white streak, are transparent, brittle, have a vitreous lustre and are non-fluorescent. The measured density is 1.81(4) g cm-³; Mohs' hardness was not determined. Cleavage is good parallel to {001} and to {100} and the fracture is uneven. Hylbrownite crystals are nonpleochroic, biaxial (-), with a = 1.390(4), β = 1.421(4), γ = 1.446(4) and 2V_calc = 82.2°. Hylbrownite is monoclinic, space group P2₁/n, with a = 14.722(3), b = 9.240(2), c = 15.052(3) Å, β = 90.01(3)°, V = 2047.5(7) Å, (single-crystal data) and Z = 4. The strongest lines in the powder X-ray diffraction pattern are [d(A)(I)(hkl)]: 10.530(60)(101̄,101), 7.357(80)(200), 6.951(100)(111̄, 111), 4.754(35)(103̄, 103), 3.934(40)(022), 3.510(45)(303̄, 303), 3.336(35)(411̄, 411). Chemical analysis by electron microprobe gave Na₂O 16.08, MgO 7.08, CaO 0.43, P₂O₅ 37.60, H₂O_calc 38.45, total 99.64 wt.%. The empirical formula, calculated on the basis of 22 oxygen atoms is Na2.93Mg₀.₉₉Ca_0.04P_2.99O _9.97·12.03H₂O. The crystal structure was solved from single-crystal X-ray diffraction data using synchrotron radiation (T= 123 K) and refined to R₁ = 4.50% on the basis of 2417 observed reflections with F₀ > 4σ(F₀). [Mg(H ₂O)3P₃O10] clusters link in the b direction to Naφ₆ octahedra, by face and corner sharing. Edge sharing Naφ₆ octahedra and Naφ₇ polyhedra form Na ₂O₉ groups which link via corners to form chains along the b direction. Chains link to [Mg(H₂O)₃P₃O1 ₀] clusters via corner-sharing in the c direction and form a thick sheet parallel to (100). Sheets are linked in the a direction via hydrogen bonds.