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

Abstract

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 nonfluorescent. 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 α = 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) ų, (single-crystal data) and Z = 4. The strongest lines in the powder X-ray diffraction pattern are [d (Å)(I)(hkl)]: 10.530(60)(101,101), 7.357(80)(200), 6.951(100)(111, 111), 4.754(35)(103, 103), 3.934(40)(022), 3.510(45)(303, 303), 3.336(35)(411, 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 Na₂.₉₃Mg₀.₉₉Ca₀.₀₄P₂.₉₉O₉.₉₇·12.03H₂O. The crystal structure was solved from single-crystal X-ray diffraction data using synchrotron radiation (T = 123 K) and refined to R ₁ = 4.50% on the basis of 2417 observed reflections with F ₀ > 4 σ(F ₀). [Mg(H₂O)₃P₃O₁₀] clusters link in the b direction to Na₆ octahedra, by face and corner sharing. Edge sharing Na₆ Octahedra and Na₇ polyhedra form Na₂O₉ groups which link via corners to form chains along the bdirection. Chains link to [Mg(H₂O)₃P₃O₁₀] 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.