Publication Date:
2013-05-09
Description:
Hylbrownite, ideally Na 3 MgP 3 O 10 ·12H 2 O, 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 α = 1.390(4), β = 1.421(4), = 1.446(4) and 2V calc. = 82.2°. Hylbrownite is monoclinic, space group P 2 1 / n , with a = 14.722(3), b = 9.240(2), c = 15.052(3) Å, β = 90.01(3)°, V = 2047.5(7) Å 3 , (single-crystal data) and Z = 4. The strongest lines in the powder X-ray diffraction pattern are [ d (Å)( I )( hkl )]: 10.530(60)(101I,101), 7.357(80)(200), 6.951(100)(111I, 111), 4.754(35)(103I, 103), 3.934(40)(022), 3.510(45)(303I, 303), 3.336(35)(411I, 411). Chemical analysis by electron microprobe gave Na 2 O 16.08, MgO 7.08, CaO 0.43, P 2 O 5 37.60, H 2 O calc 38.45, total 99.64 wt.%. The empirical formula, calculated on the basis of 22 oxygen atoms is Na 2.93 Mg 0.99 Ca 0.04 P 2.99 O 9.97 ·12.03H 2 O. 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( F 0 ). [Mg(H 2 O) 3 P 3 O 10 ] 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 Na 2 O 9 groups which link via corners to form chains along the b direction. Chains link to [Mg(H 2 O) 3 P 3 O 10 ] 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.
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
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