Tomsquarryite, NaMgAl3(PO4)2(OH)6 ● 8H2O, a new crandallite-derivative mineral from Tom's phosphate quarry, Kapunda, South Australia

Abstract

Abstract. Tomsquarryite, NaMgAl3(PO4)2(OH)6 ⚫ 8H2O, is a new secondary phosphate mineral from Tom's phosphate quarry, Kapunda, South Australia. It occurs as colourless, talc-like hexagonal platelets, with diameters of a few tens of micrometres when formed from the decomposition of minyulite and as thicker (∼ 10 µm) hexagonal crystals when formed from alteration of gordonite. Associated minerals are penriceite, elliottite, minyulite, angastonite and wavellite. The calculated density is 2.22 g cm−3. Tomsquarryite crystals are uniaxial (+) with ω=1.490(3), ε=1.497(3) (white light). Dispersion was not observed. The partial orientation is Z≈c. Electron microprobe analyses of the holotype specimen give the empirical formula Na1.02K0.02Ca0.08Mg1.26Al2.86(PO4)2.00(OH)3.82F2.48 ⚫ 7.70H2O, based on 22 anions. Tomsquarryite belongs to the trigonal crystal system, space group R–3m, with hexagonal unit-cell parameters a=6.9865(5) Å, c=30.634(3) Å and V=1294.9(4) Å3 and with Z=3. The crystal structure was refined using single-crystal diffraction data; R1=0.069 for 303 reflections with I>2σ(I) to a resolution of 0.80 Å. The crystal structure is a derivative of the crandallite structure, with Ca2+ cations replaced by hydrated magnesium ions, [Mg(H2O)6]2+, resulting in an expansion of the interlayer separation from 5.4 Å in crandallite to 10.2 Å in tomsquarryite. The results for tomsquarryite are compared with those for the chemically and structurally related minerals penriceite and elliottite.