Reaphookhillite, MgZn2(PO4)2⋅4H2O, the Mg analogue of parahopeite from Reaphook Hill, South Australia

ABSTRACT Reaphookhillite, ideally MgZn 2 (PO 4 ) 2 ⋅4H 2 O, is a new phosphate mineral from Reaphook Hill, Flinders Ranges, South Australia, Australia. Reaphookhillite occurs as colourless, bladed to thin tabular crystals to 0.6 mm across. Cleavage is perfect parallel to {010}. The mineral occur as overgrowths on parahopeite crystals and is associated with scholzite, leucophosphite and chalcophanite. The calculated density is 3.09 g/cm 3 from the empirical formula. Reaphookhillite is optically biaxial (+), α = 1.583(3), β = 1.596(3), γ = 1.611(3) and 2V calc = 88.7°. Electron microprobe analyses gave ZnO 41.57, MgO 7.96, MnO 0.40, P 2 O 5 33.72, H 2 O (calc) 16.92, total 100.57 wt.%. The empirical formula, based on 12 O apfu, is Mg 0.83 Zn 2.16 Mn 2+ 0.02 (PO 4 ) 2.01 ⋅3.97H 2 O. Reaphookhillite is triclinic, P ${\bar 1}$ , with the unit-cell parameters of a = 5.7588(12), b = 7.5341(15) c = 5.2786(11) Å, α = 93.44(3), β = 91.27(3), γ = 91.30(3)°, V = 228.49(8) Å 3 and Z = 1. The strongest eight lines in the powder X-ray diffraction pattern are [ d obs in Å ( I ) ( hkl )] 7.577 (100) (010); 4.461 (24) (01 ${\bar 1}$ ); 4.461 (24) (01 ${\bar 1}$ ); 3.771 (14) (020); 3.158 (13) (02 ${\bar 1}$ ); 2.982 (32) (021); 2.880 (27) (200); 2.775 (14) (1 ${\bar 2}$ 1, 12 ${\bar 1}$ ); and 2.668 (13) (1 ${\bar 2}{\bar 1}$ , 210). Reaphookhillite is isostructural with parahopeite, with Mg replacing Zn in the 6-coordinated site in the structure. The structure contains ZnO 4 and PO 4 tetrahedra which share corners to form a sheet in the (001) plane. Sheets are linked in the c direction by corner sharing MgO 2 (H 2 O) 4 octahedra.