Angastonite, CaMgAl_2(PO_4)_2(OH)_4 centre dot 7H_2O: a new phosphate mineral from Angaston, South Australia

摘要

Angastonite, ideally CaMgAl_2(PO_4)_2(OH)_4 centre dot 7H_2O, is a newly defined mineral from the Penrice marble quarry, South Australia. The mineral occurs as snow-white crusts and coatings up to ~1 mm thick associated with minyulite, perhamite, crandallite and apatite-(CaF). The streak is white, the lustre is pearly and the estimated hardness is 2 on the Mohs scale. Angastonite forms platy crystals with the forms {010} (prominent), {101}, {101} and {100} (rare), and also occurs as replacements of an unknown pre-existing mineral. There is one cleavage direction on {010} and no twinning has been observed. Angastonite is triclinic, P1-bar, with a = 13.303(1) A, b = 27.020(2) A, c = 6.1070(7) A a = 89.64(1) deg, beta = 83.44(1) deg, gamma = 80.444(8) deg, V = 2150.5(4) A~3, with Z = 6. The mineral is optically biaxial (+), with refractive indices of alpha = 1.566(2), beta = 1.572(2) and gamma = 1.584(2) and with 2V_(meas) = 70(2) deg and 2V_(calc) = 71 deg. Orientation: X approx = a, Y approx = b, Z approx = c; with crystals showing parallel extinction and no axial dispersion. D_(meas) is 2.47 g/cm~3, whilst D_(calc) is 2.332 g/cm~3. The strongest four powder-diffraction lines [d in A, (I/I_o), hkl] are: 13.38, (100), 020; 11.05, (25), 11-bar0; 5.73, (23), 101, 230 and 111; 8.01, (21), 130. Angastonite is likely to be related to the montgomeryite-group members and have a similar crystal structure, based on slabs of phosphate tetrahedra and Al octahedra.