Angastonite, CaMgAl(2)(PO(4))(2)(OH)(4)-7H(2)O: a new phosphate mineral from Angaston, South Australia

Stuart Mills, Lee Groat, Siobhan Wilson, William Birch, Pamela Whitfield, Mati Raudsepp

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Angastonite, ideally CaMgAl(2)(PO(4))(2)(OH)(4)center dot 7H(2)O, is a newly defined mineral from the Penrice marble quarry, South Australia. The mineral occurs as snow-white crusts and coatings Lip to similar to 1 mm thick associated with minyulite, perhainite, crandallite and apatitc-(CaF). The streak is white, the lustre is pearly and the estimated hardness is 2 on the Molls scale. Angastonite forms platy crystals with the forms 010 (prominent), ( 101), 11 101 and I 100 (rare), and also occurs as replacements of an unknown pre-existing mineral. There is one cleavage direction on 0101 and no twinning has been observed. Angastonite is triclinic, P (1) over bar, with a = 13.303(1) angstrom, b = 27.020(2) angstrom, c = 6.1070(7) angstrom alpha = 89.64(1)degrees, beta = 83.44(1)degrees, gamma = 80.444(8)degrees, V = 2150.5(4) angstrom(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)degrees and 2V(calc) = 71 degrees. Orientation: X approximate to a, Y approximate to b, Z approximate to c; with crystals showing parallel extinction and no axial dispersion. D(mcas) is 2.47 g/cm(3), whilst D(calc) is 2.332 g/cm(3). The strongest four powder diffraction lines [d in angstrom, (I/I(o)), hkI] are: 13.38, (100), 020; 11.05, (25), 1 (1) over 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.
Original languageEnglish
Pages (from-to)1011 - 1020
Number of pages10
JournalMineralogical Magazine
Issue number5
Publication statusPublished - 2008
Externally publishedYes

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