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Angastonite, CaMgAl2(PO4)2(OH)4·7H2O: a new phosphate mineral from Angaston, South Australia

Published online by Cambridge University Press:  05 July 2018

S. J. Mills ,
L. A. Groat ,
S. A. Wilson ,
W. D. Birch ,
P. S. Whitfield  and
M. Raudsepp
S. J. Mills*
Affiliation:
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver BC, Canada V6T 1Z4
L. A. Groat
Affiliation:
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver BC, Canada V6T 1Z4
S. A. Wilson
Affiliation:
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver BC, Canada V6T 1Z4
W. D. Birch
Affiliation:
Geosciences, Museum Victoria, GPO Box 666, Melbourne Victoria, 3001, Australia
P. S. Whitfield
Affiliation:
Institute for Chemical Process and Environmental Technology, National Research Council of Canada, Montreal Road, Ottawa ON, Canada K1A 0R6
M. Raudsepp
Affiliation:
Department of Earth and Ocean Sciences, University of British Columbia, Vancouver BC, Canada V6T 1Z4
*
*E-mail: smills@eos.ubc.ca
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Abstract

Angastonite, ideally CaMgAl2(PO4)2(OH)4-7H2O, 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, P1̄, with a = 13.303(1) Å, b = 27.020(2) Å, c = 6.1070(7) Å α = 89.64(1)°, β = 83.44(1)°, γ = 80.444(8)°, V = 2150.5(4) Å3, with Z = 6. The mineral is optically biaxial (+), with refractive indices of α = 1.566(2), β = 1.572(2) and γ 1.584(2) and with 2Vmeas = 70(2)° and 2Vcalc = 71°. Orientation: X ≈ a, Y ≈ b, Z ≈ c; with crystals showing parallel extinction and no axial dispersion. Dmeas is 2.47 g/cm3, whilst Dcalc is 2.332 g/cm3. The strongest four powder-diffraction lines [d in Å, (I/I°), hkl] are: 13.38, (100), 020; 11.05, (25), 11̄0; 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.

Keywords

angastonitenew mineralPenrice marble quarryAngastonSouth Australiamontgomeryiteperhamite
Type
Research Article
Information
Mineralogical Magazine , Volume 72 , Issue 5 , October 2008 , pp. 1011 - 1020
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2008

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