| Popis: |
Angastonite, ideally CaMgAl₂(PO₄)₂(OH)₄·7H₂O, is a newly defined mineral from the Penrice marble quarry, South Australia, Australia. The mineral occurs as snow-white crusts and coatings up to about 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¯ , with a = 13.303(1) Å, b = 27.020(2) Å, c = 6.1070(7) Å α = 89.64(1)º, β = 83.44(1)º, γ = 80.444(8)º, V = 2150.5(4) ų, 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)º and 2Vcalc = 71º. Orientation: X˜ a, Y˜ b, Z˜ c; with crystals showing parallel extinction and no axial dispersion. Dmeas is 2.47 g/cm³, whilstD calc is 2.332 g/cm³. The strongest four powder-diffraction lines [d in Å, (I/Io), hkl] are: 13.38, (100), 020; 11.05, (25), 11¯0; 5.73, (23), 101, 230 and 111; 8.01, (21), 130. Angastonite is likely to be related to montgomeryite-group members and have a similar crystal structure, based on slabs of phosphate tetrahedra and Al octahedra. |
