Kapundaite, (Na, Ca)2Fe4 3+(PO 4)4(OH)3·5H2O, a new phosphate species from Toms quarry, South Australia: Description and structural relationship to mélonjosephite

Stuart Mills, W Birch, Anthony Kampf, Andrew Christy, Joseph Pluth, Allan Pring, Mati Raudsepp, Yu-Sheng Chen

    Research output: Contribution to journalArticle

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    Abstract

    Kapundaite, ideally (Na, Ca)2Fe43+(PO 4)4(OH)3·5H2O, is a new mineral (IMA2009-047) from Toms phosphate quarry, Kapunda, South Australia, Australia. The new mineral occurs as cavernous aggregates of fibers up to several centimeters across, associated with leucophosphite, natrodufrenite, and meurigite-Na crystals and amorphous brown, black, and/or greenish coatings. Individual kapundaite crystals are very thin flattened fibers up to a few millimeters in length, but typically no more than a few micrometers in thickness. The main form observed is {100}; other forms in the [010] zone are present, but cannot be measured. Crystals of kapundaite are pale to golden yellow, transparent to translucent, have a yellow streak and silky luster, and are non-fluorescent. Mohs hardness is estimated to be about 3; no twinning or cleavage was observed. Kapundaite is biaxial (+), with indices of refraction α = 1.717(3), β = 1.737(3), and γ = 1.790(3). 2V could not be measured; 2Vcalc is 64.7°. The optical orientation is Z = b, Y ≈ c with weak pleochroism: X = nearly colorless, Y = light brown, Z = pale brown; absorption: Y > Z > X. No dispersion was observed. The empirical chemical formula (mean of seven electron microprobe analyses) calculated on the basis of 24 O is (Ca1.13Na0.95) Σ2.08(Fe3+3.83 Mn0.03Al 0.02Mg0.01)Σ3.89P3.92O 16(OH)3·5H2.11O. Kapundaite is triclinic, space group P1̄, a = 6.317(5), b = 7.698(6), c = 9.768(7) Å, α = 105.53(1)°, β = 99.24(2)°, γ = 90.09(2)°, V = 451.2(6) Å3, and Z = 1. The five strongest lines in the powder X-ray diffraction pattern are [dobs in Å (I) (hkl)]: 9.338 (100) (001), 2.753 (64) (21̄1), 5.173 (52) (011), 2.417 (48) (2 13, 202, 014), and 3.828 (45) (02̄1). The crystal structure was solved from single-crystal X-ray diffraction data using synchrotron radiation and refined to R1 = 0.1382 on the basis of 816 unique reflections with Fo > 4σF. The structure of kapundaite is based on a unique corrugated octahedral-tetrahedral sheet, which is composed of two types of chains parallel to a. Kapundaite is structurally related to mélonjosephite. The mineral is named for the nearest town to the quarry.

    Original languageEnglish
    Pages (from-to)754-760
    Number of pages7
    JournalAmerican Mineralogist
    Volume95
    Issue number5-6
    DOIs
    Publication statusPublished - May 2010

    Keywords

    • Crystal structure
    • Kapundaite
    • Mélonjosephite
    • New mineral
    • Octahedral-tetrahedral sheet
    • Phosphate
    • Toms phosphate quarry

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