Kapundaite, (Na,Ca)2Fe43+(PO4)4(OH)3{middle dot}5H2O, a new phosphate species from Toms quarry, South Australia: Description and structural relationship to melonjosephite

Abstract

Kapundaite, ideally (Na,Ca)₂Fe₄³⁺(PO₄)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; 2V_calc 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 (Ca_1.13Na_0.95)_∑2.08(Fe_3.83³⁺Mn_0.03Al_0.02Mg_0.01)_∑3.89P_3.92O₁₆(OH)₃·5H_2.11O. Kapundaite is triclinic, space group P1̅, 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) ų, and Z = 1. The five strongest lines in the powder X-ray diffraction pattern are [d_obs in Å (I) (hkl)]: 9.338 (100) (001), 2.753 (64) (21̅1), 5.173 (52) (011), 2.417 (48) (2̅1̅3, 202, 01̅4), and 3.828 (45) (02̅1). 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 F_o > 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.