Kapundaite, (Na,Ca)[subscript 2]Fe[subscript 4][superscript 3+](PO[subscript 4])[subscript 4](OH)[subscript 3]·5H[subscript 2]O, a new phosphate species from Toms quarry, South Australia: Description and structural relationship to mélonjosephite

Abstract

Kapundaite, ideally (Na,Ca){sub 2}Fe{sub 4}{sup 3+}(PO{sub 4}){sub 4}(OH){sub 3}{center_dot}5H{sub 2}O, 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 {l_brace}100{r_brace}; 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 = 1.717(3), {beta} = 1.737(3), and {gamma} = 1.790(3). 2V could not be measured; 2V{sub calc} is 64.7{sup o}. The optical orientation is Z = b, Y {approx} 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{sub 1.13}Na{sub 0.95}){sub {Sigma}2.08}(Fe{sub 3.83}{sup 3+}Mn{sub 0.03}Al{sub 0.02}Mg{sub 0.01}){sub {Sigma}3.89}P{sub 3.92}O{sub 16}(OH){sub 3}{center_dot}5H{sub 2.11}O. Kapundaite is triclinic, space group P{sub {bar 1}}, a = 6.317(5), b = 7.698(6), c = 9.768(7) {angstrom}, {alpha} = 105.53(1){sup o}, {beta} = 99.24(2){sup o}, {gamma} = 90.09(2){sup o}, V = 451.2(6) {angstrom}{sup 3}, and Z = 1. The five strongest lines in the powder X-ray diffraction pattern are [d{sub obs} in {angstrom} (I) (hkl)]: 9.338 (100) (001), 2.753 (64) (2{sub {bar 1}}1), 5.173 (52) (011), 2.417 (48) ({sub {ovr 21}}3, 202, 0{sub {bar 1}}4), and 3.828 (45) (0{sub {bar 2}}1). The crystal structure was solved from single-crystal X-ray diffraction data using synchrotron radiation and refined to R{sub 1} = 0.1382 on the basis of 816 unique reflections with F{sub o} > 4{sub {sigma}}F. 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 melonjosephite. The mineral is named for the nearest town to the quarry.