Description and crystal structure of a new mineral – plimerite, ZnFe³⁺₄(PO₄)₃(OH)5 – the Zn-analogue of rockbridgeite and frondelite, from Broken Hill, New South Wales, Australia

Abstract

Plimerite, ideally ZnFe 3+ 4 (PO 4 ) 3 (OH) 5 , is a new mineral from the Block 14 Opencut, Broken Hill, New South Wales. It occurs as pale-green to dark-olive-green, almost black, acicular to prismatic and bladed crystals up to 0.5 mm long and as hemispherical aggregates of radiating acicular crystals up to 3 mm across. Crystals are elongated along [001] and the principal form observed is {100} with minor {010} and {001}. The mineral is associated with hinsdalite-plumbogummite, pyromorphite, libethenite, brochantite, malachite, tsumebite and strengite. Plimerite is translucent with a pale-greyish-green streak and a vitreous lustre. It shows an excellent cleavage parallel to {100} and {010} and distinct cleavage parallel to {001}. It is brittle, has an uneven fracture, a Mohs' hardness of 3.5À4, D(meas.) = 3.67(5) g/cm 3 and D(calc.) = 3.62 g/cm 3 (for the empirical formula). Optically, it is biaxial negative with a = 1.756(5), b = 1.764(4), g = 1.767(4) and 2V(calc.) of À63º; pleochroism is X pale-greenish-brown, Y pale-brown, Z pale-bluish-green; absorption Z > X > Y; optical orientation XYZ = cab. Plimerite is orthorhombic, space group Bbmm, unit-cell parameters: a = 13.865 (3) Å , b = 16.798(3) Å , c = 5.151(10) Å , V = 1187.0(4) Å 3 (single-crystal data) and Z = 4. Strongest lines in the X-ray powder diffraction pattern are [d (Å ), I, hkl]: . Electron microprobe analysis yielded (wt.%): PbO 0.36, CaO 0.17, ZnO 20.17, MnO 0.02, Fe 2 O 3 29.82, FeO 2.98, Al 2 O 3 4.48, P 2 O 5 32.37, As 2 O 5 0.09, H 2 O (calc) 6.84, total 97.30 (Fe 3+ /Fe 2+ ratio determined by Mössbauer spectroscopy). The empirical formula calculated on the basis of 17 oxygens is Ca 0.02 Pb 0.01 Zn 1.68 Fe 2+ 0.28 Fe 3+ 2.53 Al 0.60 P 3.09 As 0.01 O 17.00 H 5.15. The crystal structure was solved by direct methods and refined to an R1 index of 6.41% for 1332 observed reflections from single-crystal X-ray diffraction data (Mo-Ka radiation, CCD area detector). The structure of plimerite is isotypic with that of rockbridgeite and is based on face-sharing trimers of (Mf 6 ) octahedra which link by sharing edges to form chains, that extend in the b-direction. Chains link to clusters comprising pairs of corner-sharing (Mf 6 ) octahedra that link to PO 4 tetrahedra forming sheets parallel to (001). The sheets link via octahedra and tetrahedra corners into a heteropolyhedral framework structure. The mineral name honours Professor Ian Plimer for his contributions to the study of the geology of ore deposits. *

Plimerite from Krásno near Horní Slavkov ore district, Czech Republic

“…The quantitative chemical analyses of five morphologically different types of plimerite from Krásno are given in Tabs 1 and 2, together with data published by Elliott et al (2009). All chemical analyses were recalculated on the basis of (P + As + Si) = 3 apfu.…”

“…All chemical analyses were recalculated on the basis of (P + As + Si) = 3 apfu. On the basis of new structural investigation and reinterpretation of structural data published by Elliott et al (2009) (Fig. 9).…”

“…Empirical formulae were calculated on the basis of (P + As + Si) = 3; H 2 O contents were obtained from the charge balance; FeO/Fe 2 O 3 were estimated from the occupancy M1 + M3 by Fe 3+ and M2 only by Fe 2+ . 1* Broken Hill (G32005), 2* Broken Hill (G32405), 3* Broken Hill (G32401), 4* Reaphook Hill, all data of Elliott et al (2009) beam diameter of 5-10 μm. In order to minimize peak overlaps, the following analytic lines and crystals were selected: K α lines: F (PC1, fluorapatite/topaz), Mg (TAP, forsterite), Na (TAP, albite), Al (TAP, sanidine), As (TAP, InAs), Si (TAP, sanidine), Cu (TAP, dioptase), K (PET, sanidine), P (PET, fluorapatite) Ca (PET, andradite), S (PET, barite), Ti (PET, TiO), Cl (PET, vanadinite), Fe (LIF, andradite), Mn (LIF, rhodonite), Zn (LIF, ZnO); L α lines: Sr (PET, SrSO 4 ); L β lines: Ba (PET, benitoite); M α lines: Pb (PET, vanadinite).…”

“…Finds of larger and better quality crystals of this mineral phase at the Block 14 Opencut (Broken Hill), provided suitable material for collection of the single-crystal Xray data and a more complete characterization of this mineral. Consequently, the mineral was approved with the name plimerite by the IMA Commission on New Minerals, Nomenclature and Classification (Elliott et al 2009). The study of Elliott et al (2009) confirmed that Zn-rich rockbridgeite from the Reaphook Hill, Flinders Range, South Australia (Johnson 1978) is also plimerite.…”

“…Consequently, the mineral was approved with the name plimerite by the IMA Commission on New Minerals, Nomenclature and Classification (Elliott et al 2009). The study of Elliott et al (2009) confirmed that Zn-rich rockbridgeite from the Reaphook Hill, Flinders Range, South Australia (Johnson 1978) is also plimerite. The aim of this paper is to describe of plimerite from Krásno and to clarify the role of Zn and possible content of molecular water in this mineral phase.…”