Description and crystal structure of a new mineral - plimerite, ZnFe43+(PO₄)₃(OH)₅ - the Zn-analogue of rockbridgeite and frondelite, from Broken Hill, New South Wales, Australia

Abstract

<jats:p>Plimerite, ideally Zn<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00005168_inline01.png" /> (PO<jats:sub>4</jats:sub>)<jats:sub>3</jats:sub>(OH)5, is a new mineral from the Block 14 Opencut, Broken Hill, New SouthWales. 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 3.5–4, D(meas.) = 3.67(5) g/cm<jats:sup>3</jats:sup> and D(calc.) = 3.62 g/cm<jats:sup>3</jats:sup> (for the empirical formula). Optically, it is biaxial negative with α = 1.756(5), β = 1.764(4), γ = 1.767(4) and 2V(calc.) of –63º; pleochroism is X pale-greenish-brown, Y pale-brown, Z pale-bluish-green; absorption Z &gt; X &gt; 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<jats:sup>3</jats:sup> (single-crystal data) and Z = 4. Strongest lines in the X-ray powder diffraction pattern are [d (A˚ ), I, hkl]: 4.638, (50), (111); 3.388, (50), (041); 3.369, (55), (131); 3.168, (100), (132); 2.753, (60), (115); 2.575, (90), (200); 2.414, (75), (220); 2.400, (50), (221); 1.957, (40), (225). Electron microprobe analysis yielded (wt.%): PbO 0.36, CaO 0.17, ZnO 20.17, MnO 0.02, Fe<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> 29.82, FeO 2.98, Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> 4.48, P<jats:sub>2</jats:sub>O<jats:sub>5</jats:sub> 32.37, As<jats:sub>2</jats:sub>O<jats:sub>5</jats:sub> 0.09, H<jats:sub>2</jats:sub>O (calc) 6.84, total 97.30 (Fe<jats:sup>3+</jats:sup>/Fe<jats:sup>2+</jats:sup> ratio determined by Mössbauer spectroscopy). The empirical formula calculated on the basis of 17 oxygens is Ca<jats:sub>0.02</jats:sub>Pb<jats:sub>0.01</jats:sub>Zn<jats:sub>1.68</jats:sub><jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00005168_inline02.png" />Al<jats:sub>0.60</jats:sub>P<jats:sub>3.09</jats:sub>As<jats:sub>0.01</jats:sub>O<jats:sub>17.00</jats:sub>H<jats:sub>5.15</jats:sub>. 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-Kα radiation, CCD area detector). The structure of plimerite is isotypic with that of rockbridgeite and is based on face-sharing trimers of (Mϕ6) octahedra which link by sharing edges to form chains, that extend in the b-direction. Chains link to clusters comprising pairs of corner-sharing (Mϕ6) octahedra that link to PO<jats:sub>4</jats:sub> 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.</jats:p>