Liraite, ideally NaCa2Mn2[Fe3+Fe2+]Mn2(PO4)6(H2O)2, a new phosphate mineral of the wicksite group from the Ceferino Namuncurá pegmatite, Córdoba, Argentina

Abstract

ABSTRACT Liraite, ideally NaCa2Mn2[Fe3+Fe2+]Mn2(PO4)6(H2O)2, is a new mineral found in the Ceferino Namuncurá pegmatite, Pocho Department, Córdoba province, Argentina. It occurs in ellipsoidal nodules up to 20 cm in diameter in the intermediate zone of a Muscovite-Rare Element class pegmatite. Secondary phosphates, such as varulite, robertsite, fluorapatite, phosphosiderite, and Sr-rich metaswitzerite, together with minor quartz in veinlets, are associated minerals. Liraite is interpreted to have formed by reaction of phosphate minerals with Na-bearing hydrothermal fluids. It is dark brown with greenish hues (nearly black) in massive aggregates and dark olive green in translucent slices with a dark brownish green streak and a vitreous luster. It is brittle with an irregular fracture, one very good cleavage, and a good cleavage orthogonal to the very good cleavage. The Mohs hardness is 5, and the measured and calculated densities are 3.52(1) and 3.529(1) g/cm3, respectively. In transmitted light it is pleochroic X = Y = olive, Z = yellowish brown with X = Y > Z and optical orientation X = 2V(calc.) = 69.2°. The refractive indicies measured with monochromatic light (λ = 589 nm) are α = 1.732 (3), β = 1.739 (3), γ = 1.754 (3). Liraite is orthorhombic (Pcab) and has unit-cell parameters a = 12.608(6) Å, b = 12.918(6) Å, c = 11.737(4) Å, V = 1911.6(14) Å3, Z = 4. The six strongest reflections in the X-ray powder diffraction pattern are as follows: [d in Å, (I), (hkl)] 2.7452, 100, (421); 2.8563, 65, (014); 2.9266, 49, (004); 2.7061, 30, (412); 2.0966, 29, (334); 2.7693, 26, (402). The crystal structure was refined to an R index of 1.94% based on 2910 observed (>4σF) reflections measured with MoKα X-radiation. Chemical analysis by electron microprobe of the structure crystal (holotype specimen) gave Na2O 1.58, FeO 5.29, Fe2O3 11.45, CaO 10.52, MgO 0.77, MnO 24.00, P2O5 41.55, SrO 0.72, ZnO 0.19, H2O (calc.) 3.50, total 99.57 wt.% where water was calculated from the crystal-structure analysis and the Fe3+/Fe2+ ratio was determined by charge balance. The empirical formula calculated on the basis of 26 oxygen atoms is (Na0.53□0.47)Σ1.00(Ca1.93Sr0.07)Σ2.00(Fe3+1.48Fe2+0.76Mn3.48Mg0.20Zn0.02)Σ5.94P6.02O24(H2O)2, ideally NaCa2M(1)Mn2M(2)[Fe3+Fe+2]M(3)Mn2(PO4)6(H2O)2. The Gladstone-Dale relation gives a compatibility index of 1 – (KP/KC) = 0.010 (superior). This new member of the wicksite group is Mn-rich, and, like bederite, has Mn dominant at the M(1) and M(3) sites. However, the Na site in liraite is Na-dominant with M(2)[Fe3+Fe2+], whereas bederite is □-dominant with M(2)Fe3+2. Liraite has a very low MgO content, and even with all available Mg assigned to the M(2) site, Fe2+ > Mg at M(2). Consequently, liraite is the first wicksite-group mineral with endmember M(2) composition [Fe3+Fe2+].