Wernerkrauseite, CaFe3+ 2Mn4+O6: the first nonstoichiometric post-spinel mineral, from Bellerberg volcano, Eifel, Germany

Abstract

Black prismatic crystals of the new mineral wernerkrauseite, ideally CaFe3+2Mn4+O6 [ Pnma , a = 9.0548(2), b = 2.8718(1), c = 10.9908(2) A; V = 285.80(1) A3, Z = 8/3], were found in altered xenoliths within alkaline basalts of the Bellerberg volcano, Eifel, Rhineland-Palatinate, Germany. Fluorellestadite, wadalite, andradite–schorlomite, perovskite, gehlenite, magnesioferrite, cuspidine, ettringite–thaumasite, hydrocalumite, jennite, katoite, and portlandite are the main associated minerals. Wernerkrauseite crystals up to 0.5 mm in size show strong submetallic lustre; the streak is black. Wernerkrauseite appears grey in reflected light. Pleochroism is very weak, bireflectance and anisotropy are weak. Reflectance data for the COM wavelengths vary from ~31 % (400 nm) to ~19 % (700 nm). The calculated density is 4.66 g/cm3, microhardness VHN25 is 154(5) kg/mm2. Wernerkrauseite is a Ca-deficient structural analogue of harmunite, CaFe2O4, and therefore is one of the four known minerals with post-spinel structures. The empirical chemical formula of the holotype wernerkrauseite is Ca0.994(Fe3+1.057Mn4+1.025 Mn3+0.878Mg0.030Al0.016)∑3.006O6. The end-member chemical formula can also be given on the basis of spinel stoichoimetry ( Z = 4): Ca2/3[Fe3+4/3Mn4+2/3]O4, which better reflects its non-stoichiometry. The crystal structure was determined using single-crystal X-ray diffraction ( R 1 = 0.0233 for 800 observed reflections [ I >2σ( I )]). The diffraction pattern shows evidence of short-range ordering of Ca-vacancies. The strongest diffraction lines of the calculated powder diffraction pattern are [ d hkl ( I )]: 2.646 (100), 2.450 (77), 2.748 (62), 4.527 (54), 4.698 (44), 1.818 (43), 2.425 (37), 1.778 (30). Raman spectra of wernerkrauseite were measured and analysed in comparison to the spectra of harmunite and marokite, CaMn2O4. Crystallisation of wernerkrauseite took place at temperatures below 850–900°C under high oxygen fugacity. Furthermore, Mn4+-bearing non-stoichiometric harmunite Ca0.862(Fe3+1.719 Mn4+0.265Ti4+0.012 Mg0.008)∑2.004O4 was found at the same locality, which suggests the existence of a continuous solid solution between wernerkrauseite, harmunite and Ca2/3Mn3+4/3Mn4+2/3O4, described by the formula Ca1−x/2(Fe3+, Mn3+)2−xMn4+xO4, with × ranging from 0 to 2/3.