Engelhauptite, KCu₃(V₂O₇)(OH)₂Cl, a new mineral species from Eifel, Germany

Abstract

A new mineral engelhauptite, KCu₃(V₂O₇)(OH)₂Cl, was found within cavities in nepheline basalts at the Auf’m Kopp quarry (“Schlackenkegel der Höhe 636 südöstlich Neroth”), Daun, Eifel region, Rheinland-Pfalz, Germany. Associated minerals are volborthite, allophane, malachite, tangeite and chrysocolla; earlier minerals belonging to the primary, high-temperature parageneses are augite, mica of the phlogopite–oxyphlogopite series, sanidine, nepheline, leucite, fluorapatite and magnetite. Engelhauptite occurs as spherulites (up to 0.2 mm in diameter) and bunches consisting of rough spindle-shaped crystals elongated parallel to [0001]. The crystals are up to 0.12 mm long and up to 0.04 mm thick. Individual grains of engelhauptite are transparent, whereas their aggregates are translucent. The mineral is yellow-brown to brown, typically with an olive green hue. The luster is vitreous. Engelhauptite is brittle, cleavage is not observed, fracture is uneven. D _calc = 3.86 g cm⁻³. Engelhauptite is optically uniaxial (+), ω = 1.978(4), ε = 2.021(4). Chemical data (wt.%, electron-microprobe, H₂O by difference) are as following: K₂O 9.63, FeO 0.05, NiO 0.29, CuO 46.11, Al₂O₃ 0.24, V₂O₅ 34.92, SO₃ 0.79, Cl 5.94, H₂O_calc 3.37, O = Cl₂ -1.34, total 100.00. The empirical formula, based on 10 (O + OH + Cl) apfu, is K_1.05(Cu_2.97Al_0.02Ni_0.02)_Σ3.01(V_1.97S_0.05)_Σ2.02O_7.23(OH)_1.91Cl_0.86. Engelhauptite is hexagonal, P6₃/mmc, a = 5.922(2), c = 14.513(5) Å, V = 440.78(3) ų and Z = 2. The eight strongest reflections of the powder X-ray diffraction pattern [d,Å(I) (hkl)] are: 7.32(98) (002), 4.224(17) (102), 2.979(100) (104, 110), 2.759(19) (112), 2.565(18) (200), 2.424(18) (202), 1.765(16) (206) and 1.481(14) (208, 220). The crystal structure of engelhauptite has been solved from the single-crystal X-ray diffraction data and refined to R = 0.090 on the basis of 135 unique observed reflections. The structure is based upon the [Cu²⁺ ₃(V₂O₇)(OH)₂]⁰ framework formed by the linkage of deficient brucite-like layers of Jahn-Teller distorted Cuφ₆ octahedra (φ = O, OH) via divanadate V₂O₇ groups. The framework contains large channels occupied by K⁺ cations and Cl⁻ anions. Engelhauptite is closely related to volborthite, Cu₃(V₂O₇) (OH)₂∙2H₂O, and can be considered as its analogue resulting from the replacement of H₂O molecules by the equal amounts of K⁺ and Cl⁻ ions. The mineral is named in honour of the German amateur mineralogist and mineral collector Bernd Engelhaupt (born 1946).