A new mineral engelhauptite, KCu3(V2O7)(OH)(2)Cl, was found within cavities in nepheline basalts at the Auf'm Kopp quarry ("Schlackenkegel der Hohe 636 sudostlich 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 (+), omega = 1.978(4), epsilon = 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, H2Ocalc 3.37, O = Cl-2 -1.34, total 100.00. The empirical formula, based on 10 (O + OH + Cl) apfu, is K-1.05(Cu2.97Al0.02Ni0.02)(I 3.01)(V1.97S0.05)(I pound 2.02)O-7.23(OH)(1.91)Cl-0.86 pound. Engelhauptite is hexagonal, P6(3)/mmc, a = 5.922(2), c = 14.513(5) , V = 440.78(3) (3) and Z = 2. The eight strongest reflections of the powder X-ray diffraction pattern [d,(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 Cu phi(6) octahedra (phi = O, OH) via divanadate V2O7 groups. The framework contains large channels occupied by K+ cations and Cl- anions. Engelhauptite is closely related to volborthite, Cu-3(V2O7) (OH)(2)a (TM) 2H(2)O, 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).
