Engelhauptite, KCu(VO)(OH)Cl, a new mineral species from Eifel, Germany.

Pekov, Igor; Siidra, Oleg; Chukanov, Nikita; Yapaskurt, Vasiliy; Britvin, Sergey; Krivovichev, Sergey; Schüller, Willi; Ternes, Bernd

A new mineral engelhauptite, KCu(VO)(OH)Cl, 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 = 3.86 g cm. Engelhauptite is optically uniaxial (+), ω = 1.978(4), ε = 2.021(4). Chemical data (wt.%, electron-microprobe, HO by difference) are as following: KO 9.63, FeO 0.05, NiO 0.29, CuO 46.11, AlO 0.24, VO 34.92, SO 0.79, Cl 5.94, HO 3.37, O = Cl -1.34, total 100.00. The empirical formula, based on 10 (O + OH + Cl) apfu, is K(CuAlNi)(VS)O(OH)Cl. Engelhauptite is hexagonal, P6/ mmc, a = 5.922(2), c = 14.513(5) Å, V = 440.78(3) Å 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 [Cu(VO)(OH)] framework formed by the linkage of deficient brucite-like layers of Jahn-Teller distorted Cuφ octahedra (φ = O, OH) via divanadate VO groups. The framework contains large channels occupied by K cations and Cl anions. Engelhauptite is closely related to volborthite, Cu(VO) (OH)∙2HO, and can be considered as its analogue resulting from the replacement of HO 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).

EIFEL (Germany); RHINELAND-Palatinate (Germany); MINERAL analysis; SPHERULITES (Petrology); PARAGENESIS; CRYSTAL structure; X-ray diffraction

Mineralogy & Petrology, 2015, Vol 109, Issue 6, p705

0930-0708

Academic Journal

10.1007/s00710-015-0400-8