Graulichite-(Ce), a new arsenate mineral from the Stavelot Massif, Belgium

Abstract

In the southeastern border of the Stavelot Massif, Belgian Ardennes, the metapelites belonging to the recently defined stratigraphic Salm Group (Verniers et al., 2001) of Cambro-Ordovician age are cross-cut by numerous quartz veins which host, among others, relatively rare Mn-rich aluminosilicates which crystallised during a low-grade metamorphism of Hercynian age (Schreyer, 1975). Besides the occurrence of these minerals, the presence of sulphides in the quartz veins is known for a long time (Hatert et al., 2002). During the last decade, these sulphides have been carefully reinvestigated (Hatert, 1996; Hatert, submitted), as well as their oxidation and alteration products (Hatert et al., 1998).These mineralogical studies reveal the presence of arsenopyrite (Hatert, 1997), particularly as new occurrences in rocks of other stratigraphic position, like rocks of the Deville and Revin Groups (Verniers et al., 2001). Recently, Hanson et al. (1999) discovered the association of gold and arsenopyrite in the quarry of Hourt, in rocks of the Deville Group. This description prompted us to investi-gate the new occurrence of arsenopyrite and to study the mineralogical association of this quarry (Lefèvre, 2001; Lefèvre & Hatert, in press).Among the secondary minerals associated with arsenopyrite from Hourt, collected by S. Puccio, M. Blondieau, J. Detaille and J. Dehove, brown to greenish coatings, constituted by well-developed rhombohedral crystals, were identified as a mineral of the crandallite group. The preliminary chemical analyses performed by one of us (F.H.) indicated that this mineral probably corresponds to the Fe 3+ -rich equivalent of arsenoflorencite, (Ce,La)Al 3 (AsO 4 ,PO 4 ) 2 (OH) 6 , not yet described. The detailed study of this species by Lefèvre (2001) confirmed the existence of a new mineral species, which was approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association (no. 2002-001) under the name graulichite-(Ce). The mineral is named in honour of Jean-Marie Graulich (1920-2001, mining engineer and honorary Director of the Geological Survey of Belgium, particularly for his contribution to the geology of the Stavelot Massif. The holotype is stored in the collection of the

Graulichite-(La), LaFe<sup>3+</sup><sub>3</sub>(AsO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>, a new addition to the alunite supergroup from the Patte d'Oie mine, Bou Skour mining district, Morocco

“…Only one very thin lamina gave some spots suitable for singlecrystal X-ray diffraction study. Similar difficulties were also encountered by Hatert et al (2003) during their study of graulichite-(Ce). They described powder-like patterns, with diffuse streaks, owing to subparallel association of single individuals.…”

“…Further studies confirmed the preliminary results, and the mineral and its name were approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (CNMNC-IMA) under voting number 2020-093. The name highlights its relations with graulichite-(Ce) (Hatert et al, 2003), in accord with the nomenclature of REE-bearing minerals (Levinson, 1966;Bayliss and Levinson, 1988). Holotype material is deposited in the mineralogical collection of the Museo di Storia Naturale, University of Pisa, Via Roma 79, Calci (Pisa), under catalogue number 19924.…”

“…Graulichite-(La) is transparent, with a very weak pleochroism from light yellow to yellow. As in graulichite-(Ce) (Hatert et al, 2003), birefringence was difficult to observe because interference colors are masked by the yellow color of the mineral. Mean refractive index, calculated according to the Gladstone-Dale relationship (Mandarino, 1979(Mandarino, , 1981, is 1.889 and 1.928 for domains #1 and #2, respectively.…”

“…Two chemically homogeneous domains (labeled #1 and #2) were identified. In agreement with Hatert et al (2003), the chemical formulae of graulichite-(La) were calculated on the basis of 6 cations per formula unit, assuming the occurrence of 14 O atoms per formula unit (apfu); H content was estimated in order to achieve the electrostatic balance. Domain #1 corresponds to the chemical formula (La 0.34 Ce 0.20 Ca 0.11 Sr 0.07 Pb 0.05 K 0.04 ) 0.81 (Fe 3+ 2.16 Al 0.84 Cu 0.20 ) 3.20 (As 1.23 P 0.39 S 0.37 ) 1.99 O 14 H 6.13 , whereas domain #2 has the composition (La 0.38 Ce 0.22 Sr 0.10 Ca 0.09 Pb 0.05 K 0.06 ) 0.90 (Fe 3+ 2.60 Al 0.49 Cu 0.20 ) Notwithstanding the low quality of chemical data, the mineral stoichiometry is consistent with the crystal chemistry of alunite-supergroup minerals.…”

“…X-ray powder diffraction data were collected using a 114.6 mm Gandolfi camera and Ni-filtered CuKα radiation. Table 2 gives the observed and the calculated X-ray powder diffraction patterns, compared with that of graulichite-(Ce) (Hatert et al, 2003). Unit-cell parameters, refined on the basis of 13 unequivocally indexed reflections using the software UnitCell (Holland and Redfern, 1997), are a = 7.2469(9), c = 16.767(2) Å, V = 762.59(17) Å 3 .…”