Fiemmeite Cu2(C2O4)(OH)2∙2H2O, a New Mineral from Val di Fiemme, Trentino, Italy

Francesco Demartin,Paolo Ferretti,Ivano Rocchetti,Italo Campostrini

doi:10.3390/min8060248

Abstract

The new mineral species fiemmeite, Cu2(C2O4)(OH)2∙2H2O, was found NE of the Passo di San Lugano, Val di Fiemme, Carano, Trento, Italy (latitude 46.312° N, longitude 11.406° E). It occurs in coalified woods at the base of the Val Gardena Sandstone (upper Permian) which were permeated by mineralizing solutions containing Cu, U, As, Pb and Zn. The oxalate anions have originated from diagenesis of the plant remains included in sandstones. The mineral forms aggregate up to 1 mm across of sky blue platelets with single crystals reaching maximum dimensions of about 50 μm. Associated minerals are: baryte, olivenite, middlebackite, moolooite, brochantite, cuprite, devilline, malachite, azurite, zeunerite/metazeunerite, tennantite, chalcocite, galena. Fiemmeite is monoclinic, space group: P21/c with a = 3.4245(6), b = 10.141(2), c = 19.397(3) Å, β = 90.71(1)°, V = 673.6(2) Å3, Z = 4. The calculated density is 2.802 g/cm3 while the observed density is 2.78(1) g/cm3. The six strongest reflections in the X-ray powder diffraction pattern are: [dobs in Å (I)(hkl)] 5.079(100)(020), 3.072(58)(112), 9.71(55)(002), 4.501(50)(022), 7.02(28)(012), 2.686(25)(114). The crystal structure was refined from single-crystal data to a final R1 = 0.0386 for 1942 observed reflections [I > 2σ(I)] with all the hydrogen atoms located from a Difference–Fourier map. The asymmetric unit contains two independent Cu2+ cations that display a distorted square-bipyramidal (4+2) coordination, one oxalate anion, two hydroxyl anions and two water molecules. The coordination polyhedra of the two copper atoms share common edges to form polymeric rows running along [100] with composition [Cu2(C2O4)(OH)2∙2H2O]n. These rows are held together by a well-established pattern of hydrogen bonds between the oxalate oxygens not involved in the coordination to copper, the hydrogen atoms of the water molecules and the hydroxyl anions.

Highlights

These rows are held together by a well-established pattern of hydrogen bonds between the oxalate oxygens not involved in the coordination to copper, the hydrogen atoms of the water molecules and the hydroxyl anions
The presence of small Cu ore deposits, in the area close to the Passo di San Lugano, Val di Fiemme, Carano, Trento, Italy is well known since the XV and XVI century, as documented by the remains of the old mining sites
The sedimentary sequence of the upper Permian consists of the continental deposits of alluvial plain of the Val Gardena Sandstone

Summary

Introduction

The presence of small Cu ore deposits, in the area close to the Passo di San Lugano, Val di Fiemme, Carano, Trento, Italy is well known since the XV and XVI century, as documented by the remains of the old mining sites. Deposits, can be explained by a genetic model given by a continental source made up of granite or acid volcanites that are eroded in an arid continental climate and transported with U and other heavy metals, such as Cu, Pb, Zn, in the form of ions dissolved in clastic aquifers, in our case the alluvial conoid deposits of the Val Gardena Sandstone The deposition of these ions is due to the strong decrease in solubility resulting from the reaction between mineralized groundwater and the strongly reducing environment given by the accumulation of trunks and organic matter in the channels or deposits of overbank. Collection of MUSE, Museo delle Scienze di Trento, sample No 5249

Mineral Description and Physical Properties

Chemical Data

Micro Raman Spectroscopy

Crystallography

Crystal Structure Description and Discussion