Multi-analytical characterization of an unusual epidote-supergroup mineral from Malmkärra, Sweden: Toward the new (OH)-analogue of dollaseite-(Ce)

Abstract

A study of a skarn sample from the Malmkärra iron mines, Norberg, Västmanland (Sweden) revealed the occurrence of a peculiar epidote-supergroup mineral. It was examined using electron microprobe, single-crystal X-ray diffraction, Mössbauer and Fourier-transform infrared spectroscopy techniques. Structure refinements combined with electron-microprobe data indicate the following cation populations: A1 = Ca0.96REE3+0.03Mn0.01; A2 = REE3+0.99Ca0.01; M1 = Mg0.40Al0.32Fe3+0.26Fe2+0.02; M2 = Al0.98Fe3+0.02; M3 = Mg0.72Fe2+0.17Fe3+0.11; T1,2,3 = Si2.93Al0.07, accounting for a total positive charge of 24.64. The presence of Fe2+ is confirmed by Mössbauer data. The remarkable amount of divalent cations at both M1 and M3 (> 1 pfu, per formula unit) demands more than one monovalent anion pfu in the structure. As the mineral lacks fluorine, charge neutrality must be achieved through additional H+ (about 0.4 apfu). Only one independent hydrogen atom is located within the structure, with O10 as donor and O4 as acceptor, as in other epidote-supergroup minerals. Nonetheless, another O–O distance is suitable for a hydrogen bond, namely O10–O2. Although the existence of the additional OH group was not directly proved by vibrational spectroscopy, FT-IR data provided information related to this potential O10–O2 bridge. In the IR-spectrum acquired, several bands are observed in the OH-stretching region, and a secondary peak at 2140 cm–1 could be assigned to the bending mode of the O10–H…O2 group. To shed light onto this puzzling observation, one single crystal was subjected to annealing experiments at temperatures from 500 to 700°C, in 50°C steps, while a second one has undergone a heat treatment at 700°C. After the heat treatment, the IR-spectrum showed a decrease in intensity of all observed bands, in agreement with a dehydrogenation occurring at high temperatures. Although the structural position of the second hydrogen is still uncertain, it is reasonable to describe the composition of the epidote-supergroup mineral from Malmkärra as a solid solution between dissakisite-(Ce) (32%), ferriallanite-(Ce) (28%) and a yet undescribed end-member (40%), the (OH)-analogue of dollaseite-(Ce) – ideally CaREE3+Mg2AlSi3O11(OH)2.