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

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 (FTIR) 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 number 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, FTIR 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 on 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 underwent a heat treatment at 700 °C. After the heat treatment, the IR spectrum showed a decreased 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)-analog of dollaseite-(Ce), ideally CaREE3 Mg2AlSi3O11(OH)2.