A multimethodic approach to the characterization of a Mn-rich celadonite from Cerchiara mine, Eastern Liguria, Italy

In the manganesiferous ores associated to the metacherts of the ophiolitic sequences at the Cerchiara mine, Eastern Liguria (Italy), a new Mn-bearing mineral belonging to the mica group has recently been found and characterized. TEM-EDT studies confirm that the mineral belongs to the mica group. Unit-cell parameters from powder diffraction pattern are: a = 5.149(1), b = 8.915(1), c = 10.304(1) Å, beta = 102.03(1)°, space group C2 or C2/m. On the basis of the EPR spectroscopic results, the Mn4+ content represents a very subordinate fraction of the total Mn, the remaining occurring as Mn3+. The Raman spectrum clearly evidences the presence of OH groups in the structure. LA-ICP-MS measurements assess the presence of considerable amounts of Li. Assuming all Mn as Mn3+ and 22 negative charges, the empirical formula can be expressed as: (K0.83□0.17)(Mn3+ 1.14Mg0.80Li0.20Fe3+0.02)(Si3.89Al0.10)O10[(OH)1.92F0.08] with the sum of the octahedral cations indicating a “transitional” character between a di- and a tri-octahedral one. This formula ideally corresponds to the Mn3+ analogue of celadonite, thus expanding the range of solid solution in the celadonite family. The ideal end-member formula KMn3+MgSi4O10(OH)2 can be easily related to celadonite by the homovalent substitution VIMn3+ → VIFe3+. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA 2015-052.