F. Di Benedetto

A multimethodic approach for the characterization of manganiceladonite, a new member of the celadonite family from Cerchiara mine, Eastern Liguria, Italy

Abstract In the manganesiferous ores associated with the metacherts of the ophiolitic sequences at the Cerchiara mine, Eastern Liguria (Italy), a new Mn-bearing mineral belonging to the mica group has been recently found and characterized. High resolution transmission electron microscopy and electron diffraction tomography studies confirm that the mineral belongs to the mica group. Unit-cell parameters from the powder diffraction pattern are: a = 5.149(1), b = 8.915(1), c = 10.304(1) Å, β = 102.03(1)°, space group C2 or C2/m. On the basis of the electron paramagnetic resonance spectroscopic results, the Mn4+ content represents a very subordinate fraction of the total Mn, the remaining occurring as Mn3+. The Raman spectrum clearly indicates the presence of OH groups in the structure. Laser-ablation inductively-coupled-plasma mass-spectrometry 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 structure. This formula corresponds ideally 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 of the International Mineralogical Association, (IMA 2015-052).

An XRPD and EPR spectroscopy study of microcrystalline calcite bioprecipitated by Bacillus subtilis

We report in this study the first XRPD and EPR spectroscopy characterisation of a biogenic calcite, obtained from the activity of the bacterium Bacillus subtilis. Microcrystalline calcite powders obtained from bacterial culture in a suitable precipitation liquid medium were analysed without further manipulation. Both techniques reveal unusual parameters, closely related to the biological source of the mineral, i.e., to the bioprecipitation process and in particular to the organic matrix observed inside calcite. In detail, XRPD analysis revealed that bacterial calcite has slightly higher c/a lattice parameters ratio than abiotic calcite. This correlation was already noticed in microcrystalline calcite samples grown by bio-mineralisation processes, but it had never been previously verified for bacterial biocalcites. EPR spectroscopy evidenced an anomalously large value of W6, a parameter that can be linked to occupation by different chemical species in the next nearest neighbouring sites. This parameter allows to clearly distinguish bacterial and abiotic calcite. This latter achievement was obtained after having reduced the parameters space into an unbiased Euclidean one, through an isometric log-ratio transformation. We conclude that this approach enables the coupled use of XRPD and EPR for identifying the traces of bacterial activity in fossil carbonate deposits.