Polyoxometalate chemistry at volcanoes: discovery of a novel class of polyoxocuprate nanoclusters in fumarolic minerals

Polyoxometalate (POM) chemistry is an important avenue of comprehensive chemical research, due to the broad chemical, topological and structural variations of multinuclear polyoxoanions that result in advanced functionality of their derivatives. The majority of compounds in the polyoxometalate kingdom are synthesized under laboratory conditions. However, Nature has its own labs with the conditions often unconceivable to the mankind. The striking example of such a unique environment is volcanic fumaroles – the natural factories of gas-transport synthesis. We herein report on the discovery of a novel class of complex polyoxocuprates grown in the hot active fumaroles of the Tolbachik volcano at the Kamchatka Peninsula, Russia. The cuboctahedral nanoclusters {[MCu$_{12}$O$_{8}$](AsO$_{4}$)$_{8}$} are stabilized by the core Fe(III) or Ti(IV) cations residing in the unique cubic coordination. The nanoclusters are uniformly dispersed over the anion- and cation-deficient NaCl matrix. Our discovery might have promising implications for synthetic chemistry, indicating the possibility of preparation of complex polyoxocuprates by chemical vapor transport (CVT) techniques that emulate formation of minerals in high-temperature volcanic fumaroles

Santabarbaraite from the oxidation zone of the Mednorudyansk field is the first finding in the Urals

In the oxidation zone of the Mednorudyansk deposit, the authors discovered and studied aqueous ferric oxide phosphate – santabarbaraite. The discovered mineral was in one of the samples of the Mednorudyansk deposit from the collection of N. I. Kozin, collector from Nizhny Tagil. As an independent mineral species, approved by the Commission on New Minerals of the International Mineralogical Association, santabarbaraite was described in 2003 in clays of the lignite sedimentary basin in the mountainous region of Santa Barbara (Italy) and in clays underlying the Pliocene basalts in the southeast of Australia. For Russia, there are few mentions of it; it appears in the deposits of Kerch and Taman iron-ore basins, as well as in bottom sediments and near Lake Baikal. Search of information on the findings of this mineral in literature was unsuccessful, although it is likely that santabarbaraite is not a rare mineral in this region, and acts as the usual product of the oxidation of vivianite in the hypergenesis zone. Santabarbaraite from the Mednorudyansk deposit forms pseudomorphs along the vivianite crystals in cavities among the nodular and cellular limonite. The pseudomorphs of santabarbaraite completely preserved the faceting of lamellar vivianite crystals. Powder X-ray examination of samples of santabarbaraite showed complete absence of diffraction reflexes, which is typical for this mineral. The mineral is characterized by the presence of impurities of magnesium, manganese, zinc, sodium and potassium. The absence in the association of metavivianite and other intermediate mineral phases indicates that the oxidation of vivianite was most likely due to the direct replacement mechanism by santabarbarite. The authors also studied this mineral using thermal analysis, infrared and Raman spectroscopy

Crystal chemistry and nomenclature of rhodonite-group minerals

This paper presents the nomenclature of the rhodonite group accepted by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA). An overview of the previous studies of triclinic (space group P) pyroxenoids belonging to the rhodonite structure type, with a focus on their crystal chemistry, is given. These minerals have the general structural formula VIIM(5)VIM(1)VIM(2)VIM(3)VIM(4)[Si5O15]. The following dominant cations at the M sites are known at present: M(5) = Ca or Mn2+, M(1-3) = Mn2+; and M(4) = Mn2+ or Fe2+. In accordance with the nomenclature, the rhodonite group consists of three IMA-approved mineral species having the following the general chemical formulae: M(5)AM(1-3)B3M(4)C[Si5O15], where A = Ca or Mn2+; B = Mn2+; and C = Mn2+ or Fe2+. The end-member formulae of approved rhodonite-group minerals are as follows: rhodonite CaMn3Mn[Si5O15]; ferrorhodonite CaMn3Fe[Si5O15]; and vittinkiite MnMn3Mn[Si5O15]