Electrophysical properties of demantoid and andradite garnets according to high-temperature impedance spectroscopy data: the influence of chemical and phase impurities (methodological aspects)

   Object of the study and methods. Electrical characteristics of demantoid from clinopyroxenites (Poldnevskoye deposit, Middle Urals) and two samples of andradite 1-2 from skarns (Verkhniy Ufaley, Middle Urals; Sokolovsky mine, Rudny, Kazakhstan) were studied by impedance high-temperature spectroscopy in the heating-cooling mode at temperatures of 200–900 °C and frequencies of 1–106 Hz using of platinum and lanthanum-strontium cobaltite electrodes. Thermogravimetric, X-ray diffraction and diffuse light scattering data are presented.

   Results. Experimental chemical formulas of andradite 1-2 and demantoid are (Mg_0.24Ca_3.16Mn_0.04)(Fe_1.63Al_0.33)Si_2.95Ti_0.05O_12.14, (Ca_3.49Mn_0.04)(Fe_1.79Al_0.51)Si_2.94Ti_0.06O_12.97, (Ca_3.51Mn_0.01) (Fe_2.49Al_0.05Cr_0.0038)Si_3.00O_13.34, respectively. Andradite 1 contains up to ~20 % clinochlore impurity and an insignificant content of ferrobustamite impurity; in andradite 2 – no more than ~8 % of isostructural impurity of hydroandradite; demantoid does not contain phase impurities, while the peaks of the garnet phase are asymmetric due to the presence of two phases with the garnet structure. In the optical spectra of andradite 1-2, a wide band is observed in the near UV region and a significant number of sufficiently wide bands in the visible region associated with the absorption of Fe²⁺, Fe³⁺ and Ti⁴⁺ ions; Spectra of annealed samples of andradites at 750 °С are similar. For demantoid, a wide absorption band of 860 nm is observed, it shifts to 700 nm after annealing; it is assumed that the 860 nm band is associated with Cr²⁺ ions, which undergo additional oxidation during annealing. The Arrhenius dependences of the electrical conductivity of andradite 1 during heating and cooling differ from each other due to the presence of impurity phases (mainly clinochlore) in the sample. Dependencies for andradite 2 and demantoid in heating-cooling mode are close to each other, while the electrical conductivity of andradite 2 is higher than that of andradite 1. At temperatures of 750–775 °C, demantoid has the highest conductivity; while the Cr impurity does not make a significant contribution to its conductivity.

   Conclusions. Electrical characteristics of demantoid were obtained for the first time; Arrhenius dependences of two andradites of different chemical and phase composition were analyzed; it was shown that the composition has a significant effect on electrical conductivity. The obtained data can be used to construct geoelectric models of fragments of the earth's crust with the corresponding minerals.

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