Crystal Structure of Potassic-Magnesio-Hastingsite KCa2(Mg4Fe3+)(Si6Al2O22)(OH)2 from Osinovy Mys (Southern Urals, Russia)

Potassic-magnesio-hastingsite with the idealized formula KCa2(Mg4Fe3+)(Si6Al2O22)(OH)2 was found in blocks of biotite-amphibolic gabbro from serpentinite mélange of the Ilmenogorsky metamorphic complex, located within the Ilmensky Natural Reserve. In this study, we report the results of the crystal structure determination of three potassic-magnesio-hastingsite samples from the type locality. These samples exhibit unusual characteristics, including the predominance of K over Na, high iron content, and approximately equal amounts of Fe3+ and Fe2+ ions. The crystal structures of three samples were refined in the anisotropic approximation of atomic displacement parameters to R of 4.01, 4.18, and 4.7% using 1043, 1116, and 1148 reflections with I > 2σ(I), respectively. The M(2) site is characterized by a diverse composition of heterovalent cations, including Fe3+, Al, Mg, Ti, Fe2+, and V. In contrast, the M(1) and M(3) sites are predominantly occupied by Mg and Fe2+. Among the trivalent C-cations, Fe3+ is dominant and primary occupies M(2) site.