3T polytype of an iron-rich oxyphlogopite from the Bartoy volcanic field, Transbaikalia: Mössbauer, infrared, Raman spectroscopy, and crystal structure.

The Mössbauer, infrared and Raman spectra of iron-rich oxyphlogopite from a new locality, the Bartoy occurrence, Transbaikalia, Russia were obtained and its crystal structure was solved. The mineral is characterized by the absence of OH groups and ordered distribution of Fe²⁺ and Fe³⁺ between sites having octahedral coordination. Unlike oxyphlogopite holotype sample which is monoclinic (1M polytype), iron-rich oxyphlogopite belongs to the 3T polytype (space group P3₁12) with the unit-cell parameters a = 5.3248(2) Å, c = 29.788(3) Å, V = 731.44(8) ų. Its refined crystal-chemical formula is (Z = 3): ^A(K_0.9Na_0.1) [^M1(Fe²⁺_0.6Mg_0.4) ^M2(Fe³⁺_0.4Ti_0.4Mg_0.2) ^M3(Mg_0.4Fe³⁺_0.3Ti_0.2Al_0.1)] [^T1,2(Si_0.7Al_0.3)₂O₅]₂^X(O_0.9F_0.1)₂. [ABSTRACT FROM AUTHOR]