Crystal Structure and Raman Spectroscopy of a New Mineral Isostructural to Lindqvistite from Xenolith of the Obnazhennaya Kimberlite Pipe (Kuoi Field, Yakutsk Kimberlite Province)

Using single-crystal X-ray diffraction analysis, we resolved the structure of a new mineral isostructural to lindqvistite with an ideal formula (K,Ba)Fe3+(Mg,Fe)4(Al,Cr3+,Ti4+)13O27. This mineral was found in a polyphase inclusion in alumochromite from a garnet–spinel lherzolite xenolith of the Obnazhennaya kimberlite pipe (Kuoi field, Yakutsk kimberlite province). The parameters of the hexagonal unit cell are as follows: a = 5.81093(15) Å, c = 32.2003(10) Å, V = 941.63(4) Å3, space group P63/mmc, and Z = 2. The mineral has a mixed layered structure, which can be described as a sequence of 14 cubic and hexagonal densely packed layers formed by oxygen atoms. In this structure, every 7th layer contains large X cations (K+ and Ba2+) occupying 1/4 of the oxygen positions. These cations are located at the 2b site and are coordinated by 12 oxygen atoms. O2– and X ions with a similar radius form the close packing, whereas ions T (Mg2+, Fe2+), M (Al3+, Cr3+, Ti4+), and A (Fe3+) occupy interstitial sites. It is found that the new mineral is isostructural to lindqvistite and W-type hexaferrite. The description of the mineral structure as a polytype of magnetoplumbite does not reflect the reversal of the layers. Our data can be used to substantiate the identification of the independent linqvistite group, because the studied new mineral is the second finding in this group.