Structure and Raman Spectroscopy of a New Mineral, Isostructural to Lindojustite, From a Xenolith of the Obnazhennaya Kimberlite Pipe (Kuoykskoye Field, Yakutsk Kimberlite Province)

The structure of a new mineral, isostructural to lindquistite, with the idealized crystal chemical formula (K, Ba)Fe(Mg, Fe)(Al, Cr, Ti)O, has been studied by single-crystal X-ray diffraction analysis. This mineral was identified in a polyphase inclusion in alumochromite from a garnet-spinel lherzolite xenolith from the Obnazhennaya kimberlite pipe (Kuoykskoe field, Yakutsk kimberlite province). The parameters of the hexagonal unit-cell were obtained: a = 5.81093(15) Å, b = 5.81093(15) Å, c = 32.2003(10) Å, V = 941.63(4) Å, space group P6/mmc, Z = 2. The mineral has layered structure of a mixed type. It can be described as a sequence of 14 cubic and hexagonal dense-packed layers formed by oxygen atoms. In each 7 layer, large cations X (K and Ba) are placed in 1/4 oxygen positions, which occupy position 2b and are surrounded by 12 oxygen atoms. O and X ions of close radii form the densest packing, while T (Mg , Fe), M (Al , Cr , Ti), and A (Fe ions occupy voids. It was found that the new mineral is isostructural to lindquistite and hexaferrite of the W-type. The refinement of the mineral structure as a polytype of the magnetoplumblite structure does not reflect the reversal of the layers. The data obtained can be used to justify the identification of a separate lindquistite group, since the studied new mineral is the second finding in this group.