Iowaite from the Udachnaya kimberlite pipe (Yakutia): crystal chemistry and post-crystallization transformations

An unusual variety of iowaite, characterized by an anomalously low content of divalent cations: (Mg+Fe2+)/Fe3+ ~2.75:1, was identified as part of the late hydrothermal mineral association in the Udachnaya kimberlite pipe (Yakutia). Its crystal structure was studied on a single crystal: polytype 3R, sp. gr. R3 ̅m, a = 3.1213(3), c = 23.621(4) Å, V = 199.30(5) Å3; R1 = 0.027. In some crystals, zoning has been revealed, indicating natural anion exchange with the main reactions 2Cl– → SO42– and 2Cl– → 2SO42– + Ca2+. This is the first reliably recorded case of natural anion exchange in minerals of the hydrotalcite supergroup. The anion exchange reaction occurs with an increase in the interlayer distance from 8.0 to 11.2 Å. The composition and character of zoning of the studied crystals provides information about the evolution of the chemistry of late low-temperature processes.