Iowaite from the Udachnaya Kimberlite Pipe, Yakutia: Crystal Chemistry and Postcrystallization Transformations

An atypical variety of iowaite with an extremely low content of divalent cations (Mg + Fe²⁺)/Fe³⁺ of ~2.75 : 1 is identified in a late hydrothermal mineral assemblage of the Udachnaya kimberlite pipe, Yakutia. Its crystal structure is studied using a single crystal: polytype 3R, space group \(R\bar {3}m\) , a = 3.1213(3) Å, с = 23.621(4) Å, V = 199.30(5) ų; R₁ = 0.027. Some crystals are characterized by zonation indicating a natural ion exchange, the leading schemes of which are 2Cl^– → SO \(_{4}^{{2 - }}\) and 2Cl^– → 2SO \(_{4}^{{2 - }}\) + Ca²⁺. This is the first reliable case of natural anion exchange in minerals of the hydrotalcite supergroup. The anion exchange reaction occurs with an increase of interlayer distance from 8.0 to 11.2 Å. The composition of the mineral and the character of its zonation bear information on the evolution of late hydrothermal processes.