Crystal chemistry of ivanyukite-group minerals, A3–xH1+x[Ti4O4(SiO4)3](H2O)n (A = Na, K, Cu), (n = 6–9, x = 0–2): crystal structures, ion-exchange, chemical evolution

Abstract

AbstractMicroporous slicates with the pharmacosiderite structure and the general formula A3–xH1+x[Ti4O4(SiO4)3](H2O)n (A = Na, K, Cu), (n = 6–9, x = 0–2) are outstanding in their ion-exchange properties. The ivanyukite mineral group consists of three species, one of which has two polymorphs. The minerals forming a progressive series: ivanyukite-Na-T → ivanyukite-Na-C → ivanyukite-K → Cu-rich ivanyukite-K → ivanyukite-Cu, have been studied by single-crystal X-ray diffraction, electron microprobe analysis and Raman spectroscopy. The microporous heteropolyhedral framework of the ivanyukite-group minerals is based on cubane-like [Ti4O4]8+ clusters that share common corners with SiO4 tetrahedra to form wide three-dimensional channels suitable for the migration of Na+, K+ and Cu2+ ions. Ivanyukite-Na-T that has a R3m symmetry loses Na+ in aqueous solutions via the substitution Na+ + O2‒ ↔ □ + OH‒, which allows for the migration of K+ ions and transformation of initial structure into the cubic (P$\bar{4}3m$) ivanyukite-Na-C polymorph or into ivanyukite-K, when most of Na is lost. Natural ivanyukite-Na-C is shown to contain domains of both R3m (subordinate) and P$\bar{4}3m$ (dominant) symmetry with the chemical composition determining the stability and dominance of cubic or trigonal forms. Incorporation of Cu into the crystal structure ivanyukite-K via the substitution K+ + OH− ↔ Cu2+ + O2− in aqueous solutions results in the formation of ivanyukite-Cu. Post-crystallisation processes (such as exchange of Na+, K+, Cu2+, and/or hydration/dehydration of primary phases) are widespread in hyperagpaitic rocks of the Kola alkaline massif and the respective mineral transformations contribute to the diversity of mineral species.