Achtarandite - sponge hibschite pseudomorph after wadalite-like phase: internal morphology and mechanism of formation

Abstract

Achtarandite, tetrahedral pseudomorph after a wadalite-like mineral, is represented by different types of pseudomorphs on the Wiluy deposit. As a result of processes and mechanisms of pseudomorphic substitution of wadalite (protoachtarandite) by hydrogarnet are white, predominantly hibschite pseudomorphs. Crystal chemical affinity of the protophase (wadalite) and metaphase (hibschite) and morphological particularities of achtarandite allowed to classify these pseudomorphs as the sponge pseudomorphs formed in the systems with isomorphic components. Peculiarities of the internal morphology, character of morphology and composition evolution of hydrogarnet from achtarandite pseudomorphs: {111} hibschite → {110} Fe-hibschite → {211} Ti-hydroandradite (GALUSKINA et al. 2001), allowed to distinguish successive formation stages of the sponge pseudomorphs and to determine mechanisms of structure formation corresponding to these stages. Sponge structure of pseudomorphs was forced by the substitution mechanisms (mechanisms of the simultaneous growth and dissolution) at the conditions of volume deficit of new-formed phase.