Vermiculite expansion through non-artificial processes in pyroclastic carbonatites from Catanda (Angola)

The pyroclastic carbonatitic rocks from the Catanda volcanic area (Angola) contain annite and phlogopite grains which have been affected to a significant extent by vermiculitization. A mineralogical study of these micas has revealed that the annite was altered to K-vermiculite while vermiculitization of phlogopite generated Ca-vermiculite. Intermediate alteration products such as intergrowths of phlogopite/Ca-vermiculite were also found, but interstratified crystals of phlogopite/ vermiculite, a common intermediate product during vermiculitization processes, are not reported in the Catanda rocks. The two types of vermiculitized micas show ‘accordion’ textures, related to the expansion of vermiculite. To the authors’ knowledge, the present study is the first report of expanded vermiculites described in natural rocks and not generated as a consequence of industrial processing.