| Summary: | Vesuvianite, a rock-forming mineral typical of skarns and rodingites, has been the object of various recent spectroscopic, structural and genetic investigations. This mineral is characterized by a complex composition and by a structure that reflects the conditions of its growth. Establishing the principles by which the vesuvianite group minerals, including wiluite, manganvesuvianite, and fluorvesuvianite, may be classified is likely to lead to the description of a new mineral species. For a long time, it was considered that minerals of the vesuvianite group in the Wiluy deposit (Yakutia, Russia), the type locality of wiluite, grossular and achtarandite (occurring as a pseudomorph after wadalite), were represented solely by wiluite. The results presented here show that minerals of the vesuvianite group that formed at different stages in the evolution of their host rock are characterized by varying compositions and morphologies, and that they are the rock-forming minerals of the achtarandite rocks. The main aims of the present work are two-fold. 1) The investigation of the morphology, crystal chemistry, structure and growth history of the vesuvianite group minerals belonging to the new isomorphic series hydrovesuvianite” (Si-deficient vesuvianite) - vesuvianite and “hydrovesuvianite” - wiluite. 2) The actualization of an earlier genetic model for the formation of achtarandite rocks.
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