Ankangite from the Monte Arsiccio mine (Apuan Alps, Tuscany, Italy): occurrence, crystal structure, and classification problems in cryptomelane group minerals

A new occurrence of ankangite, a rare member of the hollandite structural family, is described. Ankangite was found in small fractures in dolomitic marble at the Monte Arsiccio mine, a small deposit of baryte, pyrite and Fe oxides in the Apuan Alps, Tuscany, Italy. The mineral occurs as prismatic black crystals, up to 5 mm in length, associated with "apatite", anatase, arsenopyrite, Ba-rich K-feldspar, baryte, dolomite, pyrite, quartz, sphalerite, stibnite, valentinite, and zinkenite. The crystal structure of ankangite from the Monte Arsiccio mine was refined to R1 = 0.016. Ankangite is a member of the titanate subgroup in the cryptomelane group. The classification and nomenclature of the hollandite-group minerals are discussed.