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4. All minerals of the tourmaline supergroup bear six-membered cyclosilcates and a trigonal crystal symmetry. The general formula is XY3Z6(T6O18)(BO3)3V3W where X=Na+ Ca2+ K+ Pb2+and vacancy; Y=Fe2+ Mg2+ Mn2+ Al3+ Li+ Fe3+ and Cr3+; Z=Al3+ Fe3+ Mg2+ and Cr3+; T=Si4+ Al3+ and B3+; B=B3+; V=OH−and O2−; and W=OH− F− and O2−. In these minerals the ring-silicate coordinates six-fold towards Na+ K+ Ca2+or also Pb2+with its ring oxygen atoms similar as demonstrated also for cyclic siloxanes in this review. A comprehensive overview is provided in the mineralogical databasemindat(see: https://www.mindat.org/min-4003.html) and in: D. J. Henry M. Novák F. C. Hawthorne A. Ertl B. L. Dutrow P. Uher F. Pezzotta Am. Mineral.2011 96 895–913.