Crystal-chemical relations and classification problems in tourmalines belonging to the oxy-schorl–oxy-dravite–bosiite–povondraite series

Two oxy-tourmalines showing intermediate compositions between the oxy-schorl and oxy-dravite series (sample from Sludyanka crystalline complex, Baikal, Russia) and the oxy-dravite and bosiite series (sample from the Valletta mine, Piedmont, Italy) were structurally and chemically characterized by single-crystal X-ray diffraction, electron microprobe analysis, optical absorption and Synchrotron Mössbauer source spectroscopy. A comparative analysis of the results, along with data from literature on additional samples corresponding to the species oxy-schorl, oxy-dravite, bosiite and povondraite, show that the main structural variations within the Fe-bearing, Na-dominant oxy-tourmalines are related to variations in the content of Fe, which is incorporated into the structure via substitution mechanisms that involves Al. The results also show the occurrence of a complete substitution series from oxy-schorl via oxy-dravite and bosiite to povondraite, but no (or a limited) direct substitution series between oxy-schorl and bosiite (or povondraite). The substitution of Al by larger cations causes changes in 〈Y–O〉 and 〈Z–O〉 distances as well as in the unit-cell parameters. The effect of 〈Y–O〉 on the a-parameter is evident in the oxy-schorl–oxy-dravite series, but it is of marginal significance in the oxy-dravite–bosiite–povondraite series. On the other hand, a good correlation exists between 〈Z–O〉 and the a-parameter only in the oxy-dravite–bosiite–povondraite series. The c-parameter is strongly correlated with 〈Z–O〉 along the entire substitution series. Classification problems related to the naming of Fe-bearing, Na-dominant oxy-tourmalines are discussed. Application of the nomenclature rules in force resulted in ambiguous naming of the studied samples. This makes, for example, direct comparisons of physical and chemical properties of tourmalines originating from different deposits difficult.