Crystal structure and compositional evolution of vanadium-rich oxy-dravite from graphite quartzite at Bitovanky, Czech Republic

Two types of V-rich dravitic tourmaline (oxy-dravite to dravite) distinct in their color, morphology, paragenesis and composition from graphite quartzite at Bitovanky near Trebic, Moldanubicum, Bohemian Massif, were studied by means of electron microprobe, laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS), and single-crystal X-ray diffraction. Green oxy-dravite (GVD) from graphite quartzite is sporadically zoned with V-enriched rims; brown oxy-dravite to dravite (BVD) from mobilized quartz-rich nests is characterized by such V-enriched rims. The formulae derived from chemical and structure data of the two oxy-dravite varieties are (X)(Na0.534Ca0.284 square K-0.164(0.018)) (Y)(Mg1.461V0.679Al0.771Fe0.042Mn0.003Ti0.043) (Z)(Al5.074V0.237Mg0.689) (T)(Si5.782Al0.218O18) (BO3)(3) [(OH)(3.058)O0.75F0.192] for GVD and (X)(Na0.539Ca0.148 square K-0.297(0.017)) (Y)(Mg0.992V0.302Al1.039Fe0.582Mn0.011Ti0.068Cr0.007) (Z)(Al-5.339 V0.082Mg0.578) (T)(Si5.864Al0.136O18) (BO3)(3)[(OH)(3.209)O0.621F0.171] for BVD, respectively. Both types of V-rich oxy-dravite feature Al-Mg-V disorder between the Y and Z sites; V and Cr are preferentially located at the Y site. The V-rich oxy-dravite from Bitovanky has unusually high Ca contents and high proportion of vacancy at the X site. Most common trace elements include Ti, Cr, Mn, Zn, Sc, Ga, Sr, Ni, Sn, and LREE. High concentrations of V, Cr and Ti (as well as elevated Mn, Zn, Sc, Sn, and Sr) in the GVD and late BVD most probably result from hydrothermal dissolution of (Ti,V)-oxides and other accessory phases during a high-grade metamorphic overprinting of the host graphite quartzite assemblage.