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

Abstract

Two types of V-rich dravitic tourmaline (oxy-dravite to dravite) distinct in their color, morphology, paragenesis and composition from graphite quartzite at Btovnky near Teb, 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 (Na 0.534 Ca 0.284 0.164 K 0.018 ) Y (Mg 1.461 V 0.679 Al 0.771 Fe 0.042 Mn 0.003 Ti 0.043 ) Z (Al 5.074 V 0.237 Mg 0.689 ) T (Si 5.782 Al 0.218 O 18 ) (BO 3 ) 3 [(OH) 3.058 O 0.75 F 0.192 ] for GVD and X (Na 0.539 Ca 0.148 0.297 K 0.017 ) Y (Mg 0.992 V 0.302 Al 1.039 Fe 0.582 Mn 0.011 Ti 0.068 Cr 0.007 ) Z (Al 5.339 V 0.082 Mg 0.578 ) T (Si 5.864 Al 0.136 O 18 ) (BO 3 ) 3 [(OH) 3.209 O 0.621 F 0.171 ] 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 Btovnky 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.