Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Vladimír Žáček, Petrov, Hyršl

Chemistry and origin of povondraite-bearing rocks from Alto Chapare, Cochabamba, Bolivia

Journal of the Czech Geological Society, volume 43 (1998), issue 1-2, 59 - 68

Povondraite and schorl-dravite occur in an ancient, probably Cambrian, meta-evaporite known as the Locotal Breccia in Alto Chapare, Cochabamba Department, central Bolivian Andes. They originated, together with a variety of silicate minerals as reaction crust between dikes of highly alkaline volcanites, and B-rich evaporite. Chemical composition of the protolith (mainly various concentrations of alkalis) and local oxygen activity probably played decisive role in the formation either of povondraite or schorl-dravite. There are relatively small differences in SiO₂ (42-50 wt.%) and Al₂O₃ (12.2-15.5 wt.%), all rocks are relatively rich in TiO₂ (0.85-3.00 wt.%) and poor in CaO (0.0X-0.34 wt.%). The rocks rich in povondraite have extremely high concentrations of K₂O (10-12 wt.%), low Na₂O (0.4-0.7 wt.%), very low CaO(0.0X-0.13 wt.%) and increased Cr₂O₃ (about 0.1 wt.%), whereas the rock rich in dravite is slighlty richer in CaO (0.34 wt.%) and the concentrations of both Na₂O and K₂O area bout 4 wt..%. The concentrations of MgO (2.6-9.0 wt.%) and FeO^tot (5.4-13.0 wt.%) are variable and independent on given type of tourmaline.
Povondraite displays both continuous and oscillatory zoning patterns and chemical variability ranging in (wt.%): Fe₂O₃^tot = 29.5-42.4, Al₂O₃ = 2.39-10.90, TiO₂ = 0.38-3.48, Na₂O = 1.45-2.29, K₂O = 1.15-2.35, and strongly variable Cr₂O₃(0.0-1.36). Povondraite compositions differ from the ideal end-member formula NaFe₃³⁺Fe₆³⁺(BO₃)₃(Si₆O₁₈)(O,OH)₄ by a constant presence of MgO ranging from 6 to 8 wt.% and potassium X-site occupancy of 25-48 at.%. The higher TiO₂ and K₂O concentrations recorded (3.48 and 2.35 wt.%, respectively) represent the upper concentration levels of these oxides in tourmaline. Successive crystallization of povondraite (1) - intermediate dravite-povondraite (2) and dravite-schorl (3) gives evidence for decreasing oxygen activity during tourmaline crystallization.