Chemistry and origin of povondraite-bearing rocks from Alto Chapare, Cochabamba, Bolivia
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 FeOtot (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.%): Fe2O3tot = 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.
SNIP (Scopus, 2015): 0.700
IF (ISI, 2015): 1.326
Policy: Open Access