Record details

    Chemistry and origin of povondraite-bearing rocks from Alto Chapare, Cochabamba, Bolivia
Statement of responsibility
    Vladimír Žáček, Alfredo Petrov, Jaroslav Hyršl
Other titles
    Chemismus a původ hornin s povondraitem z Alto Chapare, Cochabamba, Bolívie
    Hyršl, Jaroslav
    Petrov, Alfredo
    Žáček, Vladimír
Source title - serial
    Journal of the Czech Geological Society
    Vol. 43, no. 1-2
    p. 59-67
    6 obr., 4 tab., 14 bibl.
    České resumé
    Zkr. název ser.: J. Czech geol. Soc.
Subject group
    alkalický typ
    chemické vlastnosti
Geographical name
    Alto Chapare (Bolívie)
Abstract (in english)
   Povondraite and schorl-dravite occur in an ancient, probably Cambrian, metaevaporite 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 concetrations of alkalis) and local oxygen activity probably palyed 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.X-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 slightly richer in CaO (0.34 wt.%) and the concentrations of both Na2O and K2O are about 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 patterms 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 CrO3 (0.0 - 1.36). povondraite compositions differ from the ideal end-member formula NaFe3 3 + Fe6 3 + (BO3)3 (Si6O18) (O, OH)4 by a constant presence f MgO ranging from 6 to 8 wt.% and potassium X-site occupancy of 25-48 at.wt.%.
   The highest 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
    Česká geologická služba
Contributor code
    ČGS (UNM)
Source format
Entered date
    18. 2. 2008
Import date
    8. 8. 2012