Record details

    Tourmalines of the povondraite-(oxy)dravite series from the cap rock of meta-evaporite in Alto Chapare, Cochabamba, Bolivia
Statement of responsibility
    Vladimír Žáček, Jiří Frýda, Alfred Petrov, Jaroslav Hyršl
Other titles
    Turmalíny řady povondrait-(oxy)dravit ze sádrovcového klobouku metaevaporitu v Alto Chapare, provincie Cochabamba, Bolívie
    Frýda, Jiří
    Hyršl, Jaroslav
    Petrov, Alfred
    Žáček, Vladimír
Source title - serial
    Journal of the Czech Geological Society
    Vol. 45, no. 1-2
    s. 3-12
    7 obr., 1 fot., 4 tab.
    České resumé
    Obsahuje bibliografické odkazy
    Zkr. název ser.: J. Czech geol. Soc.
Subject group
    chemismus minerálů
    zóna oxidační
Geographical name
    Alto Chapare (Bolívie)
Abstract (in english)
   An almost continuous series of Al-deficient ferrian tourmalines, corresponding mainly to povondraite-"oxy-dravite"-dravite solid solutions, was found at the type locality of povondraite (Alto Chapare, Cochabamba Department, Bolivia). These tourmalines form a component of crystalline crusts overgrowing silicate "xenoliths" or loose crystals enclosed in the meta-evaporite cap rock. Tourmaline crystals have highly variable morphology, correlated with chemical composition from acicular through prismatic to stubby. Some crystals are disconinuously zoned with Al-enriched cores. Tourmalines are typically sodic with very low Ca and low X-site vacancy. On the other hand, Fe-rich, Al-poor tourmalines have elevated K content (up to 0.6 apfu) in a trend toward a new tourmaline species. All tourmalines are magnesian (Mg=1.32-2.67 apfu) and display extreme compositional range.
   The most prominent substitution of Fe 3+ (0.31-7.58 apfu) for Al (0.03-5.81 apfu) is represented by the exchange vector Fe 3+ (Al)-1.Fe-rich members form a transitions between povondraite [NaFe3 3+ (Mg2Fe4 3+) Si6 O18 (BO3)3(OH)3 O] and hypothetical tourmaline [NaFe3 3+ (Mg1 Fe5 3+) Si6 O18 (BO3)3 (OH)2 O) O]. In accordance with this substitution mechanism, the Oh calc ranging from 1.78-3.52 pfu is positively correlated with Mg and inversely proportional to the sum of R3+ (=Al+Fe3+). Incorporation of Ti4+ in tourmaline solid solution is coupled with a decrease of OH and an increase of O. The highly variable Ti (0.0-0.56 apfu) negatively correlates with the sum of R3+. However, there also is an excellent negative correlation (R=0.99) among R3+ and Mg+Ti. This consistent with exchange vectors R3+O (Mg OH)-1 and TiOR3+ -1(OH)-1. Such substitutions suggest a new possible Ti-bearing tourmaline Na(TiR3+ 2)(Mg2R3+4) Si6 O18 (BO3)3 ((OH)2O)O.
   The povondraite-(oxy)dravite solid solutions are known in meta-evaporite tourmalines but only the tourmalines from the Bolivian Alto Chapare attained such an extensive compositional range
    Česká geologická služba
Contributor code
    ČGS (UNM)
Source format
Entered date
    19. 3. 2007
Import date
    8. 8. 2012