Record details

Title
    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
Author
    Frýda, Jiří
    Hyršl, Jaroslav
    Petrov, Alfred
    Žáček, Vladimír
Language
    anglicky
Source title - serial
    Journal of the Czech Geological Society
Vol./nr.
    Vol. 45, no. 1-2
Pages
    s. 3-12
Year
    2000
Notes
    7 obr., 1 fot., 4 tab.
    České resumé
    il.
    Obsahuje bibliografické odkazy
    Zkr. název ser.: J. Czech geol. Soc.
Subject group
    chemismus minerálů
    dravit
    evapority
    metamorfóza
    složení
    turmalín
    zóna oxidační
Geographical name
    Alto Chapare (Bolívie)
    Bolívie
Keyword
    Alto
    Bolivia
    Cap
    Chapare
    Cochabamba
    Meta-evaporite
    Povondraite-(oxy)dravite
    Rock
    Series
    Tourmalines
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
Contributor
    Česká geologická služba
Contributor code
    ČGS (UNM)
Source format
    U
Entered date
    19. 3. 2007
Import date
    8. 8. 2012