Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Vladimír Žáček, Jiří Frýda, Petrov, Hyršl

Tourmalines of the povondraite - (oxy)dravite series from the cap rock of meta-evaporite in Alto Chapare, Cochabamba, Bolivia

Journal of the Czech Geological Society, volume 45 (2000), issue 1-2, 3 - 12

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 discontinuously 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 Fe³⁺ (0.31-7.58 apfu) for Al (0.03-5.81 apfu) is represented by the exchange vector Fe³⁺(Al)_-1. Fe-rich members form a transitions between povondraite [NaFe₃³⁺ (Mg₂Fe₄³⁺) Si₆O₁₈ (BO₃)₃ (OH)₃ O] and hypothetical tourmaline [NaFe₃³⁺ (Mg₁ Fe₅³⁺) Si₆O₁₈ (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 is 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)Si₆O₁₈ (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.