Geologic Origin of Opals Deduced from Geochemistry
Résumé
Seventy-seven opals from 11 countries were characterized then chemically analyzed by inductively coupled plasma-mass spectrometry (ICP-MS), in order to establish the nature of the impurities, correlate the mode of formation with the physical properties of the opals, and evaluate the use of geochemistry for establishing geographic origin. The main impurities present were, in order of decreasing concentration, Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts were Ba, Zr, Sr, Rb, U, and Pb. For the first time, a distinction was found between various kinds of opal deposits according to their geochemistry. Compared to those from sedimentary deposits, volcanic opals were characterized by relative anomalies in Eu and Ce in their rare-earth element (REE) patterns. Opals from each volcanic deposit could be distinguished mostly according to their Ca content (or, if necessary, using Mg, Al, K or Nb). For example, volcanic opals from Ethiopia could be separated by a high Ca content, the presence of Nb, and a positive Ce anomaly in their REE patterns. The opals could also be separated according to their Ba content; sedimentary opals had Ba concentrations higher than 110 ppm, while volcanic opals were generally poor in Ba. The restricted range of all element concentrations for play-of-color opals around the world indicates that they must have very specific conditions of formation compared to common opals. An initial interpretation of the "crystallochemistry" of this amorphous material looked at the crystallographic site of certain impurities as well as their substitutions. The main replacement is the exchange of Si4+ by Al3+ and Fe3+. This modification involves a charge imbalance neutralized by the presence of additional cations (mainly Ca2+, Mg2+, Mn2+, Ba2+, K+, and Na+). It was also shown for the first time that the chemistry of an opal influences its physical properties. For example, greater concentrations of iron correlated to darker colors (from yellow to "chocolate brown"). This element inhibits luminescence, too, whereas only trace amounts of U induce a green luminescence (1 ppm, sometimes less). Host rocks from Mexico and Brazil were analyzed to understand the conditions of opal genesis and the mobilization of elements during the weathering process. The geochemistry of an opal depends mostly on the host rock, although it may be modified by processes of dissolution during the weathering.