The geochemistry of gem opals as evidence of their origin

Abstract

International audienceSeventy-seven gem opals from ten countries were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS) through a dilution process, in order to establish the nature of the impurities. The results are correlated to the mode of formation and physical properties and are instrumental in establishing the geographical origin of a gem opal. The geochemistry of an opal is shown to be dependant mostly on the host rock, at least for examples from Mexico and Brazil, even if modified by weathering processes. In order of decreasing concentration, the main impurities present are Al, Ca, Fe, K, Na, and Mg (more than 500 ppm). Other noticeable elements in lesser amounts are Ba, followed by Zr, Sr, Rb, U, and Pb. For the first time, geochemistry helps to discriminate some varieties of opals. The Ba content, as well as the chondritenormalized REE pattern, are the keys to separating sedimentary opals (BaN110 ppm, Eu and Ce anomalies) from volcanic opals (Bab110 ppm, no Eu or Ce anomaly). The Ca content, and to a lesser extent that of Mg, Al, K and Nb, helps to distinguish gem opals from different volcanic environments. The limited range of concentrations for all elements in precious (play-of-color) compared to common opals, indicates that this variety must have very specific, or more restricted, conditions of formation. We tentatively interpreted the presence of impurities in terms of crystallochemistry, even if opal is a poorly crystallized or amorphous material. The main replacement is the substitution of Si4+ by Al3+ and Fe3+. The induced charge imbalance is compensated chiefly by Ca2+, Mg2+, Mn2+, Ba2+, K+, and Na+. In terms of origin of color, greater concentrations of iron induce darker colors (from yellow to "chocolate brown"). This element inhibits luminescence for concentrations above 1000 ppm, whereas already a low content in U (=1 ppm) induces a green luminescence

Play-of-Color Opal from Wegel Tena, Wollo Province, Ethiopia

“…Among the trace elements, white opal contained abundant Ba (140-226 ppm [by weight]), Sr (127-162 ppm), and Rb (44-73 ppm). The orange fire opal portion of one sample showed Ba, Sr, and Rb contents consistent with those of opal-CT (Gaillou et al, 2008a). For the concentrations of other elements, see table 4 and the G&G Data Depository.…”

“…From our preliminary measurements of two samples, trace-element composition was comparable to opals from Mezezo (Gaillou et al, 2008a), with the following notable exceptions: Y (0.07-0.3 ppm), Nb (0.3-2.8 ppm), and Th (<0.2 ppm) were lower in the Wegel Tena opals, whereas Sc (1.5-2.3 ppm), Rb (39-73 ppm), Sr (72-162 ppm), and Ba (82-226 ppm) were higher. The enriched Ba concentration in opals from Wegel Tena is surprising.…”

“…Iron was not detected in the white samples. These compositions are typical for opal (Gaillou et al, 2008a). Among the trace elements, white opal contained abundant Ba (140-226 ppm [by weight]), Sr (127-162 ppm), and Rb (44-73 ppm).…”

“…The fluorescence is a mix of intrinsic silica surface-related violetish blue emissions and extrinsic uranium-related green emission. The latter is often more visible with short-wave UV (Fritsch et al, 2001;Gaillou, 2006;Gaillou et al, 2008a).…”

“…The enriched Ba concentration in opals from Wegel Tena is surprising. Looking at opals from elsewhere (Gaillou et al, 2008a), we see that those that formed in a volcanic environment always have low Ba (<100 ppm), whereas those from a sedimentary environment contain 100-300 ppm Ba. However, we note that the relatively high Ba contents in some Wegel Tena opals are consistent with the geologic environment that also resulted in the presence of Ba-Mn oxides in fissures mentioned above.…”