『Abstract
Seventy-seven gem opals from ten countries were analyzed by inductively
coupled plasma-mass spectrometry (ICP-MA) 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 chondrite-normalized
REE pattern, are the keys to separating sedimentary opals (Ba>110
ppm, Eu and Ce anomalies) from volcanic opals (Ba<110 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
id 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.
Keywords: Opal; Chemical composition; Trace element analysis;
Genesis; ICP-MS』
1. Introduction
2. Background
3. Materials and methods
3.1. Materials
3.2. Methods
3.2.1. Determination of gemological properties
3.2.2. Chemical analyses
4. Results
4.1. Main features of opal trace element chemistry
4.2. Opal colored by other phases
4.3. Luminescence and chemical composition
4.4. Geochemistry of opal host rocks
4.5. Opal-A versus -CT
4.6. Play-of-color versus common opal
4.7. Volcanic versus sedimentary opal
4.8. Variations with geographic origin
4.8.1. Orange to brown opals
4.8.2. White opals
4.8.3. Pink opals
4.9. Further work
5. Conclusions
Acknowledgements
References