The geochemistry of gem opals as evidence of their origin

被引:80
|
作者
Gaillou, Eloise [1 ,2 ]
Delaunay, Aurelien [1 ]
Rondeau, Benjamin [3 ]
Bouhnik-le-Coz, Martine [4 ]
Fritsch, Emmanuel [1 ]
Cornen, Guy [3 ]
Monnier, Christophe [3 ]
机构
[1] Univ Nantes, Nantes Atlantique Univ, CNRS, Inst Mat Jean Rouxel IMN,UMR 6502, F-44000 Nantes, France
[2] Smithsonian Inst, Dept Mineral Sci, Washington, DC 20560 USA
[3] Univ Nantes, Nantes Atlantique Univ, Lab Planetol & Geodynam, UFR Sci Tech, F-44322 Nantes, France
[4] Lab Geochim, F-35042 Rennes, France
关键词
Opal; Chemical composition; Trace element analysis; Genesis; ICP-MS;
D O I
10.1016/j.oregeorev.2007.07.004
中图分类号
P5 [地质学];
学科分类号
0709 ; 081803 ;
摘要
Seventy-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 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 Mail) 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. (C) 2008 Published by Elsevier B.V.
引用
收藏
页码:113 / 126
页数:14
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