The geochemistry of gem opals as evidence of their origin

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

An examination of the Napoléon diamond necklace

International audienc

Boron in natural type IIb blue diamonds: Chemical and spectroscopic measurements

International audienc

Luminescence of opals: a witness to their geochemistry

International audienc

Cathodoluminescence of Natural, Plastically Deformed Pink Diamonds

International audienc

Geologic Origin of Opals Deduced from Geochemistry

1 p.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

Gem News International: An accidental corundum and lead glass triplet?

International audienc

Study of the Blue Moon Diamond

International audienceThe Blue Moon diamond, discovered in January 2014 at the historic Cullinan mine in South Africa, is of significance from both trade and scientific perspectives. The 29.62 ct rough yielded a 12.03 ct Fancy Vivid blue, Internally Flawless gem. The authors were provided the opportunity to study this rare diamond at the Smithsonian Institution before it went on exhibit at the Natural History Museum of Los Angeles County. Infrared spectroscopy revealed that the amount of uncompensated boron in the diamond was 0.26 ± 0.04 ppm, consistent with measurements of several large type IIb blue diamonds previously studied. After exposure to short-wave ultraviolet light, the Blue Moon displayed orange-red phosphorescence that remained visible for up to 20seconds. This observation was surprising, as orange- red phosphorescence is typically associated with diamonds of Indian origin, such as the Hope and the Wittelsbach-Graff. Time-resolved phosphorescence spectra exhibited peaks at 660 and 500 nm, typical for natural type II blue diamonds.As with most natural diamonds, the Blue Moon showed strain-induced birefringence

Characterization of electronic properties of natural type IIb diamonds

International audiencePrecision admittance spectroscopy measurements were carried out over wide temperature and frequency ranges for a set of natural single crystal type IIb diamond samples. Peaks of conductance spectra vs. temperature and frequency were used to compute the Arrhenius plots, and activation energies were derived from these plots. The capacitance-voltage profiling was used to estimate the majority charge carrier concentration and its distribution into depth of the samples. Apparent activation energies between 315 and 325 meV and the capture cross section of about 10− 13 cm2 were found for samples with uncompensated boron concentrations in the range of 1 to 5 × 1016 cm− 3 (0.06–0.3 ppm). The obtained boron concentrations are in good coincidence with FTIR results for the samples. Also, a reason for the difference between the observed admittance activation energy and the previously reported ionization energy for the acceptor boron in diamond (0.37 eV) is proposed

The nanostructure of fire opal

International audienceire opal, a transparent orange variety of opal, which does not diffract visible light, is built from the random accumulation of granular particles of hydrated silica, about 20 nm in size. This opal variety does not present the structure most commonly associated with precious opal, that is, a regular three-dimensional network of amorphous silica spheres about 200 nm in diameter. About 60 samples (from Mexico, Brazil, Kazakhstan, Ethiopia, Tanzania, Slovakia and USA) were documented using scanning electron and atomic force microscopy. This work demonstrates that nanograins are the elementary building blocks of this variety of opal