Recently, the author discovered new deposits of common gem opal (figure 1) in the hills near Lake Cuitzeo, in Mexico’s Michoacán State. These opals were embedded in volcanic andesitic rocks. They were generally characterized by a medium to light orange to brown color, with no visible inclusions. While these new deposits are considered “common opal” since the material does not show play-of color, some gemologists may also consider the specimens to be fire opal due to the orangy bodycolor. The samples were studied at the Institute of Earth Sciences at the University of Michoacán in Morelia.
Standard gemological testing yielded refractive indices between 1.440 and 1.457 and hydrostatic specific gravities (SGs) ranging from 2.11 to 2.14. The material was inert to both long- and short-wave UV radiation. These properties suggested common opal, which we later confirmed with scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman microprobe, and infrared spectroscopy.
Previous research (E. Fritsch et al., “Mexican gem opals: Nano-and micro-structure, origin of colour, comparison with other common opals of gemmological significance,” Australian Gemmologist, Vol. 21, No. 6, 2002, pp. 230–233) has shown that Mexican opals with play-of-color exhibit a higher degree of organization. In these specimens, nanoparticles measuring about 10–50 nm in diameter form pseudospheres (lepispheres) of appropriate size to diffract visible light (about 200 nm) in a matrix of less acid-soluble silica particles. When examined with a scanning electron microscope, fresh broken surfaces show no particular organization, but after etching with diluted hydrofluoric acid (10% vol. HF), the microscopic arrangement of spheres is revealed. There is a continuum of structures between opals with and without play-of color. In addition to the role of particle size, rounder spheres with a more organized structure display a more noticeable play-of-color with a wider range of colors.
From SEM and AFM images, it is clear that these newly discovered common opals from Michoacán are built from a random aggregation of small, near-spherical grains, averaging 60 to 100 nm in size. (The software we used was ImageJ, an open-source image processing program designed to generate scientific multidimensional images.) The apparent diameter of some grains actually ranges from about 120 to 130 nm (figures 2 and 3). The nanostructure of this common opal explains the absence of play-of-color. In this case, we can consider the following general explanations for the lack of play-of-color:
- The spheres do not have the same size (heterogeneous structure).
- The spheres are not perfectly spherical.
- The spheres are the same size but not well organized.
- The spheres are too small (<150 nm) or too large (>300 nm) to diffract light.
In some cases two or more of these conditions occur in the same sample.
All Mexican volcanic CT-opals have similar Raman spectra characterized by a very strong general band (apparent maximum toward 325 cm–1) that shows a complex structure with lines of weak to medium intensity (M. Ostrooumov et al., “Spectres Raman des opales: aspect diagnostique et aide a la classification,” European Journal of Mineralogy, Vol. 11, No. 5, 1999, pp. 899–908). According to theoretical calculations, the normal modes in the 300–350 cm–1 range consist mainly of Si-O-Si bending vibrations of ring atoms. Other principal bands in the Raman spectra of opal from these new deposits are found at about 800 and 960 cm–1, 1069–1086 cm–1, and 1600 and 3200 cm–1. These bands belong to α-tridymite, α-cristobalite, α-quartz, and groups of H2O and OH.
Infrared absorption bands were observed between 4000 and 400 cm–1, which is typical for all varieties of micro- and non-crystalline opals (C, CT, and A). The three strong bands near 1100, 790, and 480 cm–1 are common to all silicates with tetrahedrally coordinated silicon (M. Ostrooumov, “A Raman, infrared and XRD analysis of the instability in volcanic opal from Mexico,” Spectrochimica Acta, Part A, Vol. 68, No. 4, 2007, pp. 1070–1076). Broadly speaking, in opal-CT only localized Si-O-Si stretching and bending vibrations remain. The distinction between opal-CT and opal-A requires careful inspection of the frequencies of the three strong Si-O bands. In particular, the band at 790 cm–1 is always at higher frequency in opal-A than in opal-CT. Broad absorption bands between 3700 and 2700 cm–1 are due to fundamental O-H stretching vibrations. For example, a very broad band is present at around 3448–3458 cm–1, with a shoulder at about 3250 cm–1 that is generally considered to be related to hydrogen-bonded molecules of water.
Based on TEM results, we have been able to prove that the orange to brown bodycolor of Mexican common opal is due to nanoinclusions of an iron-containing material. It is probably related to hematite, which is often found associated with common opal in nodules. It typically appears as needles measuring 10 to 20 nm wide by 100 to 200 nm long, seen only with TEM.