Arthropod entombment in weathering-formed opal: new horizons for recording life in rocks.

Animal fossils preserved in various geological materials, such as limestone, claystone, or amber, provide detailed information on extinct species that is indispensable for retracing the evolution of terrestrial life. Here, we present the first record of an animal fossil preserved in opal formed by weathering with such high-resolution details that even individual cuticle hairs are observed. The fossil consists of the exoskeleton of a nymphal insect belonging to the order Hemiptera and either the family Tettigarctidae or the Cicadidae. This identification is based on anatomical details such as the tibial and femoral morphology of the forelegs. The exoskeleton of the insect was primarily zeolitized during the alteration of the host rocks and later sealed in opal deposited by silica-rich fluids derived from the continental weathering of the volcanic host rocks. Organic matter is preserved in the form of amorphous carbon. This finding makes opal formed by rocks weathering a new, complementary source of animal fossils, offering new prospects for the search for ancient life in the early history of Earth and possibly other terrestrial planets such as Mars, where weathering-formed opal occurs.

Reverse dichromatism in gem andalusite related to total pleochroism of the Fe2+-Ti4+ IVCT.

We document a dichromatism effect in gem andalusite that shifts from light brownish pink at low thickness to medium green at high thickness. This is roughly reverse to that usually observed in other dichromatic materials. We show that this is due to a very strong pleochroism in andalusite, when dichromatism in other materials is due to an alexandrite effect. Here, colors varies from a strongly absorbing orangey-brown in the c direction to much less absorbing medium green in a and b directions. The brown absorption is so strong that it becomes opaque (total absorption) for thickness above about 1.5 mm (this value may vary according to concentration of the chromophore and intensity of the light source), and hence does not contribute to coloration anymore. For such high thicknesses, only the a and b directions remain transparent and hence only the green component contributes to the color: the sample appears green. For lower thicknesses, the green color combines with the orangey-brown color that becomes transparent along the c direction, resulting in a light brownish pink. From trace elements analysis in zoned samples, we also confirm that the dark orangey-brown color is due to Fe2+-Ti4+, and bring evidences that the light green color is mostly due to Fe, either in the form of isolated Fe3+ or Fe2+-Fe3+ IVCT, with possibly some contributions from isolated Mn3+.

Investigation of hidden periodic structures on SEM images of opal-like materials using FFT and IFFT.

We have developed a method to use fast Fourier transformation (FFT) and inverse fast Fourier transformation (IFFT) to investigate hidden periodic structures on SEM images. We focused on samples of natural, play-of-color opals that diffract visible light and hence are periodically structured. Conventional sample preparation by hydrofluoric acid etch was not used; untreated, freshly broken surfaces were examined at low magnification relative to the expected period of the structural features, and, the SEM was adjusted to get a very high number of pixels in the images. These SEM images were treated by software to calculate autocorrelation, FFT, and IFFT. We present how we adjusted SEM acquisition parameters for best results. We first applied our procedure on an SEM image on which the structure was obvious. Then, we applied the same procedure on a sample that must contain a periodic structure because it diffracts visible light, but on which no structure was visible on the SEM image. In both cases, we obtained clearly periodic patterns that allowed measurements of structural parameters. We also investigated how the irregularly broken surface interfered with the periodic structure to produce additional periodicity. We tested the limits of our methodology with the help of simulated images.