Sur la présence de fergusonite et d'euxénite en Guyane

Cruys A., Parfenoff Alexandre, Fauquier Daniel. Sur la présence de fergusonite et d'euxénite en Guyane. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 87, 4, 1964. pp. 625-626

Stable and ultrastable heavy minerals of alluvial to nearshore marine sediments from Central Portugal: Facies related trends

The Pliocene to Pleistocene sedimentary record from the western Iberia Coastal Margin is a thin succession (maximum thickness around 70 m) derived from the metamorphic and igneous Variscan Iberian Massif and from older sedimentary deposits. It comprises a wide variety of facies representative of deposition in environments ranging from inner shelf to alluvial fan. The facies are stacked in an overall regressive sequence. This sequence may be divided into lower order sequences with thin transgressive portions (not always present) and thicker prograding portions. Given the limited burial diagenetic transformation, the differences in heavy mineral (HM) assemblage can be explained by provenance, hydraulic segregation and chemical and mechanical selection. Transparent heavy minerals susceptible to acid treatment were not considered in this heavy mineral analysis. After the acid treatment, the HM signal is relatively monotonous. Except for the mica-rich assemblages in alluvial facies fed from the Variscan Iberian massif by short streams, the HM assemblages are dominated by tourmaline, andalusite, zircon and staurolite. However, it is still possible to recognize two opposite trends in HM composition linked to facies evolution. The first trend is widespread along the coastal margin, although typical of the earlier deposits. It is characterized by a vertical increase in staurolite and andalusite proportions coincident with prograding and regressive facies evolution. Relative sediment starvation during retrogradation, when compared to progradation, favoured the recycling of previous deposits, leading to higher proportions of ultrastable minerals (tourmaline, zircon and rutile). Hydraulic segregation in the nearshore zone also contributes to increased zircon content in the very fine sand fraction. The second trend is observed in stratigraphically higher sediments, deposited when environments were displaced basinward. This trend is common in relatively uplifted western sectors of the coastal margin, where sand and gravel dominate the record. It is characterized by a vertical increase in the content of ultrastable minerals (mostly tourmaline and zircon) throughout the regressive sequence. It may be explained by alluvial storage and the recycling of previous nearshore and coastal plain facies. Recycling, favoured by relative sediment starvation or low accommodation conditions, plays a major role in these HM assemblages.http://www.sciencedirect.com/science/article/B6V6X-4NKJ0HY-2/1/cba0fe9da6b5d36a639b794ef722b4a

Intricate tunnels in garnets from soils and river sediments in Thailand - Possible endolithic microborings

Garnets from disparate geographical environments and origins such as oxidized soils and river sediments in Thailand host intricate systems of microsized tunnels that significantly decrease the quality and value of the garnets as gems. The origin of such tunneling has previously been attributed to abiotic processes. Here we present physical and chemical remains of endolithic microorganisms within the tunnels and discuss a probable biological origin of the tunnels. Extensive investigations with synchrotron-radiation X-ray tomographic microscopy (SRXTM) reveal morphological indications of biogenicity that further support a euendolithic interpretation. We suggest that the production of the tunnels was initiated by a combination of abiotic and biological processes, and that at later stages biological processes came to dominate. In environments such as river sediments and oxidized soils garnets are among the few remaining sources of bio-available Fe2+, thus it is likely that microbially mediated boring of the garnets has trophic reasons. Whatever the reason for garnet boring, the tunnel system represents a new endolithic habitat in a hard silicate mineral otherwise known to be resistant to abrasion and chemical attack. The authors acknowledge funding from the Swedish Research Council (Contracts No. 2007-4483 (SB), 2010-3929 (HS), 2012-4364 (MI), and 2013-4290 (SB), 2015-04129 (SS)), Danish National Research Foundation (DNRF53), and Paul Scherrer Institute (20130185) (MI) as well as Swedish National Space Board (Contract No. 83/10 (MI), 121/11 and 198/15 (SS)).</p