Minéralogie | EPFL Graph Search

La minéralogie est une science multidisciplinaire qui a pour objet les minéraux, leurs identifications, leurs caractérisations et descriptions, leurs analyses, leurs variétés et habitus, leurs classements, classifications et collections, leurs gîtologie, gisements et répartitions, leurs origines et leurs divers modes de formation, leurs usages par l'Homme, leurs intérêts pour la végétation ou la faune, leurs histoires dans l'univers des écrits ou discours savants ou des savoirs profanes ou traditionnels, les diverses modalités de connaissances... La minéralogie descriptive étudie les minéraux des milieux naturels, constituants diversement associés dans les roches. Elle peut être considérée, tout en gardant son statut autonome et réglementé au niveau international, comme une discipline de base de la géologie, une science auxiliaire de la pétrologie, ou encore une branche de la planétologie ou une matière d'approfondissement en prolongation de la chimie minérale dans le sens étendu où

Cataclastic fault rocks in underground excavations

Christoph Bürgi

The present study makes part of a research project on the geological and geomechanical characterisation of weak cataclastic fault rocks encountered in underground excavation sites in the Alps. The project is composed of a geological and a geomechanical part, split up in two PhD theses and realised in parallel at the Laboratory of Geology (GEOLEP) and the Laboratory of Rock Mechanics (LMR) at the Swiss Federal Institute of Technology in Lausanne. By their very contrasting and unfavourable mechanical behaviour in underground excavations, weak cataclastic fault zones influence directly the excavation method, the safety of the working site, the choice of tunnel support, the long-term behaviour and the costs of the infrastructure. The construction of galleries across cataclastic fault zones bears furthermore the potential risk to modify the local and regional hydrogeological regime. Weak cataclastic fault rocks show a large scatter of geological and geomechanical properties. Due to their low cohesion, they alter rapidly when exposed to weathering. They can hence be studied and sampled in natural outcrops and exploratory boreholes only with great difficulty. Analyses and laboratory tests are laborious and require special sample preparation techniques and equipments. For these reasons still little is known about their geological, hydrogeological and geomechanical behaviour. The purpose of this study is to develop a detailed and objective differentiation and characterisation method for weak cataclastic fault rocks. Classifications and terminologies of (weak) cataclastic fault rocks are in fact still controversial. After a review of fault rock definitions and rock deformation mechanisms, the term kakirite has been retained in agreement with Heitzmann (1985) and Wyder (1997) as a general designation for weak, cohesionless cataclastic fault rocks. This term has been approved recently by the Swiss Society of Engineers and Architects (SIA) to designate fault zones made up of "loose, structureless material" (SIA Norm 199, 1998). Cataclastic fault zones have been studied and sampled in galleries currently under construction in different geological and tectonic contexts in the Alps. Research has been concentrated mainly on fault zones occurring in quartzo-phyllitic host rocks. For comparison, fault rocks from greenstones and serpentinite have been analysed too. To improve the understanding of the geomechanical and hydrogeological behaviour of weak cataclastic fault rocks, a new quantitative characterisation method has been developed. In a simplified approach, the geomechanical behaviour of (fault) rocks is considered to be controlled by two factors, namely the: mineralogical composition (mwVh), and rock fabric (TC, MC) The proposed mineralogical and structural characterisation of cataclastic fault rocks is referring to the meso- to microscopic scale, respectively to a scale comparable with common geomechanical laboratory tests. Mineralogy is expressed quantitatively by means of the mean weighted Vickers hardness, mwVh (Calembert et al., 1980a). The relative proportions of the principal rock forming minerals have been determined by semi-quantitative XRD-analyses. The mineralogical composition of kakirites, adjacent host rock samples and sieved kakirite grainsize fractions has been determined. Comparing the composition of the different grainsize fractions a clear dependency of grainsize on mineralogical composition has been found in kakirites. Quartz content is in general considerably higher in the clast than in the matrix fraction. The later is in fact composed mainly of phyllosilicates. This mineralogical grainsize fractionation is interpreted to be controlled by the original host rock fabric, the strength contrast between the involved mineral phases and alteration processes. Except kaolinite (Varzo), no other alteration mineral phases have been identified. The presence of (swelling) clay-minerals in very low proportions which cannot be detected by the performed whole rock XRD-analyses cannot be excluded however. Kakirite samples sieved for XRD-analyses show an extended grainsize distributions with in general high proportions of silt/clay grainsize fraction (mean 45%). This may explain the often observed very low permeability of kakirite zones. Their hydrogeological role is illustrated in the last part of this work by the means of a case history of a fault zone crossed by the gallery of Cleuson-Dixence. It demonstrates the often observed groundwater flow barrier-effect of cataclastic fault zones and its impact on the excavation work and the local hydrogeological regime. Rock fabric of kakirites has been characterised by thin section analyses with subsequent semiautomatic image analysis. It is based on a two-phase "clast-matrix" model, considering kakirites to be composed of varying proportions of "hard" clasts and "weak", fine-grained matrix. Clast properties (density, shape and relative orientation) are quantified by the texture coefficient (TC) of Howarth & Rowlands (1987). Pre-existing discontinuities in the matrix are characterised by a newly developed matrix coefficient (MC), involving the discontinuity density, roughness, mean length and relative orientation. The combined structural approach involving the MC and TC has shown to be appropriate to characterise and differentiate a wide range of weak cataclastic fault rocks by their microscopic rock fabric. Two opposite kakirite fabric types can be distinguished: first, a granular fabric, characterised by clasts taken in a fine-grained, isotropic matrix. Second, a discontinuity dominant, often schistose fabric wherein no clasts can be identified. A gradual transition is observed in-between these two end-member types. In order to assess the validity of the developed characterisation method, a first attempt of correlation has been made between the mineralogical-structural parameters and corresponding rock strength of kakirites tested by Habimana (1999). The found correlation is a strong motivation to continue the started research on cataclastic fault rocks with the presented mineralogical and structural approach.

EPFL1999

The modern analogue of a Cretaceous coral with a calcitic skeleton

Anders Meibom, Jaroslaw Hubert Stolarski

It was argued that, in contrast to all known modern scleractinian corals that form aragonite skeletons, the original mineralogy of the Cretaceous "Coelosmilia" (ca. 70-65 Ma) was calcite during a period when the Mg2+/Ca2+ ratio of the seawater was presumably the lowest in Phanerozoic (Science 2007 318: 92-94). This is consistent with the idea that scleractinians extend only partial control over their skeletal mineralogy, because calcite is the preferred calcium carbonate polymorph (over aragonite) during abiotic CaCO3 precipitation in low Mg2+/Ca2+ seawater conditions. However, subsequently it was found that a number of different Cretaceous corals formed purely aragonite coralla in the same low Mg2+/Ca2+ conditions, suggesting instead that biological control over the skeletal mineralogy differs between taxonomically different corals (Geology 2017 45:319-322) and posing the question why was "Coelosmilia’s" skeletal mineralogy affected by the seawater chemistry changes and not that of the other corals. An unusual feature of "Coelosmilia" is the infilling of a significant portion of the calice with dense, banded skeletal deposits. They are crystallographically continuous with earlier formed layers of septa, wall and axial structures, but show distinct biogeochemical signatures. To date, no modern analogs of such corals were known, and it was tempting to argue that this skeletal infilling represents secondary, purely abiotic deposits (similar to marine cements, which would be calcitic in Cretaceous seas). Here we document a living scleractinian coral that exhibits a structural organization identical to Cretaceous "Coelosmilia", including dense calicular infilling that is a species-specific feature. Surprisingly, the inner parts of these skeletal elements are composed of fibrous calcite, whereas the rest of the skeleton is aragonitic. We suggest that this modern coral is phylogenetically related to "Coelosmilia" and capable of formation of strictly biologically controlled skeletal mineralogy, which can include both aragonitic and calcitic structures. This original bipartite skeletal organization may explain several diagenetic phenomena that we observe in "Coelosmilia", and is highly relevant for the proper geochemical sampling of corals with such skeletal organization.

2019

Microbial community evolution in a marine shore porphyry tailings deposit throughout wetland remediation

Nouhou Diaby, Christof Holliger, Pierre Rossi

Background Two porphyry copper mines located in southern Peru initiated the discharge of mine wastes (tailings) for a final shore deposition in the Bay of Ite (Pacific Ocean). 780 millions metric tons of tailings were deposited, which formed a delta with a surface of ca. 16 km2. A low-pH oxidation zone characterized by the presence of iron hydroxides developed on the surface, along with a strong build-up of efflorescent salts rich in heavy metals. Objectives A 3 years investigation was carried out in order to understand the biogeochemical processes resulting from the remediation of the oxidizing tailings by the implementation of a wetland cover1. Methods Analysis of the bacterial communities was carried out by molecular techniques (T-RFLP and cloning-sequencing). A combination of physical, chemical and mineralogical methods was applied for the characterization of solid tailings and water samples. Results The non-remediated tailings was characterized by a low-pH oxidation zone (pH 3 – 4), in which the oxidative dissolution of sulphide was identified as the dominant process, carried out mainly by Leptospirillum spp, Acidithiobacillus spp and Sulfobacillus spp. This dissolution led to an increase in metal concentrations in the pore water. Below the oxidation zone, the neutrophilic primary zone was characterized by SO42- and Fe reductions induced by sulphate- and iron-reducing bacteria (SRB and FeRB). Drastic changes were observed along the remediation process, as the heavy metal concentrations decreased progressively close or below the detection limits together with the gradual installation of a near neutral pH and more reducing conditions. SO42- and Fe reductions were still observed in the deep section of the remediated tailings. Close to the surface, the presence of lithoautotrophic neutrophilic iron oxidizing bacteria such as Mariprofundus ferroxidans, Galionella spp, Leptotrix mobilis suggested that iron oxidation played an important role in the iron cycle. Other ubiquitous bacteria related to fresh water and marine environment were also detected in the wetland. Conclusions The implementation of a wetland promoted neutralization of the acid mine drainage and induced favourable geochemical conditions for the development of both SRB and FeRB, which lead to the metal removal from solution. References Diaby, N. 2009. Biogeochemical evolution of a marine shore tailings deposit during bioremediation. Ph. D. Thesis, University of Lausanne, Switzerland. 217 pp.

2009