Asbestos consists in natural minerals crystallized in a specific habit and possessing in particular properties. In the case of Naturally Occurring Asbestos, usual methods applied to the identification of mineral fibers and the determination of their possible asbestiform nature seems not efficient, especially in the case of mineral fibers included in mineral matrix.We present a new in-situ method based on the use of confocal Raman-in-SEM imaging implemented in a Scanning Electron Microscope as an efficient method for in-situ mineralogy. The limitation of conventional methods is discussed. We applied 2D-Raman imaging to the identification of sub-micrometric fibers included in different mineral matrix. We were able to identify actinolite fibers down to 400 nm in diameter, included in feldspar, quartz and/or calcite matrix. HighlightsNew methods are requested for characterizing asbestos fibers in a mineral matrix SEM-Raman imaging is efficient for characterizing mineral fibers in-situ Confocal Raman imaging makes 3D analysis possible 3D analysis provides information on the aspect ratio and volume fraction of asbestos Fibers thinner than 400nm can be identified by confocal Raman in SEM ( = 532 nm) Keywords in-situ asbestos diagnosis; Raman-in-SEM imaging; naturally occurring asbestos; 3D analysis; fibrous amphiboles;
<p>The event geological maps consist in innovative numerical maps that were just designed and produced for the first time, as part of the RGF (&#8220;French Geological Referential&#8221;) mapping program in the Pyrenees. Rocks acquire their mineralogical, structural and textural characteristics through a complex geological history reflecting successive stages of transformation (i.e. metamorphism, deformation, alteration&#8230;), so called &#8220;geological events&#8221;. Classical geological maps can only represent some of these events.&#160; In the Pyrenean orogenic belt, which results from a polyphase tectono-metamorphic history over 600 Ma (from Precambrian to present), 3400 geological events were identified. Such geological events were classified by types (e.g. deposit, volcanism, intrusion, metamorphism, weathering, hydrothermal alteration&#8230;) and time periods. They were referenced into a database and associated to mapped features (120,000 polygons and lines), coming from a compilation of 60 geological maps at 1: 50,000 scale.</p><p>In the Pyrenees, Naturally Occurring Asbestos (NOA) mostly occur in specific lithologies such as ultrabasic, basic and intermediate plutonic rocks, and meta-limestones. These rocks may be affected by different metamorphic events (i.e. hydrothermal alteration, greenschist and/or HT-LP regional metamorphism, contact metamorphism). We performed a GIS treatment to produce a predictive map of potential NOA hosting lithologies. This treatment crosses lithological and selected geological event informations (e.g different metamorphic and alteration events).</p><p>Subsequent geological field investigations with associated sampling and laboratory analyses (combining optical microscopy, microprobe and SEM analyses) allowed us to identify and characterize fibrous and asbestiform mineralogical species. Results of this work particularly emphasize: (i) the importance of actinolite-asbestos in doleritic rocks, and (ii) the occurrence of fibrous actinolite/tremolite in different marbles and skarns. Finally, we present a 1: 50,000 scale map of potential NOA occurrences in the Pyrenees.</p><p>Conversely, field observations allowed us to improve both the lithostratigraphic and the event geological maps, in particular with the identification of geological domains where intense hydrothermal alteration was not previously mapped. All the data (maps of potential NOA occurrences, field observations and results of laboratory analyses) are stored in a geospatial database, partly accessible to the public. This work illustrates a possible use of geological event maps as a powerful innovative and predictivity tool. This approach will be useful in the context of the evolution of French regulations now imposing the search for asbestos before all types of works in natural environments.</p>
<p>Asbestos is a commercial term which refers to six minerals that crystallize as fibrous bundles made of very thin and easily separable fibrils. Asbestos fibers have been exploited for a long time and voluntary added in a very large set of manufactured products. In France, asbestos is prohibited since an official decree published in 1997 that prohibits the manufacture, processing, sale and import of asbestos. The asbestos ban has been the subject of an European directive published in 1999.&#160;<br />Following this ban, a standard was defined in order to specify the sampling, preparation and identification methods for asbestos fibers in samples of commercial origin (ISO 22262-1). For natural materials, no specific analytical protocol is currently defined in France. Searching for asbestos in a rock sample, the commonly used protocols require the reduction of the sample, the grinding of a sub-sample (1 to 2 g) and its calcination in order to eliminate organic matter, then an acid attack to dissolve some constituents (calcite, gypsum). The final test portion (~ 20 mg) is mixed in water, stirred using ultrasound, filtered through a metallized membrane and covered with a new layer of carbon before it can be examined using a transmission electron microscope.<br />The protocols currently used are long and complex and require the grinding of the sub-sample. This grinding operation is a critical step because it can lead, starting from non-asbestiform minerals, to the artificial formation of more or less fine and elongated fibriform particles (cleavage fragments), quite similar in some cases to asbestos fibers. Grinding is therefore an operation liable to affect the quality of the final diagnosis.<br />The new protocol presented here was built with the aim of developing an analytical approach specific to coherent rock samples. This protocol does not involve the grinding of the sample and allows the in-situ morphological and chemical characterization of fibrous minerals. It is based on the use of combined analytical techniques (MOLP, EPMA, FESEM-EDS, FIB-SEM, and confocal RAMAN in SEM) from a single support corresponding to a polished thin section. This protocol allows to observe the natural morphologies of the fibers, to measure their dimensions, to characterize the relationships between fibers and the other mineralogical constituents while preserving the texture of the rock and to acquire precise chemical analyzes of the fibers. It also overcomes problems related to the grinding of the sample and the formation of cleavage fragments. This protocol has been tested through the study of several types of massive rock samples. It provides a representative and reliable in-situ diagnosis of the initial state of the fibers in solid rocks.</p>
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