Morphological and chemical characterization of asbestos fibers in solid rocks: Towards an in-situ and combined analytical approach

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. 
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.
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.
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.