Abstract
All six minerals defined as “asbestos” by the existing regulation on asbestos hazard, i.e., actinolite, tremolite, anthophyllite, crocidolite and amosite amphiboles, and the serpentine-group mineral chrysotile are typical constituents of mafic and ultramafic magmatic rocks of ophiolitic sequences. However, little is known about the presence and distribution of naturally occurring asbestos (NOA) in plutonic felsic rocks. The Isadalu magmatic complex outcropping in central Sardinia and belonging to the post-variscan Permian volcanic cycle, is described here as an interesting occurrence of fibrous amphiboles in granitoid rocks. Field work and collected mineralogical/petrological data show that NOA fibers from the Isadalu complex belong compositionally to the actinolite-tremolite series. They were generated by metasomatic growth on pristine magmatic hornblende, at ca. 470 °C at 1 kbar, during sodic-calcic hydrothermal alteration. In terms of environmental hazard, the Isadalu complex represents a high-value case study, since the actinolite-bearing felsic rocks outcrop in a strongly anthropized area. Here, towns with local and regional strategic infrastructures (dams, pipes, hydroelectric power plants, water supply, roads) have been developed since the last century, also using the granitoid asbestos-rich stones. The aim of this study is to demonstrate that NOA and relative hazard are not univocally connected to a restricted typology of rocks. This result should be taken into account in any future work, procedure or regulation defining asbestos occurrences in natural environments.
Highlights
The occurrence in nature of fibrous amphiboles, and chrysotile, i.e., the six silicate minerals defined by the existing regulation as “asbestos”(e.g., [1,2,3,4,5,6,7,8,9]) is increasingly attracting attention in environmental hazard evaluation [1,10]
Following the guidelines proposed in Gunter et al [1], we examined the habit and morphology of the actinolite/tremolite overgrowth with two independent methods based on the use of (i) a polarizing-light microscope and (ii) a scanning electron microscope
The Isadalu magmatic complex is composed of felsic hypabyssal dykes, stocks and minor extrusive bodies associated with mafic dykes
Summary
The occurrence in nature of fibrous amphiboles (anthophyllite, actinolite, tremolite, crocidolite and amosite), and chrysotile, i.e., the six silicate minerals defined by the existing regulation as “asbestos”(e.g., [1,2,3,4,5,6,7,8,9]) is increasingly attracting attention in environmental hazard evaluation [1,10]. The NOA (natural occurring asbestos) hazard arises when physical/mechanical processes produce airborne fibers that, due to their low density and small size, can be widely dispersed and, contaminate the atmosphere and water supplies e.g., [2,3] (Figure 1). Flowchart processesproducing producingasbestos asbestos fibres distribution in the environments (modified after [3,11]). Naturalprocesses processes(erosion (erosion and and transport) transport) as crushing, Natural as well well as as anthropic anthropicactivities activities(mining,. (mining, crushing, grinding and milling), represent an important source of asbestos. Both processes may induce a grinding and milling), represent an important source of asbestos Both processes may induce strong environmental environmental hazard ofof the concentration of of a strong hazard[2,3,11]
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