Nucleation of naturally occurring calcic amphibole asbestos.

This article proposes an initial model of natural asbestiform minerals growing in four stages. Structures dating from the early stages of the development were observed in the damaged zone surrounding meso-fractures, more particularly in microfractures, microcavities and microcracks that lie in front of and along mesofractures. This study is limited to calcium amphiboles cross-fibers, which develop from altered calcium amphiboles. The observations were made using PLM, with some using TEM. The samples are amphibolites, dolerites and skarns from France. All these rocks have in common that they have been exposed to hydrothermal circulation which gives them a propylite character. The earliest phenomenon was the development of metasomatic veins. In these veins, actinolite form pseudomorphs after hornblendes. The new amphiboles preserve the original morphology of hornblende, in particular the appearance of the cleavages. In the second stage, hydrothermal Fluid circulation promotes the development of subgrains (DSG) with boundaries generally parallel to the cleavage. Some sub-grains become thinner and more and more individualized due to dissolution by the hydrothermal fluid. The third stage is fracturing. The irregular ends of DSGs and amphibole debris can form the substrate of asbestiform mineral nuclei. Further dissolution of DSGs can also lead to the creation of substrates. The last stage is the nucleation and growth. The nuclei have a conical shape and variable widths, from a few microns to about ten microns. The basal parts of the asbestos minerals (BPAMs) extend the DSGs along the c axis. BPAMs have variable widths and can divide during their development at the level of transverse microcracks. BPAMs when not dividing have a morphology comparable to that of whisker nanocrystals synthesized using the vapor-liquid-crystal mechanism. The shape of the fragments from BPMAs is close to that of DSGs as both have variable widths and both have lengths controlled by microcracks.