In situ study of nano-chrysotile growth

In the past decades, the extraordinary properties of asbestos fibres (heat resistance, non-combustibility, unusual tensile strength and resistance to selective chemical attack) favoured chrysotile to be widely used in practical industrial applications . However, health hazards associated with natural asbestos have been well established and are clearly documented in the medical and general health literature (pulmonary fibrosis, lung cancer and pleural mesothelioma are common diseases associated to inhalation of natural asbestos fibres). These pathologies cannot be ascribed exclusively to the shape of asbestos fibres (dangerous are those with length/diameter > 3): the chemical, structural and physico-chemical properties of the material play in fact a key role in the interaction with biological systems. The crystalline cell of Mg3Si2O5(OH)4 chrysotile (accounting for approximately 90% of asbestos world production) is composed of sheets of Si-centred tetrahedra in a pseudo-hexagonal network joined to brucitic sheets. Those layers are curved concentrically or spirally, usually around the x axis (clinochrysotile and orthochrysotile) and seldom around the y axis (parachrysotile), into a tubular structure (rolls) of about 22-27 nm in diameter. These rolls are hollow because the layers energetically cannot withstand a very small curvature. The curvature is necessary to accommodate the dimensional misfit (stress) between the octahedral brucitic layer (a=0.54 nm, b=0.94 nm) and the tetrahedral layer (a=0.50 nm, b=0.87 nm). Pure and stoichiometric chrysotile nanocrystals have been synthesised as unique phase under controlled hydrothermal conditions. The absence of any poisoning with foreign ions, especially those considered responsible for asbestos toxicity, suggests the synthetic chrysotile nanocrystals as possible standard reference sample to investigate chemico-physical proprieties and interactions with biological systems of mineral chrysotile. Moreover, the availability of stoichiometric chrysotile nanocrystals with constant morphology and structure allows to consider the nanotubes as possible starting material to prepare e.g. quantum wires with optical-electron properties or fibrous materials for innovative applications. A peculiar tube-in-tube uniform microstructure is obtained and there is no trace of any planar product. Modifications of the basic reaction scheme led to the formation of lizardite, the planar analogue of chrysotile. The main modification concern the Si/Mg molar ratio and precisely the Si/Mg,Al molar ratio. In this study we propose an in situ analysis of a set of hydrothermal reactions carried out in glass capillaries at increasing concentration of Aluminum, as to follow the transformation kinetics and establish the reaction path. The experiment was conducted in glass capillary at 300 °C and at the saturated vapour pressure on the ID11 beamline of the ESRF synchrotron. Powder diffraction patterns were collected at regular intervals of time up to the point where chrysotile forms. The combination of pattern fitting and line profile analysis, as well as the use of the DIFFaX+ code (devised for the structural/microstructural analysis of layered materials), allowed the modification of the structure and therefore the reaction path to be followed.