# A Systematic Study of the Cryogenic Milling of Chrysotile Asbestos
## 4 Discussion
The rate-limiting step of the reaction was found to be one-dimensional  
Figure 6: Percentage of long (L > 5 \(\upmu\)m) and short (L < 5 \(\upmu\)m) chrysotile fibres vs. milling time (from 0 to 40 min). (a) Chrysotile fibre length fraction > 5 \(\upmu\)m. (b) Chrysotile fibre length fraction < 5 \(\upmu\)m.  
diffusion with an instantaneous or decelerator rate of nucleation [54]. This result was explained by the peculiar structure of chrysotile: this mineral is a hydrated magnesium silicate with the ideal chemical formula Mg3(OH)4 Si2O5. The double-layered structure is composed of an octahedral Mg(OH)2 sheet (O) bonded to a tetrahedral SiO2 (T) sheet, with the Mg cations fully occupying the octahedral cation sites. The fibrous nature of chrysotile is due to a lattice misfit between the two layers, giving rise to a strain gradient that leads to self-rolling and the formation of a fibre structures with the Mg(OH)2 surface exposed externally. [PERSON] et al. [54] concluded that the most probable one-dimensional escape paths for the water molecules were along the fibre axis. In this work, the FTIR analyses suggested that this process does not occur during cryogenic milling (Figure 5). As shown in Figure 5b, the FTIR spectrum of chrysotile milled for 40 min preserved the characteristic bands assigned to the OH groups (i.e., 3643 and 3682 cm\({}^{-1}\)). In the literature, various methodologies have been proposed to produce size-selected fibre samples as discussed in the introduction. Two-step procedures are generally applied, including comminution followed by size separation. Currently, gravitational wet separation of hand-milled fibres is the most common method of obtaining groups of fibres differing in length. This process is relatively time-consuming due to slow settling velocity (ca. 3 h for asbestos fibres according to [55]) and the fact that the solvent must be evaporated from the sorted fibres prior to use. The results shown in this work evidenced that the two desired size classes for _in vitro_ toxicology studies (i.e., L \(<\) 5 um and L \(>\) 5 um), could be directly obtained by cryo-milling without a subsequent separation step [25]. In fact, milling for 40 min directly delivered a batch consisting only of short fibres (95% \(<\) 5 um), whereas 10 min of cryo-milling resulted in a batch containing mainly long fibres (mean L = 14.32 um and 90% of fibres with L \(>\) 5 um). These two sets of fibres are suitable as they are, without the need for an additional size separation step. Another important advantage of the cryo-milled fibres (especially considering the upper length size class) was that stable fibre suspensions, ready to be added to the culture media for _in vitro_ testing, could easily be prepared by ultrasonification of the cryo-milled fibres in a biological solution. In fact, even though most of the fibres were still arranged in bundles following mechanical pre-treatment under cryogenic conditions, they were easy to split by ultrasonic treatment. This was shown to be difficult to achieve with the very long and curly pristine fibres that, instead, tended to agglomerate in large clusters (not shown here).