The dehydroxylation behaviour of amphiboles

Abstract

Summary.The temperature at which dehydroxylation occurs in amphiboles is thought to be largely dependent on the type of cation that occupies the M a and M1 sites. Four amphibole types have been studied in which these sites are occupied, in various proportions, by Mg 2+ or Fe 2+. The temperature of dehydroxylation, in an inert atmosphere, is shown to rise with increasing Mg 2+ content in these sites.ECENTLY Hodgson (1965) has discussed the relationship between the chemical composition and the temperature of thermal decomposition of some crocidolites. He concluded from d.t.a, evidence that the position of the high temperature endotherm, that is, the temperature at which structural breakdown of the crocidolite occurs, was dependent on the Fe2+/Mg ~+ ratio, the temperature of the endotherm rising with rising Mg 2+ content.Croeidolite may be unique amongst the amphiboles in that the dehydroxylation process and the structural breakdown process are consecutive rather than concurrent (Freeman, 1962). Thus, in seeking correlations between the composition and thermal behaviour of amphiboles it is important to consider the temperature of dehydroxylation as well as the temperature of final structural breakdown.In the amphibole structure the hydroxyl group is bonded to the M 3 cation and to the M 1 cations (using Whittaker's (1949) notation). Whittaker (1949) found that in Bolivian crocidolite the MI-OI-I and Ms-OH bond distances were 2-10 A. The MI-OH bond distance was 2-10 3~ and the Ms-OH distance 2.07 3~ in eummingtonite (Ghose and I-Iellner, 1959). It is to be expected, then, that the strength of the M-OH bond, or the MO-I-I bond (where M represents the cation on the M a or M 1 sites), and consequently the temperature at which bond breakage occurs, will be dependent on the cations which occupy these sites. In order to show the etteet of site occupancy on stability the temperature of dehydroxylation of four amphibole types has been determined.Experimental results. The four amphibole types considered were:

TG/DSC study of the thermal behaviour of hazardous mineral fibres

“…According to Freeman [27] the decomposition temperatures of anthophyllite asbestos as well as tremolite asbestos were higher than those reported for amosite and crocidolite. The weak endothermic peaks at 246 and 509 °C (Fig.…”

“…DTG curve shows one main peak of maximum weight loss (2.30 %) at 868 °C due to anthophyllite dehydroxylation in correspondence with the shoulder at 824 °C on DSC curve. However, DDSC confirm the presence of an endothermic peak at 861 °C which corresponds to the structural breakdown of this phase [22,27] followed by recrystallization of enstatite as showed by the exothermic peak at 915 °C (Fig. 14).…”

“…As a matter of fact, only few studies were devoted to the thermal decomposition of pure amphibole asbestos up to 1100 °C and rare data report on the new phases appeared after thermal treatment. Thermal decomposition of fibrous amphibole minerals have been generally limited to asbestos minerals of commercial value or health concern such as crocidolite [20][21][22][23][24][25] and amosite [23,26,27]. Moreover, in some papers regarding the thermal analysis of amphiboles asbestos, a full picture of their thermal behaviour is not given [28].…”

Study on the thermal decomposition of crocidolite asbestos

“…In comparison with the K1 sample, the dehydroxylation temperature for the K2 samples is higher. This phenomenon can be explained as follows: according to Freeman [33], the temperature at which dehydroxylation occurs in amphiboles is largely dependent on the type of cation that occupies the M 3 and M 1 sites in the crystal structure, because in the amphibole structure the hydroxyl group is bonded to the cations that occupy these sites. The temperature of dehydroxylation is shown to rise with increasing Mg 2?…”

“…The authors in [32] showed thermal analysis curves of crocidolite asbestos and stated that the major mass loss occurs before 700°C. In turn, the authors in [33] stated that the dehydroxylation temperature of crocidolite asbestos in an inert atmosphere is 520°C. This temperature was connected with the maximum rate of mass loss due to the water release.…”