Thermochemistry of some pyroxenes and related compounds

Calorimetric investigation of nanotubular hydrosilicates in the Mg3Si2O5(OH)4-Ni3Si2O5(OH)4 system

“…In our previous study [7], we used the same method of hightemperature oxide melt solution calorimetry and determined the enthalpy of formation of the natural magnesian chrysotile asbestos of the composition Mg 3 Si 2 O 5 (OH) 4 : ∆ f, el H 0 (298.15 K) = -4353 ± 17 kJ/mol. This value agrees well with the results obtained from acid (HF) calorimetry (-4364.6 ± 3.3 kJ/mol) [8], as well as with the data calculated from the equilibria for this mineral (-4363.35 kJ/mol [9] and -4358.46 ± 2.50 kJ/mol [10]).…”

“…The corresponding parameters of the Ni 3 Si 2 O 5 (OH) 4 nanotubes synthesized under the same conditions are as follows: length, 10 µ m; outer diameter, 10-15 nm; and inner diameter, 2-3 nm. The dehydration temperatures of the Mg 3 Si 2 O 5 (OH) 4 and Ni 3 Si 2 O 5 (OH) 4 nanotubes are equal to 953 to 993 K, respectively. Table 1 presents the results of the chemical analysis, which was carried out by the analyst L.A. Doronina (Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, Russia).…”

“…Earlier [1], it was shown that the substitution of nickel for magnesium leads to a decrease in the nanotube size and a slight increase in the dehydration temperature. For example, the Mg 3 Si 2 O 5 (OH) 4 nanotubes have the following parameters: length, 10-20 µ m; outer diameter, 20-30 nm; and inner diameter, 4-5 nm. The corresponding parameters of the Ni 3 Si 2 O 5 (OH) 4 nanotubes synthesized under the same conditions are as follows: length, 10 µ m; outer diameter, 10-15 nm; and inner diameter, 2-3 nm.…”

“…These studies provide important information on the thermal stability and chemical durability of the aforementioned materials and permit one to determine the optimum conditions for their synthesis. The physicochemical conditions of the formation of nanotubes based on compounds of the variable composition (Mg,Ni) 3 Si 2 O 5 (OH) 4 with a chrysotile structure and their influence on the microstructure and properties were investigated in [1]. The purpose of the present work was to perform a calorimetric investigation of the synthesized nanotubular hydrosilicates in the Mg 3 Si 2 O 5 (OH) 4 -Ni 3 Si 2 O 5 (OH) 4 system.…”

Manganoan deerite and calderitic garnet from high-pressure metamorphic Fe-Mn-rich quartzites on Andros Island, Greece

“…Maresch and Mottana, 1976;Chopin, 1978). The change in the slope of reaction (1) resulting from the presence of rhodonite instead of pyroxmangite is probably small, because the AS and A V of the rhodonite-pyroxmangite transition is small (Navrotsky and Coons, 1976).…”