High-temperature Melt Calorimetry Research Articles

Thermochemical study Mg–Fe amphiboles

The paper presents data on the thermochemical study (high-temperature melt calorimetry in a Tian–Calvet microcalorometer) of two natural Mg–Fe amphiboles: anthophyllite Mg2.0(Mg4.8Fe0.2 2+)[Si8.0O22](OH)2 from Kukh-i-Lal, southwestern Pamirs, Tajikistan, and gedrite Na0.4Mg2.0(Mg1.7Fe0.2 2+Al1.3)[Si6.3Al1.7O22](OH)2 from the Kola Peninsula, Russia. The enthalpy of formation from elements is obtained as–12021 ± 20 kJ/mol for anthophyllite and as–11545 ± 12 kJ/mol for gedrite. The standard entropy, enthalpy, and Gibbs energy of formation are evaluated for Mg–Fe amphiboles of theoretical composition.

Calorimetric determination of the enthalpy of formation for pyrophyllite

A calorimetric study of the natural pyrophyllite was performed by high-temperature melt calorimetry on a Tian-Calvet calorimeter. Based on experimentally determined in this work for pyrophyllite and gibbsite, as well as previously obtained for corundum and quartz, the total value of the enthalpy increment for the sample heated from room temperature to 973 K and the enthalpy of dissolution at 973 K by Hess’s law, the enthalpy of formation of pyrophyllite of Al2[(OH)2/Si4O10] composed of elements was calculated at 298.15 K: ΔfHelo(298.15 K) = −5639.8 ± 5.7 kJ/mol.

Enthalpy of formation of natural heulandite group zeolite—clinoptilolite

The minerals of the heulandite–clinoptilolite series are the most widespread zeolites in nature. The large deposits of clinoptilolite are explored around the world (in US, Russia, Japan, and other countries). The specific properties and various fields of its application provoked attention to this mineral and its intense study. Clinoptilolite is characterized by variable cationic composition, Si/Al ratio, and water content. Owing to the changeable composition of extra-framework cations, clinoptilolites are subdivided into Ca-, Na-, and K-clinoptilolites, with possible presence of Sr, Ba, and Mg, as well as Fe 2+ and Fe 3+ . The general formula of the mineral is ( Na , K , Ca 0.5 Sr 0.5 Ba 0.5 Mg 0.5 ) 6 [ Al 6 Si 30 O 72 ] · ~20 H 2 O [1]. The Si/Al ratio in clinoptilolites is more than 4.0. Minerals with the same framework topology but Si/Al (K + Na) are classified as heulandite, which forms continuous solid solutions with clinoptilolite. The thermal behavior of clinoptilolite within a wide temperature range, processes of its dehydration and rehydration, and structural changes during heating of natural samples were studied in detail by many authors [2–7], whereas thermodynamic properties of this compositionally complex zeolite are significantly less studied. Among reported thermodynamic data are the low-temperature heat capacity measured for Ca-clinoptilolite within a range of 15–305 K [8] and for Na–Ca clinoptilolite within a range of 5.5–344 K [9]; values of S °(298.15 a) – S ° (0 K) calculated from these data differ by ~3%, which can be related to the significant difference in the composition of the studied natural zeolites. The values of the Gibbs free energy of the formation of Naand K-exchanged varieties of natural clinoptilolite were determined in [10, 11] during study of their solubility in the weakly alkaline aqueous solutions. The calorimetric study of hydration of natural clinoptilolite and its modified forms and determination of their enthalpies of hydration were determined in the works [12–14]. The enthalpy of formation of clinoptilolite from elements (Table 1) was determined by acid (HF) calorimetry as (298.15 K ) = –41290 ± 27 kJ/ mol [9] and by high temperature melt calorimetry as (298.15 K ) = –40232 ± 34 kJ/mol [14]. Strong (by approximately 1000 kJ/mol) difference in these values simulates interest in the new experimental determination of enthalpy of formation of natural clinoptilolite. ∆ f Hel °