CALORIMETRIC STUDIES AND THERMODYNAMIC CALCULATIONS ON THE TRIPLE-CHAIN SILICATE NA-CLINOJIMTHOMPSONITE

Na-clinojimthompsonite (ideally Na₄Mg₈Si₁₂O₃₂(OH)₄ = Na-cjt), the sodium analogue to the naturally occurring triple-chain silicates jimthompsonite and clinojimthompsonite, is a valuable resource for studying triple-chain silicate minerals because it is readily synthesized using traditional hydrothermal techniques. A thorough study of the thermodynamics of this phase is a crucial first step to understanding the stability of natural triple-chain silicates.

To elucidate this issue, solution calorimetry (performed at the University of California in Davis) and adiabatic calorimetry (performed at Brigham Young University) were performed on a synthetic Na-cjt of the composition Na_3.4Mg_8.3Si₁₂O_30.6(OH)_6.8 to determine the enthalpy of formation and third law entropy of this phase, respectively. Using existing data on the heats of solution of the simple oxides to complete the thermochemical cycle, the measured enthalpy of drop solution of 1360.9 ± 11.4 kJ/mol provides an enthalpy of formation at 298K and 1 bar (ΔH_f,) of -18096.6 ± 22.5 kJ/mol. Adiabatic calorimetry measurements of the heat capacity of Na-cjt were made over the range of 20 to 333 K and extrapolated to zero Kelvin using a linear fit in Cp/T vs. T² space. The resultant third-law entropy (S˚) at 298.15 K and 1 bar is 939.4 ± 4.7 J/K•mol.

Thermodynamic calculations were made in an effort to predict the conditions under which Na-cjt would be stable in nature. Natural triple-chain silicate minerals often occur with talc and anthophyllite in ultramafic bodies. Using the thermodynamic data above for Na-cjt and existing calorimetric data for anthophyllite and talc (Robie and Hemingway 1995), Na-cjt would not be expected to form in seawater at 298K and 1 bar from these phases. However, Na-cjt may form from hydrothermal alteration of mid-ocean ridge basalts by the reaction 8 diopside + 4 Na⁺ + 12 H⁺ = Na-cjt + 4 quartz + 8 Ca²⁺ + 4 H₂O. It is suggested that Na-cjt, with a Na content as high as 10.3 wt. %, could act as an important mechanism for sequestering sodium in seawater, and this offers a possible reaction by which this could occur.