Thermochemistry of a synthetic sodium-magnesium rich triple-chain silicate: Determination of thermodynamic variables and geological applications

An understanding of the thermodynamics of triple-chain silicate phases can offer insight into their formation in nature, including the steps through which they form by the alteration of pyroxenes and layer silicates. Because the more common Mg-triple chain silicates have not yet been synthesized, this study focused on the Na-analogue to clinojimthompsonite (Na-cjt), which forms readily in a laboratory setting. It was characterized using electron microscope analysis, scanning electron microscopy, thermogravimetric analysis, transmission electron microscopy, infrared spectroscopy, and manual heating to determine its water content. The results of these studies indicated a formula for Na-cjt of Na3.34Mg8.16Si12O30.46(OH) 6.74, although there was substantial uncertainty associated with the water content. A series of experimental reversals were performed for the reaction Na3.34Mg8.16Si12O30.46(OH) 6.74 (Na-cjt) = 1.5 Na2.26Mg5.45Si8O 21.26(OH)2.64 (Na-Mg amph) + 1.39 H2O over a temperature range of 350--650°C and a pressure range of 0.1--1 GPa, in which the equilibrium boundary was found to be a sharply back-bending curve above 0.5 GPa. The third law entropy and enthalpy of formation of both Na-cjt and the associated Na-Mg amphibole were measured using adiabatic and solution calorimetry, respectively. Calculating the equilibrium boundary for the reaction using the calorimetrically determined values did not produce the same boundary found through the experimental reversals. The discrepancy was attributed to water variability with pressure. This variability and its effect on Na-cjt's thermodynamic variables can be expressed by the experimentally fitted equations nH2O &parl0;molesinNa-c jt&parr0;=10.01+21.30P-2.97P2 -22.48P0.5 PinG Pa DH° f &parl0;kJ/mol&parr0;=-294.4 nH2O,Na-cjt-17313.2 S°&parl0;J/K-mol&parr0;=48.2 nH2O,Na-cjt+745.9 V&parl0;J/bar-mol&parr0;=1.46 nH2O,Na-cjt+36.94. Using these equations and the thermodynamic database of Holland and Powell (1998) to examine several reactions for the potential formation of Na-cjt, I propose that basaltic rocks near a mid-ocean ridge or ultramafic rocks at locations more distant from the ridge would provide a likely setting for its natural occurrence.