INVESTIGATION OF THE STABILITY BOUNDARY OF NA-CLINOJIMTHOMPSONITE, A SYNTHETIC TRIPLE-CHAIN SILICATE, TO NA-MG AMPHIBOLE

Na-clinojimthompsonite (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 standard hydrothermal techniques. In this study, reversal experiments were performed in cold-seal and gas vessels to bracket the equilibrium conditions for the reaction between Na-cjt and a Na-Mg amphibole. Chemical data determined using an electron microprobe indicates the balanced reaction Na_1.69(Na_1.74Mg_0.26)Mg₈Si₁₂O₃₂(OH)₄ + 0.3 H₂O = 1.535 Na_0.6(Na_1.62Mg_0.38)Mg₅Si₈O₂₁(OH)₃. The experimentally determined equilibrium conditions (0.1 GPa/439ºC, 0.2 GPa/468ºC, 0.3 GPa/541ºC, 0.5 GPa/511ºC, 0.7 GPa/506ºC) indicate a lower set of temperatures than those reported by Franz and Althaus (Contr. Min. Pet. 1974).

Rietveld techniques were used to refine the crystal structure of the amphibole based on Cámara et al.'s (Phys. Chem. Min. 2003) structure of Na(NaMg)Mg₅Si₈O₂₂(OH)₂, resulting in a unit cell volume of 909.6 ų, or 273.9 cm³/mol. A similar analysis was done with Na-cjt using Tateyama et al.'s (Contr. Min. Pet. 1978) crystal structure solution, and a volume of 1399.3 ų, or 421.3 cm³/mol, was determined. With these data, the thermodynamic values for ΔS and ΔH at 298K and 1 bar for the reaction can be derived using a G prime vs. temperature plot. The derived values of ΔS and ΔH, assuming that heat capacities of the solids will cancel each other, are -0.0445 kJ/K•mol and -10.747 kJ/mol, respectively.

The reversal experiments indicate that Na-cjt does indeed have a stability field of its own at geologically relevant conditions. If Na-cjt does occur naturally, its high sodium content (10.3 wt. %) suggests it could act as an important mechanism for sequestering sodium in mid-ocean ridge rocks, possibly offering a partial explanation for the cause of ocean salinity shifts throughout geologic time.