PREDICTING THE FORMATION CONDITIONS OF THE TRIPLE-CHAIN SILICATE SODIC-CLINOJIMTHOMPSONITE

Although triple-chain silicates are recognized in nature with growing frequency, there is considerable uncertainty concerning the stability of this new class of silicates. The fundamental question concerning their origin is whether they have their own stability fields, or if they are intermediate phases in the reaction path to more stable minerals (i.e., sheet or double-chain silicates). Thermodynamic data (DH°_f = -18,417 kJ/mol, S° = 919.2 J/K·mol at 298K, 1 bar) were recently reported by Ams et al. (2009, Am Min) for a sodic triple-chain silicate of the composition Na_3.74Mg_8.16Si₁₂O₃₂(OH)₄·1.4H₂O (= Na-cjt). This sodic analogue of the naturally occurring clinojimthompsonite can be made with high purity at 0.2 GPa and 550°C. To determine the conditions at which Na-cjt should form in nature, its thermodynamic data can be combined with data for other common minerals to examine possible reactions.

Gibbs free energies at 0.2 GPa and 450°C were calculated for six different reactions for the formation of Na-cjt, involving synthetic enstatite, synthetic talc, and natural anthophyllite (Na_.06Ca_0.26Mg_4.92Fe_1.62Al_0.17(Al_0.18Si_7.82)O₂₂(OH)₂) reacting with Na (either as NaOH or NaCl) and involving quartz as either a reactant or product. In addition, the reactions were experimentally investigated at the same P-T conditions for durations of 140-335 h. For all but one reaction the experimental results matched the thermodynamic prediction that Na-cjt can be readily formed from either enstatite, talc, or chrysotile in the presence of NaOH, but not from NaCl. The one reaction that contradicted the thermodynamic prediction involved anthophyllite and produced a Na-Mg-amphibole instead of Na-cjt, probably because anthophyllite provides a kinetically favorable pathway (i.e., nucleation sites) for the formation of the structurally similar Na-Mg-amphibole over the thermodynamically stable Na-cjt.

This thermodynamic analysis can be extended to predict what reactions might form Na-cjt in nature. Calculations of activity-pH relationships involving common minerals at mid-ocean ridge hydrothermal conditions suggest that Na-cjt could form as an alteration product of clinopyroxene (diopside) at temperatures of about 100-300°C and 0.1 GPa at a pH of 3.5 or less, or from enstatite at a pH of 7.6 or higher.