Stability and Thermochemistry of Sodic Triple-Chain Silicates

Suny At Binghamton, Binghamton NY

Investigators

Abstract

Jenkins EAR-0228975 This project will examine the crystal structure, pressure-temperature stability, and thermochemical properties of several members of a new class of silicate minerals known as the triple-chain silicates. Research will begin with the most readily made triple-chain silicate, Na4Mg8Si12O32(OH)4 = Na-Cjt, which is the sodium-bearing analogue of the naturally occurring mineral clino-jimthompsonite. The experimental investigation of the pressure-temperature stability of Na-Cjt breaking down to a Na-Mg-bearing double chain silicate (amphibole) will provide a valuable foundation for studying other triple-chain silicates. An important aspect of this study is the measurement of the third-law entropy and enthalpy of formation of Na-Cjt, which will be made in the labs operated by Prof. J. Boerio-Goates at Brigham Young University and by Prof. A. Navrotsky at the University of California, Davis, respectively. The thermodynamic properties of Ca-clinojimthompsonite (Ca2Mg8Si12O32(OH)4) will be determined, to a first approximation, by experimentally investigating the shift of the Na-Cjt stability curve as Ca is added to both the triple- and double-chain phases and using simple ideal-activity expressions to estimate the activities of the end-member phases. The broader scientific contribution of this study is that it is the first attempt to provide experimental and calorimetric data for a class of silicate minerals that is becoming increasingly recognized in nature but for which there is no solid information on their pressure-temperature-composition field of stability. This study may also draw attention of Earth scientists to the role that Na-Cjt may play in storing Na in the oceanic lithosphere.