Effect of Water on the Equation of State of Nominally Anhydrous Minerals

Abstract

In order to address this question, it is necessary to know, quantitatively, the effects of water (or more precisely structurally bound hydroxyl) on the elastic moduli of mantle minerals. Also required are pressure and temperature derivatives of the elastic moduli for more direct comparison with seismological observation. This chapter will review what is known about the elastic properties of “hydroxylated” NAMs from experimental studies. From a crystal chemical perspective, hydrated NAMs are defect structures because hydrogen is usually incorporated through charge balance by cation vacancies. Therefore, small variations in water content can have a dramatic effect on thermoelastic parameters, more so than any other major geochemical substitution such as iron or aluminum. For example, at one atmosphere the addition of ~1 wt% H₂O into ringwoodite has a similar effect on the shear modulus as raising the temperature by 800–1000 °C (Wang et al. 2003a; Jacobsen et al. 2004). However, due to elevated pressure …