Mechanisms of OH defect incorporation in naturally occurring, hydrothermally formed diopside and jadeite

Abstract

The IR spectrum of an alpine, hydrothermally formed diopside containing 17 wt ppm H2O consists of three main OH absorption bands centred at 3647, 3464 and 3359 cm−1. Jadeite from a Californian vein occurrence is characterised by bands at 3616 and 3557 cm−1 and contains about 197 wt ppm H2O. Based on the pleochroic scheme of the OH absorption bands in diopside, OH defect incorporation models are derived on the basis of fully occupied cation sites and under the assumption of M1 and M2 site vacancies; OH defects replacing O2 oxygen atoms are most common. The less pronounced OH pleochroism and the broad band absorption pattern of jadeite indicate a high degree of OH defect disordering. The pleochroic scheme of the main absorption bands at 3616 and 3557 cm−1 implies partial replacement of O2 oxygen atoms by OH dipoles pointing to vacant Si sites. Under the assumption of M1 and M2 site vacancies, O1–H and O2–H defects are also derivable. OH incorporation modes assuming Si-vacancies should be considered for jadeite-rich clinopyroxenes formed in deep crust and upper mantle regions.