Types of OH-bearing point defects in olivine and their transformation under changing petrological conditions

Abstract

In this paper, we characterize the most common coupled point defects in the olivine structure of the hydrous olivine {vMe, 2OH−}, humite {vSi, 4OH−}, laihunite {vMe, 2Fe3+}, hydroxyl {vMe, Fe3+, OH−}, and Schottky {vMe, vO} types, where v is a vacancy. Their segregation and evolution under changing physicochemical conditions were considered. During the development of deformation textures, the segregation of {vMe, 2OH−} and {vSi, 4OH−} defects is accompanied by the formation of planar features in olivine consisting of hydrous olivine, n(Mg,Fe)2SiO4 · MgH2SiO4, and humite-like layers, n(Mg,Fe)2SiO4 · Mg(OH)2, respectively. Under the conditions of low-temperature oxidation, the segregation of {vMe, 2Fe3+} defects in anhydrous olivine results in the formation of planar laihunite-type defects. The OH-bearing defects undergo dehydrogenation and oxidation during heating with decompression, which results in the transformation of hydrous olivine-type defects into laihunite-type {vMe, 2Fe3+} or hydroxyl-type {vMe, Fe3+, OH−} defects. The mobility of coupled point defects and their ability to segregate and form new phases in the host olivine, the possibility of releasing a proton with transformation into defects of a different type, and potential role as an oxidizer in dehydrogenation processes are manifested upon changes in physicochemical conditions and can be considered as indicators of geologic processes accompanied by variations in P, fO2, and aH2O.