Mineral replacement rate of olivine by chrysotile and brucite under high alkaline conditions

Abstract : Olivine mineral replacement by serpentine is one major alteration reaction of oceanic hydrothermalism. In the present experimental study, olivine grains were replaced by chrysotile and brucite under high alkaline conditions. In our study, olivine replacement implied a spatial and temporal coupling of dissolution and precipitation reactions at the interface between olivine and chrysotile-brucite minerals. Coupled dissolution-precipitation led to the alteration of starting olivine grains (so-called primary or parent mineral) to a porous mineral assemblage of chrysotile and brucite with preservation of the initial olivine morphology. This mineral replacement reaction of olivine (serpentinization) has been characterized using XRD, FESEM and FTIR measurements. Moreover, a simple and novel method is here proposed to quantify the mineral replacement rate (or serpentinization rate) of olivine by using thermogravimetric (TG) and differential TG (DTG) analyses. Serpentinization extent depends on the grain size: it is complete after 30 days of reaction for the smallest olivine grains (<30µm), after 90 days of reaction for the intermediate olivine grains (30 µm-56 µm) and reaches 55% of olivine replacement after 90 days for the largest fraction (56-150 µm). Based on the fitting of the serpentinization extent (t) versus time (t) by using a kinetic pseudo-second-order model, the serpentinization rates vary from 3.6x10-6 s-1 to 1.4x10-7 s-1 depending on the olivine grain size. An additional correlation between FTIR spectra analysis and TG measurements is proposed. The mineral replacement reactions frequently observed in natural alteration processes could be a powerful synthesis route to design new porous and/or nanostructured materials.