FROM EXPERIMENTS TO REACTION MECHANISM: APPLICATION OF MICROBEAM TECHNIQUES TO UNDERSTAND ALBITISATION REACTIONS

摘要

One aim of this study was to investigate the mechanism behind the transformation of plagioclase to albite, as this is one of the most common fluid-mineral reactions that take place in the earth's shallow crust [1]. A field emission scanning electron microscopy (SEM) equipped with traditional energy-dispersive X-ray spectrometry (EDS) and electron probe microanalysis (EPMA) were used to examine reaction products after high temperature high pressure albitisation experiments, and as preliminary to advanced characterisation of product texture and porosity. In this study, preliminary plagioclase albitisation experiments were conducted at 600 °C and 2 kbar, with labradorite (Nao.4Cao.6Al1.6Si2.4O8). sodium metasilicate and water as basic reactants, but with varying activity of water and quartz in each experiment. EDS analysis was done with a FEI Quanta 3D FEG FIB, operated at 15kV and 16 nA. The products' texture and EDS spectra are shown in Fig. 1. With different amounts of water added into the system, reaction products remained the same, i.e., pectolite (NaCa_2Si_3O_8(OH)), and analcime (NaAlSi_2O_6•H_2O) (Figs, 1a and 1c). However, as the activity of quartz was increased in the system, albite formed as a reaction product (Figs, 1b and 1c).