Microstructure of calcined Mg-rich clay minerals of Egyptian serpentinites

The thermal behaviour of Mg-rich clay minerals in two serpentinites was studied by means of DSC-TG, XRD, IR and SEM. Experiments are carried out heating at temperatures in the 600- 1200 °C range for 2-4 hours. Analyses revealed that antigorite, talc and clinochlor are the main Mg-rich clay minerals. Dolomite and magnesite are accessory minerals. Chromite, magnetite and hematite are in small amounts. DSC-TG results of the antigorite - rich sample reveal that most of the loss of free water and water absorbed at surface (approx. 13%) occurs at approximately 750 °C. The decomposition into the high-temperature products forsterite + enstatite proceeds via an intermediate assemblage of forsterite and a talc-like phase, observed within a temperature interval of 100 ± 20 °C. The breakdown of antigorite and the talc-like phase is kinetically controlled by surface growth processes at the edges of grains. The decomposition of the talc-clinochlore-antigorite assemblage took place at higher temperature and produced enstatite and spinel. It is suggested that their formation dependents on how clinochlore dehydrates and on the talc trioctahedral arrangement, which provides rapid transformation into enstatite. The increase of the reaction rate with temperature is confirmed by XRD and IR data as well as by SEM. Results of this work, indicate that highly crystalline forsterite and enstatite form by dehydroxilation of Mg-rich minerals. riassunto. — Il comportamento termico di minerali argillosi ricchi di Mg presenti in due serpentiniti è stato analizzato tramite l’uso di apparecchiature quali DSC-TG, XRD, IR e SEM. Gli esperimenti sono stati condotti riscaldando il materiale a temperature comprese tra 600 e 1200 °C per intervalli di tempo variabili da 2 a 4 ore. Dalle analisi è emerso che i principali minerali ricchi di Mg sono antigorite, talco e clinocloro. Dolomite e magnesite figurano come accessori mentre cromite, magnetite ed ematite sono presenti in quantità modeste.