THERMAL REACTIVITY OF JADEITE AND NEPHRITE

The thermal reactivity of the naturally occurring silicates jadeite, N aAl[Si2O6], and nephrite, a variety of actinolite, Ca2(Mg,Fe)5[(OH,F)S i4O11]2, have been investigated by thermogravimetric and thermomechani cal analysis as well as temperature-dependent X-ray diffraction and an alytical electron microscopy. TG shows that nephrite undergoes a weigh t loss at around 900-degrees-C. Mass spectrometry reveals that this ir reversible reaction corresponds to the evolution of H2O, and XRD shows that a phase related to diopside CaMg[Si2O6] is formed. Jadeite does not undergo any observable weight changes up to 1000-degrees-C. Thermo mechanical analysis indicates a reversible phase transition at about 9 50-degrees-C. Temperature-dependent X-ray diffraction shows that jadei te is again present on cooling (peak temperature: 1000-degrees-C), but that this is accompared by an additional unidentified phase. The mech anism of this process is not yet clear although it has been observed i n several samples from different origins and with different metal impu rities.