Thermal behavior of scapolite Me79.6 and Me33.3

Scapolite-group minerals have the general formula (Na,Ca,K)(4)[(Al,Si)(12)O-24](Cl,CO3,SO4). Thermal analyses (DTA, DSC, and TG) were carried out on two samples (Me-79.6 and Me-33.3) using a Netzsch STA 409 EP/3/D simultaneous TG-DTA apparatus and a Netzsch STA 449C simultaneous TG-DSC instrument. The DTA-TG results indicate that Me-79.6 loses 5.7 wt.% in two steps at different rates: at 1171degreesC, CO2(g) and minor NaCl are lost together, followed by a small amount of SO3(g) at about 1232degreesC, and the residue melts at 1407degreesC. High-temperature X-ray-diffraction (XRD) data also indicate that the Me-79.6 residue melts between 1300 and 1400degreesC. Me-33.3 loses 5.2 wt.% of NaCl, CO2(g), and minor SO3(g) in a single step at 1199degreesC, and the residue melts at 1319degreesC. The residue of Me-79.6 melts at a higher temperature than Me-33.3 because its framework composition is closer to that of anorthite, whereas that of Me-33.3 is closer to that of albite. The cage clusters [Na-4.Cl](3+) and [NaCa3.CO3](5+) in Me-33.3 are ordered, whereas the clusters [NaCa3.CO3](5+) and [Ca-4.CO3](6+) in Me-79.6 are disordered. The DSC curve for scapolite Me-33.3 shows a transition at about 299degreesC arising from disordering of [Na-4.Cl](3+) and [NaCa3.CO3](5+) clusters on heating.