Thermal behaviour of fibrous zeolites of the natrolite group

The western region of the Deccan Volcanic Province (DVP) is constituted of tholeiitic lava flows. They host numerous cavities, varying in size and shape, largely occupied by zeolites and a variety of secondary minerals, amongst which the fibrous zeolites (natrolite group), are particularly gorgeous and attractive. Scolecite and mesolite are the only two members commonly occurring in this region. Our data on their thermal behaviour significantly differs from that reported in previous literature. Scolecite, Ca₈(Al₁₆Si₂₄O₈₀)·24H₂O shows three distinct steps instead of two and water loss is observed even beyond 550°C. It is held that the first step corresponds to the expulsion of water from the site farthest away from Ca and the next two steps in succession correspond to the two sites nearer to Ca. Mesolite, Na₁₆Ca₁₆(Al₄₈Si₇₂O₂₄₀)· 64H₂O has much more complex behaviour with four or five steps of water expulsion and a major loss around 248–270°C in a double reaction attributed here to the expulsion of water from scolecite type channels. It is further held that the natrolite type channels are emptied in further steps. Loss of water in steps even beyond 400°C is particularly noted and such reactions are well reflected in the TG and the DTG. Peak temperature dependence on sample amounts is also evident. Successive phase transformations above 700°C, up to 1000°C are reflected in the DTA curves.