Order-disorder in natrolite group minerals

The structure refinement of a natrolite from Palagonia (Sicily, Italy), closely intergrown in gonnardite, has been carried out in order to study order-disorder in tetrahedral sites of natrolite-group minerals. About 20% of Al has been found in Sil and Si2 sites and 30% of Si in the Al site. The results of this structural refinement and those of literature, together with chemical data, suggest that complete disorder cannot be reached in hydrothermal natrolites. The complete disorder found in gonnardites, where Si/Al < 1.5, is a consequence of Loewenstein's rule, which imposes the alternation of Al and Si in tetrahedral sites. Tetranatrolite, where Si/Al > 1.5, is interpreted as the dehydration product of paranatrolite, whose high H2O content and complete disorder in tetrahedral sites is attributed to unusual genesis (high temperature and H2O pressure). Accordingly, a definition of natrolite, gonnardite, tetranatrolite, and paranatrolite is suggested