Order-disorder in natrolite-group minerals

The refinement of the crystal structure of a natrolite from Palagonia, Sicily, closely intergrown in gonnardite, was carried out to study order-disorder in tetrahedral sites of natrolite-group minerals. About 20 % of Al has been found in Si1 and Si2 sites and 30 % of Si in the Al site. These results, 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 > or = 1.5, is interpreted as the dehydration product of paranatrolite, whose high H (sub 2) O content and complete disorder in tetrahedral sites is attributed to an unusual genesis (high T and PH (sub 2) O). Accordingly, a definition of natrolite, gonnardite, tetranatrolite and paranatrolite is suggested.