THE EXTREME COLLAPSE OF A FRAMEWORK OF NAT TOPOLOGY - THE CRYSTAL-STRUCTURE OF METANATROLITE (DEHYDRATED NATROLITE) AT 548-K

Heating of natural natrolite, Na2Al2Si3O10. 2H(2)O, to 548K yields an anhydrous Na2Al2Si3O10 called metanatrolite. The symmetry is reduced f rom space group Fdd2 in natrolite to space group F112. The volume of t he F112 cell of metanatrolite (a = 16.277(17) Angstrom, b = 17.045(9) Angstrom, c = 6.434(4) Angstrom, gamma = 90.08(6)degrees, V = 1785.1(2 ) Angstrom(3)) is much reduced compared with the original hydrated nat rolite. The agreement factor is R1 = 0.041 for 954 F-obs measured on a heated single crystal. After dehydration, the crystal (originally unt winned) is twinned on a plane normal to [100]. The Na-atoms in metanat rolite occupy similar positions to those of the water molecules in hyd rated natrolite. The contraction of the framework due to the loss of w ater is caused by tilting and twisting of the chains within themselves as well as by a rotation of the chains as a whole around their hinges between the fibers. The most remarkable thing about mecanatrolite are the angles Al-O(2)-Si of about 116 degrees which are the smallest eve r observed in a framework aluminosilicate.