How the structure of natrolite is modified through the heating-induced dehydration

Natrolite [1318-95-2], Orthorhombic with space group Fdd2, at 350-500° dehydrates and contracts to a new phase, called metanatrolite [1302-73-4], whose space group is monoclinic F2. The cell parameters of natrolite from Faeroer Islands (a = 18.29, b = 18.64, c = 6.59 Å) after heating to 350° become a = 16.01, b = 16.73, c = 6.40 Å, γ = 90° in metanatrolite. In metanatrolite, the tetrahedral chains are rotated 11° wiyh resp. to natrolite, thus producing the obsd. decrease in a and b tarameters. Rotations of each tetrahedron around an axis perpendicular to c cause twisting of the chain and thus a decrease of the c parameter. This chain-twisting is the consequence of rotation of the tetrahedra in 2 different ways, which gives rise to 2 different configurations, 1 being more frequent. In metanatrolite Na atoms occupy positions near those occupied by H2O in natrolite. One of the 2 sym. independent Na sites can be 4-, 5-, 6-, or 7-fold coordinated, depending on the 2 possible twistings of the chains, whereas the other is always 5-fold coordinated.