The crystal chemistry of natrolite, mesolite and scolecite

Natrolite [1318-95-2], mesolite [12043-66-2], and scolecite [1319-20-6] samples were analyzed by electron microprobe and their unit cell dimensions were detd. by x-ray powder diffractometry. K, Mg, Sr, and Ba are nearly absent in these zeolites. The Si/(Si + Al + Fe3+) ratio is 0.598-0.624, its av. being similar in the 3 phases. The (Na + K)/(Mg + Ca + Sr + Ba) ratio is 0.971-1.000, 0.455-0.517, and 0.162 for natrolite, mesolite, and scolecite, resp. The narrow field of existence of these zeolites is probably due to the Si-Al order and the distribution of the exchangeable cations. In scolecite the exchangeable cations are related to the crystallog. parameters; this relation is attributed to the presence of antiphase domains when the content of exchangeable cations exceeds the theor. value of 8 atoms per unit cell. In natrolite, the strong correlation between (b-a) and c is explained by the relation existing between these parameters and the degree of disorder in the Si-Al distribution. Analogous correlations in scolecite and mesolite are interpreted in the same way.