SMECTITE-GROUP MINERALS IN DEEP-SEA SEDIMENTS - MONOMINERALIC SOLID-SOLUTIONS OR MULTIPHASE MIXTURES

Results of XRD, TEM, and chemical analyses are combined to characteriz e the true chemistry and structural formula of the smectite-group mine rals present in selected samples of deep-sea sediments from the north- central Pacific and Angola, Lau, and North Fiji Basins. In most cases, the abundant Fe and K in the <0.2-mum fraction is associated with acc essory minerals such as mica, phillipsite, and iron oxides. Bulk analy tical data obtained from the <0.2-mum fraction, corrected for the pres ence of these minerals, reveal a structural formula very close to that of ideal montmorillonite. The extensive chemical variability among me mbers of the clay-mineral group usually is attributed to the occurrenc e of solid solution. The variability in smectite-group minerals also c ould be due to (1) the occurrence of mixed-layer structures, (2) the p resence of compositionally distinct domains in the structure, or (3) a mixture of two or more distinct phases. A clear distinction between a monomineralic solid-solution phase and a mixture of coexisting end-me mbers based on chemical composition or even conventional XRD and HRTEM techniques is of crucial importance. The data presented in the litera ture concerning the nature of the smectite-group phase and related I/S mixed layers do not necessarily reflect the true nature of the indivi dual clay-mineral phases. The application of the term ''smectite'' to a material characterized by a structural formula should be avoided.