Abstract: The conditions for the formation of various heteropolytypic or mixed-layer phases were determined in a hydrothermal study of synthetic mixtures along the join tale—Na-saponite—Na-phlogopite composition. Heteropolytypes of talc and saponite from compositions between tale and a saponite solid solution and of saponite and a 10 Å sodium mica (Na-phlogopite?) from compositions lying between saponite solid solutions and Na-phlogopite have been prepared reproducibly.
At 1 kb in the temperature range 450–575°C, randomly stacked heteropolytypic phases were formed and persisted. At higher pressures the regular heteropolytypes (regularly interstratified or mixed-layer minerals), generally 1:1 phases, were formed. The genesis of heteropolytypes in nature is discussed on the basis of these results.