Clay minerals have been prepared by synthesis in solutions of comparable strength to their solubility (10–20 mgm./l). Syntheses have been carried out in boiling water and at normal temperatures, at various pH's, and with various components; the products have been examined with X-rays and the electron microscope. In general it is found that the proportion of the components has only a small effect on the product, this being determined mainly by the pH. The low temperature gives less crystalline products. Some results are:

Si-Mg syntheses.—Montmorillonite: “crushed paper” type at pH 9 (Fig. 1); “star” type at pH 8 (Fig. 2 from boiling solutions, Fig. 3 at ordinary temperature).

Si-Mg-Al syntheses.—Al as aluminate at pH 8 gives montmorillonite; with more Al, “swelling chlorite” (and 7Å mineral?) (Fig. 4). With cationic Al at pH 8·5 a filmy, almost amorphous product (sepiolite?) (Fig. 5); at pH 7·7 boeh-mite (Fig. 6). Anionic Al with KCl in great excess gives micas (Fig. 7).

Syntheses with iron.—Iron-rich saponites, montmorillonite minerals, and Fe2O3+“swelling chlorite” have been obtained. In NaCl excess, mica-like (but isotropic, poorly crystalline) products (Fig. 8).

The authors consider that the brucite-gibbsite layer forms first in these syntheses, and the silicate layer attaches itself to it: analogous products have been made with aluminate and chromate in place of silicate.