The classification for the layer silicates (including most of the clay minerals) proposed here is based on structural studies of numerous investigators on natural materials. In many instances, however, the limits of chemical composition have been determined by research on synthetic layer silicates.

The layer lattice silicates can be subdivided by the successive application of three criteria: (1) height of fundamental repeat unit or “thickness of layer”; (2) gross composition, whether diocta-hedral or trioctahedral, and ionic content of layers; (3) stacking sequence of layers and degree of orderliness of stacking.

The relationship between compositional and structural divisions is clearly indicated in compositional diagrams. These show that some structurally similar families may be widely separated in composition (kaolinite and septechlorite), whereas others —e.g., phengites (micas) and beidellites—are not far apart with respect to bulk composition.

A scheme of successive operations that will permit the identification of layer lattice silicates singly or in simple mixtures is outlined, particularly for the nonspecialist. The essential apparatus includes only that for powder X-ray diffraction and a small laboratory furnace.

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