Abstract: The dominant clay mineral in an amygdaloidal basalt from Mozambique is a mixed-layer variety having a highly regular alternation of montmorillonite-like and chlorite-like layers. The montmorillonite-chlorite occurs in the amygdules as green rosettes having a fine fibrous structure.
X-ray examination of this mineral shows a long spacing of about 29 A in air and a fairly well-developed sequence of basal spacings. The long spacing shows some variation which is dependent on the relative humidity of the air surrounding the sample. The montmorillonite-like layers expand with glycerol and contract with heat treatment similar to montmorillonite, while the chlorite-like layers show modified reflection intensities on heating similar to chlorite. The mineral shows a 31.7 A spacing when glycerol saturated and a 23.3 A spacing when fully dehydrated.