Argillaceous mineral

Argillaceous minerals are minerals containing substantial amounts of clay-like components (Greek: ἄργιλλος = clay). Argillaceous components are fine-grained (less than 2 μm) aluminosilicates, and more particularly clay minerals such as kaolinite, montmorillonite-smectite, illite, and chlorite. Claystone and shales[1] are thus predominantly argillaceous. Argillaceous minerals may appear silvery upon optical reflection.

The adjective "argillaceous" is also used to define rocks in which clay minerals are a secondary but significant component.[2] For example, argillaceous limestones are limestones[3] consisting predominantly of calcium carbonate, but including 10-40% of clay minerals: such limestones, when soft, are often called marls. Similarly, argillaceous sandstones are sandstones consisting primarily of quartz grains, with the interstitial spaces filled with clay minerals.