There are several different types of clay minerals, including kaolinite, smectite and illite. These minerals are differentiated from one another using an x-ray diffractometer.
Color: light color (white, tan, light green, etc.)
Luster: dull, earthy
Diaphaneity: too fine grained to tell
Specific gravity: 2.6
Cleavage/fracture: 1, but it's too small to tell
Other distinguishing properties: occurs in aggregates of dust-fine grains. Some clay minerals expand in the presence of water, and these expansive clays constitute a considerable hazard to buildings whose foundations rest on them. Clay is comm on in sedimentary rocks such as sandstone, arkose, argillaceous sandstone, graywacke, mudstone, shale, claystone, siltstone.
Klein, C., and Hurlbut, C.S., Jr., 1999, Manual of Mineralogy (after James D. Dana) [21st edition, revised]: New York, John Wiley & Sons, 682 p.
Los Angeles County Museum of Natural History, The Photo Atlas of Minerals: nhm.org/pam/
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