clay - teaching hand specimen of non-expanding calcium montmorillonite clay
There are very few 100% montmorillonite clays. This clay is composed of montmorillonite, quartz and calcite and was formed by alteration of volcanic ash in a Plio-Pleistocene lake bed. It was mined, ground and air dried for use as a binder in alfalfa cubes shipped to Japan. The mine is no longer active. This clay is an attractive white clay that shows phyllosilicate cleavage.
The name montmorillonite was given to a smectite clay found near Montmorillon, France. Smectite clays are phyllosilicates (from the Greek phyllon for leaf) with a molecule like a mineral sandwich, having an octrahedral silicate layer between two tetrahedral silicate layers. Montmorillonite clays can absorb water between the layers, allowing the clay to swell greatly, though this clay does not swell. Other swelling clays are bentonite and hectorite. Their names are often used interchangeably.
Near the Four Corners of the U.S. Southwest, there is one source of montmorillonite for potters. Native Americans used this clay as a slip, painted over the surface of their pottery. When they applied a black paint made from bee weed, that paint burned off the surface and went into the outer layer of clay when the pottery was fired in a reducing flame. Two examples of faithful prehistoric Southwestern pottery reproductions made using this technique are shown here, courtesy of the potter, Erik Ludwig.
Phyllosilicates are characterized by their layered structure. Serpentine, talc and micas are in this group. Most products of rock weathering - the constituents of soils - have this silicate layering. The ability to stockpile water in the soil, to retain and release plant nutrients, and the accessibility of the soil to atmospheric gases and organisms are all due to the sheet silicate structure.
The specimens of this clay will stand up to student inspection, but will break if dropped.
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