“Darn that old Gumbo!” is a phrase you hear all too much this time of year. But what is that “old Gumbo”. Geologically speaking, “gumbo” is a garbage basket term that refers to sticky soil that usually very high in clay. The term can refer to any kind of clay but some kinds of clay are more sticky than others. Remember that clay is actually a size term in geology. It has to do with very small particles less that 1/256 of a mm in size (darn small as there are 25 mm to the inch!). This is smaller than silt and considerably smaller than sand sized particles. These size terms have no relationship to composition. For example, you can have sand that is rich is quartz, or feldspar or even exotic material like magnetite or garnet. So you can have a quartz sand or a garnet sand. Clays have the same thing going on. A rare clay can contain particles made of quartz grains that are very small or the small particles can be made of some other exotic material as well.
Almost all clays are weathering by products of another material called Feldspar. Feldspar is the second most common mineral on the planet (after quartz) and easily breaks down into clay minerals. These clay minerals have exotic names like Illite, Kaolinite, and Smectite. Montmorillinite is another exotic name in the geologic literature. It is the last clay, Montmorillinite that I will focus on here. The common name for Montmorillinite is Bentonite. Bentonite is actually a byproduct of weathered volcanic ash. The ash was originally wind sorted from its original volcanic source. Bentonite actually contains as many as 33 different clay minerals in the mix. Most of these minerals consist of booklets of flat sheets (like mica) but are obviously very small. This gives the material very interesting properties. Besides medicinal uses (which I will not get into), the clay has a tremendous capacity for absorbing water and expanding its surface area (swelling.) A single quart bottle of Bentonite (when hydrated and a total surface area of 960 square yards). The individual platelets of Bentonite also have a negative charge on the flat surface and a positive charge on the edges. This gives it great ability to adsorb toxins like heavy metals and pesticides by literally attracting the toxins like flies to sticky fly paper. This ability to attract and hold impurities make Bentonite a valuable mineral for industry. What a great place to live when the rest of the world want to buy our dirt! Colony, Wyoming, for example, is a place build on and paid for by Bentonite production.
One of the less than desirable effects of Bentonite occurs when you try to put a building on soils rich in the clay. When the soil gets wet, it swells which is only a problem if you want your building to stay level, not have cracked concrete floors. Engineers are trained to look at the tendency of certain soils to swell and often suggest avoiding certain building sites based on the presence of Bentonite in the soil. The only choice you have if you have build on a Bentonite rich soil, is to keep it dry. This same property make Bentonite valuable for sealing ponds and dams. The swelling property seals leaks automatically as soon as the material gets wet.
Some of you may know that I like to play with extreme 4X4 trucks. There isn’t a 4X4 truck made (or any other vehicle for that fact) that can drive up a hill covered in wet Bentonite. My advice is, if you have Bentonite on your place, let it dry out before you try to pass over the “Gumbo”.