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Methylene blue adsorption (MBA) is found to be an effective method of indicating when smectite (group name for the mont-morillonite minerals) is present in major amounts in mudrocks or along discontinuities. The basic relationship is that MBA values greater than 15 indicate that smectite is present as 15 is the maximum value found for illite, the next most adsorptive mineral as compared with smectite. For example, the Pierre shale, which typically contains 30 to 40% smectite along with 25 to 45% mixed-layered smectite-illite has a MBA value of 48.5; whereas, the Palastine shale, which has illite rather than smectite as its chief component has a MBA value of only 7.0. The abundant smectite constituted samples are of materials involved either in slope stability or slaking in foundations, or both. MBA requires only a 2-g specimen, and thus clays along discontinuities and clays along thin slip zones of slides can also be analyzed as to type of clay. The method is less expensive than X-ray diffraction analyzes but also can supplement X-ray diffraction investigations.
Retired petrographer-engineering geologist, Portland, OR
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