Mountain soap
old name for a large group of minerals, soap clays
Mountain soap (Latin: Argilla saponiformis, German: Bergseife, French: Savon de montagne), rock-soap or bolus — a partially outdated trivial name for a large group of clay minerals similar in properties from the group of hydrous layered aluminum silicates with variable composition. Minerals from the mountain soap group contain primarily silicates (44-46%), alumina (17-26%), iron oxides (6-10%) and water (13-25%). The mountain soap group included at different times up to two dozen mineral species and varieties. In different cases, this name could mean different minerals, most often halloysite (from the proper name), saponite (soapstone), bentonite or montmorillonite (from the French: Montmorillon, toponym). The last mineral is a large group, each of which could be called mountain soap. All these minerals received their common colloquial name, which gradually penetrated into mineralogy, for their ability to lather and serve as a detergent for various purposes.
Mountain soap has some common properties that are characteristic of all minerals included in the group. All of them have very low hardness (from 1 to 2 on the Mohs scale) and are a typical weathering product of aluminosilicates. In the form of its main component (bentonite, which is formed during the weathering of volcanic rocks — tuffs and ash), mountain soap is one of the main minerals in many types of soils, and it is also found in many sedimentary rocks. Thanks to its layered "batch" structure, mountain soap has the ability to absorb water, swell strongly and has pronounced sorption properties.
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This entry incorporates text from “Mountain soap” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.