Said of an igneous rock having a silica content lower than that of a basic rock. Percentage limitations are arbitrary; the upper limit was originally set at 44%. The term is frequently used interchangeably with ultramafic. Although most ultrabasic rocks are also ultramafic, there are some exceptions; e.g., monomineralic rocks composed of pyroxenes are ultramafic but are not ultrabasic because of their high silica content. A monomineralic rock composed of anorthite would be considered ultrabasic (SiO2 = 43.2%) but not ultramafic. Ultrabasic is one subdivision of a widely used system for classifying igneous rocks on the basis of silica content; the other subdivisions are acidic, basic, and intermediate. CF: ultramafic; silicic. AGI
Source:
Dictionary of Mining, Mineral, and Related Terms












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