A fine-grained, igneous rock consisting of albite or oligoclase as the major component, accompanied by chlorite, epidote, or calcite. Phenocrysts of unaltered augite are sometimes present. The mineral assemblage is almost entirely secondary in origin, indicating that the original rock was a tholeiitic andesite, the volcanic equivalent of plagiogranite. Keratophyres are found on the ocean floor, and in ophiolite complexes where they are associated with spilites. Like that of the spilites, the mineral assemblage of keratophyres may result from low-grade, ocean-floor metamorphism.