a. A texture sometimes observed in a thin section of a rock, in which a
ring of one mineral occurs with another mineral or minerals inside and
outside the ring.
b. In mineral deposits, the surrounding of one mineral by a ring of one or
more other minerals; commonly results from replacement of pyrite by
another mineral, with the outermost pyrite unaffected and constituting the
"atoll." Syn: core texture
Dictionary of Mining, Mineral, and Related Terms