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A polyhedron all of whose vertices lie in one of two parallel planes.

[Greek prīsma, prīsmat-, prism + -oid.]

pris′ma·toi′dal (-toid′l) adj.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
References in periodicals archive ?
William Haidinger (1826) wrote in the Edinburgh Journal of Science that "Few species in mineralogy have been so incorrectly described as the ores of manganese, and, in particular, the most common one among them, the prismatoidal manganese-ore." The various black oxides and hydroxides of manganese have indeed proven difficult for professional mineralogists and casual mineral collectors alike to distinguish, as all are black, many are poorly crystallized, and their compositions can include several subordinate components (such as OH, Ba and K).
Until 1828, mineralogists customarily used the term "prismatoidal manganese ore" for manganite (Mohs, 1825; Haidinger, 1826)--considering it among other black manganese species including "pyramidal manganese ore" (hausmannite), "brachytypous manganese ore" (braunite), "prismatic manganese ore" (pyrolusite) and "uncleavable manganese ore" ("psilomelane," now discredited in favor of cryptomelane).