A porous, flinty, and calcareous sedimentary rock, with conchoidal or irregular fracture, consisting of fine-grained opaline silica (up to 90%), and hardened by the presence of silica of organic origin.



a tough, porous, flinty sedimentary rock. Opoka consists of up to 97 percent fine-grained hydrous amorphous silica, usually with admixtures of clay, sand, glauconite, and other substances. Poorly preserved remains of diatoms and sponge spicules are present.

Opoka is distinguished from tripoli, which is similar in structure, by its greater uniformity and its conchoidal fracture. Its color ranges from light gray to dark gray and nearly black. Pure varieties of opoka have good adsorption properties. The Soviet scientist Ia. V. Samoilov, who introduced the term “opoka” into Russian geological literature in its modern meaning (1917), classifies opoka with rocks of organogenic origin; the American geologist W. H. Twenhofel and others classify it with rocks of chemogenic origin. Opoka is found primarily in Paleogene deposits and, to some extent, in Upper Cretaceous deposits. It is used in construction and as an adsorbent.

References in periodicals archive ?
Tenders are invited for delivery of pupils to primary schools in dabrowa, grabwka, opoka duza, swieciechw duzy and annopol
George EC, Walker AS, Kiguli S, Olupot-Olupot P, Opoka RO, Engoru C, et al.
Montini coach Brian Opoka said he's excited for the next two matches against Marmion, both in Aurora.
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Opoka, "The role of zinc in the pathogenesis and treatment of central nervous system (CNS) diseases.
Opoka et al., "Mortality after fluid bolus in African children with severe infection," The New England Journal of Medicine, vol.
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Their nine member team includes three returning incumbents: Susan Nakawuki, Fred Mukasa Mbidde and Chris Opoka.
[22.] Holmberg D, Franzen-Roh E, Idro R, Opoka RO, Bangirana P, Sellgren CM, et al.