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 ?
Their nine member team includes three returning incumbents: Susan Nakawuki, Fred Mukasa Mbidde and Chris Opoka.
George EC, Walker AS, Kiguli S, Olupot-Olupot P, Opoka RO, Engoru C, et al.
10% of cement content was replaced (by weight) with modifying admixtures: opoka (Composition III), silica fume suspension (Composition IV) and dolomite dust (Composition VI).
Robert Johnson (3) Carolyn Mueller and Sarah Opoka (4) Kevin Woods, Lois Woods and Eric Saczawa (5) Jay DiGeronimo and Dean Hickey (6) Mainsouth, from left, James Mercier, Joel Helander and Mike Tierney
internet publications by authors associated with the Catholic Church, particularly those posted on popular Catholic portals (such as opoka.
Maitland K, Kiguli S, Opoka RO, Engoru C, Olupot-Olupot P, Akech SO et al.
Abdinasir Abubakar, MD, South Sudan, World Health Organization, Martin Opoka, MD, Eastern Mediterranean Region, World Health Organization.
Nowack G, Szewczyk B, Polszak E, Librowski T, Opoka W and A Pilc Zinc and magnesium as natural antidepressants: basic and clinical data: Macro, Micro and Trace Elements, 22nd Workshop 24th-25th September 2004, JENA, GERMANY.
Manome, kad ir dolomitas kartu su kitomis Lietuvoje randamomis ar importuojamomis uolienomis (klintimis, opoka, granitu, trepeliu) gali buti panaudotas kaip mineralinis betono priedas (mikrouzpildas).
Today, as well as earlier, 10 kinds of resources are extracted, such as dolomite, peat, limestone, clay, gravel, oil, marl, opoka (gaize), sapropel, however, extraction of the last three kinds is carried out discontinuously, and that of oil--was started only in 1990.
trepelis, opoka, iskaitintas molis, dolomitas, klintys, skaluno pelenai, smelis, kalkes ir ivairiu medienos atmainu sviezios pjuvenos (23 %--lapuociu ir 77 %--spygliuociu).
But if there is unsubtypable (UNSUBTYPABLE) H1N1 A influenza virus, the samples have to be sent to WHO designated laboratories for confirmation," Dr Martin Opoka, an epidemiologist with the EMRO said.