Opoka

opoka

[ō′päk·ə]
(petrology)
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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Opoka

 

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.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.
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George EC, Walker AS, Kiguli S, Olupot-Olupot P, Opoka RO, Engoru C, et al.
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(12.) George EC, Walker AS, Kiguli S, Olupot-Olupot P, Opoka RO, Engoru C, et al.
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[22.] Holmberg D, Franzen-Roh E, Idro R, Opoka RO, Bangirana P, Sellgren CM, et al.