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Cherts from the Mississippian/Pennsylvanian Lisburne Limestone of northern Alaska can be divided into 2 distinct fabric types. These are (1) intragranular cherts in which the silica has started nucleation within the skeletons of crinoids and pelecypods, and (2) matrix chert which occupies the intergranular spaces and shows replacement of contained bioclastic debris only from the exterior. The matrix chert is considered to be of direct biogenic origin, but preservation of the original skeletons is very rare because of their progressive recrystallization since deposition. The intragranular chert began nucleation early in the diagenetic process, probably as a result of the reaction of the organic tissue in the crinoid pore-spaces with the silica of the interstitial waters. T e organo-silicic acid produced by this mechanism then could react with the calcium carbonate of the crinoid plates and deposit silica on a piecemeal basis.
Dolomite is a common accessory of chert even when dolomite is absent from the adjacent limestone host-rock. The dolomite usually is distributed homogeneously through the chert either as euhedral or corroded rhombs. In places iron-rich dolomite forms a reaction-rim at the chert-calcite interface. A process of progressive solution of dolomite in the interior of the chert and reprecipitation at its edge is necessary to explain this distribution. Dolomite probably is only locally soluble in organo-silicic acids and the source of its magnesium is almost certainly the high-Mg calcite skeleton of the crinoid.
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