Pemmatites macroporus Dunikowski, 1884

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Discussion:

This sponge has a radial and cross-hatched arrangement of major spicules
that are intergrown, producing a strong skeletal framework for the sponge.

The preservation is distinctive, consisting of a silicified matrix in which the
original spicules (of opaline silica) have been dissolved, leaving many cavities
where the spicules had been located. The broken surface shows that some
spicule cavities remain unfilled while others have been filled with secondary
calcite. The spicules shown in the thin sections are composed of a mixture
of silica and replacement calcite. The dark patches on the specimen are
areas where silicification has formed a solid chert mass that destroys the
original fabric, contrasting with the incomplete silicification of the matrix.
The dark area on the left side of the broken surface is partly chertified
and the right area is completely chertified.

Acknowledgment:

Thanks to Dr. Yuri V. Yashunsky (Moscow, Russia)
for microphotography of this specimen.

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