Page 91 - Petrology of Sedimentary Rocks

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Dashed lines represent contours on homogeneity of weathering, varying from very
homogeneous (at the upper left) to very heterogeneous (at lower right); solid, curved
lines indicate average amount of weathering (all fresh at lower left, all weathered at
upper right) and also indicate roughly the abundance of feldspar, increasing amounts
being found toward the lower left.

Authigenic feldspar is common in marine sediments. Orthoclase, microcline, and
albite have been formed authigenically, either as overgrowths on feldspar sand grains in
clean sandstones, or as overgrowths or new crystals without nuclei in carbonate rocks.
In limestones they grow by replacing carbonate (cut across fossils, oolites, etc.) under
low temperatures and probably low pressures; in sandstones they form by direct
precipitation, again usually under conditions of low temperature and pressure. Authi-
genie feldspar frequently contains inclusions of carbonate, liquid, or organic matter;
sometimes authigenic feldspar of one species may grow over a nucleus of a different
feldspar type, e.g., microcline on an oligoclase nucleus. Occasionally the overgrowths
are complexly twinned, but usually they are free of twinning.

Occasionally feldspar forms in non-marine, clay-rich rocks (shales or clayey
sandstones) by replacement of the clay. Potassic solutions are required, supplied either
by connate or hydrothermal solutions. In some tectonic non-marine arkoses, the
original red clay matrix has been replaced by feldspar in the form of overgrowths and
felted masses of crystals. On burial, some zeolites alter to feldspars especially in
volcanic sediments.

Volcanic Rock Fraqments. These are a quite diagnostic constituent of sediments,
and represent sand-sized bits of the aphanitic groundmass of lava flows hypabyssal
intrusives or volcanic ejectments. They are made up usually of a felted mass of tiny
lath-like feldspar crystals, occasionally with some ferromagnesian minerals. They are

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