Authigenic feldspars occur in both marine limestones and dolostones, and no correlation has been found between the amount of MgCO3 in the carbonate rocks and any chemical or physical properties of authigenic feldspars.

Optical and unit-cell parameters indicate that authigenic feldspars are highly ordered but not as ordered as low albites and microclines from low-grade metamorphic rocks and pegmatites. The albites contain less than 1 mole percent potassium and calcium and the microclines contain less than one mole percent sodium and calcium. Authigenic feldspars show no detectable cathodo-luminescence.

The composition of the carbonate inclusions that are present in all authigenic feldspars, is the same as that of the rock matrix.

Authigenic albite is far more abundant than authigenic microclin e. In a few localities the two feldspars coexist, apparently in equilibrium as indicated by textural relations.

A mass balance between the maximum total percentage of authigenic feldspars and mica components in carbonates and the amount of fluid phase needed to provide the alkalies for the feldspars, suggests isochemical formation of authigenic feldspars. Calculations based on ion exchange and thermodynamic data from the literature show that the K/(K+Na) ratio in the fluid phase in equilibrium with both albite and microcline at 100°C is approximately equal to the K/(K+Na) ratio in sea water. Thus the chemical environment of formation of authigenic albite does not necessarily differ appreciably from the environment of authigenic microcline.

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