Low temperature feldspars in sedimentary rocks

Abstract

Authigenic feldspars form in many different kinds of sediments as well-crystd. grains of albite [12244-10-9] or K-feldspar. They are pure end-members, occurring either as homogeneous grains or more frequently as overgrowths around detrital cores of detrital feldspar, quartz, and non-silicates. Textural relations indicate growth in place by replacement; most have formed early after deposition. Albite is more abundant than K-feldspar in carbonate rocks, but the reverse is true for authigenic feldspars in sandstones, except for graywackes, in which albite dominates. Authigenic feldspars are assocd. with zeolites in some sandstones, in volcaniclastic rocks, of alk. lake deposits. Authigenic feldspars occur in rocks of all ages. Albites are triclinic euhedra that are frequently twinned. K-feldspars are triclinic or monoclinic, with various degrees of Al/Si ordering. Pure compn., lack of cathodoluminescence, nature of twinning, and high pos. d18O values are diagnostic of authigenic origin of feldspars. Thermodn. stable assemblages in the system SiO2-Al2O3-K2O-Na2O-H2O, plus the addn. of CO2, CaO, MgO, and FeO, include feldspars, clay minerals, micas, carbonates, and several oxides and/or hydroxides. A kinetic anal. of the many geochem. processes accompanying shallow and deep burial leads to several geochem. models for authigenic feldspar formation that depend on the supply of dissolved SiO2, either from biogenic SiO2 or from the hydrolysis of volcanic glass, and the supply of alkali metals, either from seawater or from detrital and diagenetic aluminosilicates. Al is considered in most sedimentary environments to be present mainly in solid form, and in extremely low concns. as dissolved species, except in some alk. lakes. The preferred models are for isochem. change, for most petrog. evidence favors this origin. Some occurrences, however, must be analyzed in terms of exchange reservoir models. [on SciFinder (R)]