Detrital feldspar assemblages (1 to 4 km burial depth) in Oligocene sandstones from South Texas are extensively altered by dissolution. In some samples, cementation in secondary pores, principally by albite but also by authigenic K-feldspar and other minerals, occurs subsequent to the dissolution and can lead to wholesale replacement textures. Solid-state diffusion of cations has not been important in the replacement process. Information from conventional light microscopy, SEM examination, and back-scattered electron imaging suggests that relative rates of dissolution versus replacement varied widely among different detrital grains through time, even on the scale of a thin section. Dissolution of detrital feldspars and precipitation of relatively pure albite and K-feldspar take place over a large depth interval and thus can be markedly separated in time. Vacuolized albite occurring as irregular veins within detrital grains is associated with little visible secondary porosity and forms when precipitation is rapid relative to dissolution of the detrital grain. More rapid dissolution results in formation of larger-scale intragranular secondary pores, commonly filled by subsequent precipitation of clear albite within the former grain volume. Electron microprobe analyses show that the composition of vacuolized albite and authigenic K-feldspar deviate somewhat from ideal compositions presumed typical for authigenic feldspars. Regardless of their cause, these subtle variations in composition have important ramifications for basin-scale mass balance calculations requiring accurate estimates of the volume of authigenic feldspar.

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