Plagioclase detrital

Figure 8.9. Log of molar concentration of K+ in equilibrium with K-feldspar in 3m NaCl solution, and the log of the molar concentration of Ca2+ in equilibrium with two kinds of typical detrital plagioclase in 3m NaCl solution. (After Land, 1987.)...

Detrital feldspars in modern sediments have a compositional range that reflects their composition in the source rocks (Trevena and Nash, 1981). During diagenesis, sodium-rich K-feldspars and more calcium-rich detrital plagioclase grains are more subject to dissolution (and replacement) than potassium-rich K-feldspar or sodium-rich plagioclases (Maynard, 1984 Milliken, 1988, 1992 Milliken et al, 1989). 

Morad S., Bergan M., Knarud R., and Nystuen J. P. (1990) Albitization of detrital plagioclase in Triassic reservoir sandstones from the Snorre Field, Norwegian North Sea. J. Sedim. Petrol. 60, 411-425. 

The parent Na—Ca—Cl brine originates deep in the Gulf of Mexico basin, at temperatures between 200 and 250°C, by the reaction halite -1- detrital plagioclase -1- quartz -t- water albite -I- brine... 

Authigenic albite occurs as discrete euhedral crystals (10 pm), overgrowths on detrital feldspars, and most commonly as replacement of detrital K-feldspar and plagioclase in the proximal and middle domain. K-feldspar grains in these areas commonly show partial albitization, whereas the detrital plagioclase is totally replaced by albite. Albitized feldspar grains display the typical optical, textural and chemical characteristics outlined by... 

In thin section, calcite appears most commonly as scattered euhedral crystals from 10 pm up to 500 pm (Fig. 4C,D). Completely cemented samples have an interlocking mosaic of crystals that fill pore space but typically do not replace detrital grains. Some sandstones with abundant calcite cements are relatively uncompacted, with uncrushed biotites, and these generally do not have partially dissolved detrital plagioclase grains, whereas adjacent compacted and uncemented sandstones may exhibit up to 5% secondary porosity after plagioclase. These cements are inferred to be relatively early. 

Lithic fragments are dominantly of metamorphic and plutonic origin. The majority of samples are classified as arkoses (Folk et al. 1970) with an average QFR ratio of 68 22 10. The detrital feldspar grains consist of approximately equal amounts of K-feldspar and plagioclase. The sandstones are extremely clean and lack any recognizable detrital clay matrix. 

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