Albite detrital

Figure 7 Replacement driven by force-of-crystal-Uzation is characterized by authigenic phases that develop euhedral faces that are not plausibly constmed as crystal growth within pore spaces (a) sphalerite replaces albitized detrital feldspar and adjacent portions of clay-rich matrix, Frio Formation, Oligocene, South Texas and (b) siderite crystal (s) attacks a detrital K-feldspar (K) in sandstone, Breathitt Formation, Pennsylvanian, eastern Kentucky.

Even detrital or authigenic minerals in bulk equilibrium under some specific diagenetic condition (e.g., detrital albite in contact with albite-saturated water) may still be at disequilibrium with respect to their trace element and isotopic composition, as weU as their structural state. Such disequilibrium favors dissolution, even if fluids are near equilibrium with respect to the bulk mineral. 

Figure 6 Patchy calcite (c) replacing detrital feldspar (now albite, ab) in sbale. Similar microscale distributions of replacement phases are observed widely in sandstones and sbales. Frio Formation, Oligocene, South Texas. Backscattered electron image.

Aagaard P., Egeberg P. K., Saigal G. C., Morad S., and Bj0rlykke K. (1990) Diagenetic albitization of detrital K-feldspars in Jurassic, Lower Cretaceous, and Tertiary clastic reservoir rocks from offshore Norway 11. Formation water chemistry and kinetic considerations. J. Sedim. Petrol. 60, 575-581. 

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... 

Fig. 7. Present and original detrital composition of representative Lunde sandstones plotted on McBride s (1963) diagram A, considering albitized feldspars as diagenetic constituents (not in F pole)—observe the shift from the arkose (original composition white dots) to the subarkose field (present composition black dots) B, plotting albitized feldspars in F pole—a less substantial field dislocation is seen from original (white squares) to present composition (black squares).

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... 

Albitization is another possible source of Ca (e.g. Land et al., 1987), but albitization is not uniform across the formations examined (Cibin et al., 1993 Milliken McBride, unpublished data). Isotopically evolved formation water from deeper in the basins, interpreted from oxygen isotopic data from calcite cement, probably contributed the bicarbonate component of cement (see below) the same water may have contained calcium released by albitization of detrital feldspar assemblages in deeper, more altered sandstones. The amount of calcium introduced from deeper in the basins, however, was probably miniscule compared with locally derived calcium. 

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