Feldspar authigenic

MacKenzie and Garrels equilibrium models. Most marine clays appear to be detrital and derived from the continents by river or atmospheric transport. Authigenic phases (formed in place) are found in marine sediments (e.g. Michalopoulos and Aller, 1995), however, they are nowhere near abundant enough to satisfy the requirements of the river balance. For example, Kastner (1974) calculated that less than 1% of the Na and 2% of the K transported by rivers is taken up by authigenic feldspars. 

Kastner, M. (1974). The contribution of authigenic feldspars to the geochemical balance of alkalic metals. Geochim. Cosmochim. Acta 38, 650-653. 

KASTNER (M.), 1971. Authigenic feldspars in carbonate rocks. Amer. Min. 

Milliken K. L. (1989) Petrography and composition of authigenic feldspars, Oligocene Erio Formation, South Texas. J. Sedim. Petrol. 59, 361—374. 

Other minor constituents are authigenic pyrite which appears as framboides or separate crystals secondary gypsum, mainly as vein filling detrital quartz grains and authigenic feldspar (microcline). 

The sample suite from each core can be divided into four factors on the basis of similarities in chemical compositions as defined by Q-mode factor modeling. Factor-1 samples are rich in trace metals due to adsorption onto clay, altered tuffaceous material, and (or) organic matter and precipitation as sulfides. The relatively high concentration of boron is probably related to its inclusion in authigenic feldspars. Factor-2 samples are rich in elements commonly associated with minerals of detrital or volcanic origin. These samples contain relatively high concentrations of analcime, dawsonite, and (or) potassium feldspar, all of which are associated with alteration of tuffaceous material. Siderite and ferroan or ankeritic... 

The formation of such minerals and authigenic feldspar under highly saline conditions is sometimes termed reversed weathering. This is appropriate in the sense that ions weathered in other locations are thus incorporated into new minerals instead of flowing to the sea. The minerals formed, however, are considerably different from the original igneous aluminosilicates, although their chemical compositions may be similar. 

Occasionally the course of weathering can reverse in the sense that feldspar weathering will create secondary silicates that are unstable under the leaching and weathering conditions of well drained soils. Such reversal occurs because of the accumulation of K+, Na+, Ca2+. Mg2, and Si(OH)4 in arid and poorly drained soil solutions. These secondary silicates include zeolites, evaporites, and the authigenic feldspars. The area denoted as soil solution in Fig. 7.6 shows the extreme concentrations that have been reported in soil solutions. Within this range, several silicate minerals are stable. 

Feldspars are the most abundant minerals of igneous rocks, where their ubiquity and abundance of their components influence normative classifications. They are also abundant in gneisses, and may be observed in several facies of thermal and regional metamorphic regimes. Notwithstanding their alterability, they are ubiquitously present in sedimentary rocks, as authigenic and/or detritic phases. Only in carbonaceous sediments is their presence subordinate. 

Desborough G. A. (1975). Authigenic albite and potassium feldspar in the Green River formation, Colorado and Wyoming. Amer. Mineral, 60 235-239. 

MOIOLA (R.J.), 1970. Authigenic zeolites and K-feldspar in the Esmeralda formation, Nevada. Amer. Min. 5J5, 1681-91. 

Some of the lMd material (either illite or mixed-layer illite-montmorillonite) presumably formed authigenically on the sea bottom or on land from the weathering of K-feldspars however, much of it was formed after burial. Studies of Tertiary, Cretaceous, and Pennsylvanian thick shale sections (Weaver, 1961b) indicate that little lMd illite was formed at the time of deposition. These shales and many others contain an abundance of expanded 2 1 dioctahedral clays with a lMd structure, some of which is detrital and some of which formed by the alteration of volcanic material on the sea floor. With burial the percentage of contracted 10A layers systematically increases. 

Sedimentary pools of P have generally been divided into the following fractions (1) organic P, (2) Fe bound P, (3) authigenic P minerals (e.g., CFA, struvite, and vivianite), and (4) detrital P minerals (e.g., feldspar). 

The concentrations of potassium in the samples obtained from the Norphlet Formation in the central Mississippi Salt Dome Basin are those expected from the Louann Salt bittern potassium values in other samples obtained from reservoirs of Jurassic age are lower by a factor of —2 (Kharaka et al., 1987). The decrease in the dissolved potassium in these samples is attributed to the formation of authigenic illite and potassium feldspar (Carpenter et al., 1974 Kharaka et al., 1987). 

Figure 5 Highly microporous aggregate of authigenic albite replacing a K-feldspar (evidenced by the adularia overgrowth, K). Frio Formation, Oligocene, South Texas. SEM image (source Milliken, 1989) (reproduced by permission of SEPM (Society for Sedimentary Geology) fromi. Sedim. Petrol, 1989, 59, 364).

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.

Other common, though volumetrically minor, feldspar-replacing minerals include titanite, ana-tase, sphalerite, barite, ankerite, siderite, and fluorite. With the exception of replacement driven by force of crystallization, feldspar replacements have intracrystalline distributions that are strongly localized at sites of surface-controlled dissolution. Interestingly, replacement of detrital feldspars by authigenic clays is rarely observed in late diagenesis. 

Clays are volumetric ally the most abundant mineral group in coal. They can be authigenic or detrital in origin. Kaolinite is the most common clay and the most common authigenic mineral in coals. The silicon and aluminum in kaolinite are, perhaps, residual from the dissolution of ferromagnesian minerals and feldspars. Illite and mixed layer clays in coal are almost exclusively detrital in origin. Chlorites, smectites, and other clay minerals may be abundant locally. 

The sources of Si for cave-authigenic silicates were most likely the silicates disseminated in the dolostone units of the Permian Capitan reef complex. Silt-sized quartz and minor feldspar, and clays such as illite and dickite, made up approximately 1-5% of the dolostones. 

Hearn, P. P., and Sutter, J. E., 1985, Authigenic potassium feldspar in Cambrian carbonates evidence of Alleghanian brine migration Science, v. 228, no. 4707, p. 1529-1531. 

Fig. 10. SEM pictures of a Melke Formation siltstone sample (well 6506/12-4 depth 3979.3 mKB). (A) Detrital potassium feldspar undergoing dissolution. (B) Authigenic kaolinite.

The Maastrichtian (latest Cretaceous) Israeli oil shales consist of four main groups of components organic matter biogenic calcite and apatite detrital clay minerals and quartz, with a little amount of authigenic pyrite and feldspar. The main chemical characteristics of the oil shales are reviewed,with an emphasis on those which may affect future utilization techniques. 

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