Feldspar detrital

Very simply, a sandstone has a grain composition of stone detritus (quartz, feldspar, detritic mica/clays) and a binder that may be clayey, quartzitic, or carbonatic (or mixtures thereof) [51, 55]. The mineral composition of limestone is mostly calcium carbonate, often derived fi om precipitated calcium carbonate and fossil shell remnants fi-om marine organisms and a calcitic binder matrix [51, 56, 57]. 

Silicification Through Transformation of Feldspars, Detrital lllite and Other Micaceous Components... 

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. 

The cations become a component of river water and are eventually transported to the sea. About 45% of the dissolved solids entering the ocean are derived from the weathering of detrital silicates. Feldspars are the most important somce rock for terrigenous clays as illustrated by the following reaction... 

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. 

Many investigations have reported the presence of zeolites at the deep ocean bottom (Biscaye, 1965 Heath, 1969 Bonatti, 1963 Sheppard and Gude, 1971 Jacobs, 1970 Morgenstein, 1967 among others). Most of the alkali zeolites are represented except the silica-poor species natrolite and analcite. Rex and Martin (1966) indicate that detrital potassium feldspar is not stable under ocean floor conditions. Zeolites are found in most ocean basins where wind-carried volcanic ash predominates over detrital river-born clay mineral sediments. In these sediments phillipsite is particularly evident and it is known to continue to grow in the sediment column to depths of more than a meter (Bernat, t al.,... 

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. 

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.)...

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). 

Detrital minerals minerals introduced by weathering of parent rock materials (e.g., feldspar). 

Copeland P. and Harrison T. M. (1990) Episodic rapid uplift in the Himalaya revealed by Ar/ Ar analysis of detrital K-feldspar and muscovite, Bengal Fan. Geology 18, 354-357. 

Hemming S. R., McDaniel D. K., McLennan S. M., and Hanson G. N. (1996) Pb isotope constraints on the provenance and diagenesis of detrital feldspars from the Sudbury basin, Canada. Earth Planet. Sci. Lett. 142(3— 4), 501-512. 

Figure 1 Detrital orthoclase grains (K) in subarkosic sandstone, from Moodies Group, Barberton Mountains, South Africa. Apart from dissolution that may relate to modem outcrop weathering (blue), these feldspars have survived since the Archean (at least 3.2 Ga).

On a large scale, replacements may appear pseudomorphic after original detrital grains on a small scale, replacements are very imperfect. Typically they consist of microporous aggregates that only partially mimic the original crystal (Figure 5 Milliken, 1989). Many cements and replacements have a close spatial association, because they are both localized on the same substrate. For example, an ankerite crystal that replaces a detrital feldspar will often also fill an adjacent primary pore space. 

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.

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.

Detrital feldspars in both sandstones and shales are subject to dissolution and replacement during late diagenesis. As previously discussed, replacement processes during diagenesis commence with... 

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). 

Small secondary pores are frequent in feldspars of modem sediments (Passaretti and Eslinger, 1987 Brantley et al., 1999). Additional dissolution of detrital feldspars proceeds through the earliest stages of diagenesis (Milliken, 1988). Sandstones flushed with large quantities of meteoric water are especially prone to loss of feldspars through dissolution (Mathisen, 1984). 

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. 

In the Guff of Mexico, the increase in Pqq occurs to a greater degree (up to 16 mol.%) and at shallower depths in Eocene units (Wilcox formation) that contain smaller quantities of less-reactive detrital feldspars (of plutonic as opposed to volcanic provenance) (Lundegard and Land, 1986). Apparently, as the detrital feldspar supply is exhausted, pH buffering shifts to the carbonate assemblage. The FcOo rises... 

Diffusional transfers of potassium and silicon between sandstones and shales may be sufficient to accomplish feldspar dissolution, illitization, and quartz cementation (Thyne, 2001 Thyne et al, 2001). Losses of the magnitude observed for detrital carbonates in shales exceed the capacity of diffusion-mediated transfer. Large-scale advection seems required, although our understanding of shale permeabilities seems to preclude this (Bjprlykke, 1989, 1993 and Lynch, 1997). The possibility of convection driven by salinity heterogeneity within thick shale sequences has been demonstrated by Sharp et al (2001), who note that more information for rock properties and fluid compositions within deep basinal shales is needed before the generality of their results can be assessed. 

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