Silicate minerals, authigenic

Zeolite deposits that formed by the above mechanism commonly show a vertical zonation of authigenic silicate minerals similar to that in the John Day Formation. Tertiary tuffs at the Nevada Test Site in southern Nevada were altered after burial by subsurface water (59), but the authigenic mineral zonation is more complex than that in the John Day Formation. The upper zone consists of unaltered glass with local concentrations of chabazite or clay minerals. Zeolitic tuff continues downward for as much as 6000 feet. A zone rich in clinoptilolite underlies the zone of unaltered glass and is succeeded downward by zones rich in mordenite and analcime, respectively. 

McDowell, S.D. and Elders, W.A. (1980) Authigenic layer silicate minerals in borehole Elmore 1. Salton Sea geothermal field, California, U.S.A. Contr. Mineral. Petrol., 74, 293-310. 

Finally, some authigenic clay minerals are produced by the reaction of seawater with fresh volcanic glass. This commonly occurs near mid-ocean ridges and rises or where lava from coastal volcanoes flows into the sea. Clay minerals produced by this process are primarily smectites, such as montmorillonite, and a type of framework silicate called zeolites of which phillipsite and clinoptilite are the most common marine examples. Zeolites are characterized by three-dimensional frameworks with large cavities that... 

Rex, R.W., 1967. Authigenic silicates formed from basaltic glass by more than 60 million years contact with sea water, Sylvania Guyot, Marshall Islands. Clays Clay Miner., Proc., 15 195-203. 

It should be noted that reactive Fe is operationally defined as the dithionate-extractable Fe minus the Fe associated with FeS—this definition incorporates crystalline Fe(III) minerals and authigenic silicates that may become solubilized during the extraction procedures (Kostka and Luther, 1994). Much of the reactive Fe(III) in surface sediments appears to be represented by amorphous Fe(III) minerals as opposed to crystalline Fe(II) at depth (Kostka and Luther, 1995). For sediments that have high concentrations of AVS a different parameter called the degree of sulfidization (DOS) was first introduced by Boesen and Postma (1988) and recently modified by Rozan et al. (2002) as follows ... 

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. 

Livi KJT, Veblen DR, Ferry JM, Frey M (1997) Evolution of 2 1 layered silicates in low-grade metamorphosed Liassic shales of Central Switzerland. J Metamor Geol 15 323-344 Masuda H, O Neil JR, Jiang W-T, Peacor DR (1996) Relation between interlayer composition of authigenic smectite, mineral assemblages, 1/S reaction rate and fluid composition in silicic ash of the Nankai Trough. Clays Clay Minerals 44 460-469... 

Weathered fragments of continental crust comprise the bulk of marine sediments. These particles are primarily detrital silicates, with clay minerals being the most abmidant mineral type. Clay minerals are transported into the ocean by river runoff, winds, and ice rafting. Some are authigenic, being produced on and in the seafloor as a consequence of volcanic activity, diagenesis and metagenesis. 

Coal contains detrital minerals that were deposited along with the plant material, and authigenic minerals that were formed during coalification. The abundance of mineral matter in coal varies considerably with its source, and is reported to range between 9.05 and 32.26 wt% (Valkovic 1983). Minerals found in coal include (Table 2) aluminosilicates, mainly clay minerals carbonates, such as, calcite, ankerite, siderite, and dolomite sulphides, mainly pyrite (FeS2) chlorides and silicates, principally quartz. Trace elements in coal are commonly associated with one or more of these minerals (see Table 2). 

In weathering situations, saturation of fluids with SiC relative to any species of pure silica is probably only rarely achieved. In continental and shallow sea deposits, silica is precipitated in some initially amorphous form, opaline or chert when lithified or extracted by living organisms. Authigenically formed silicates are probably not in equilibrium with quartz when they are formed. As compaction increases in sediments, silica concentrations in solution are again above those of quartz saturation (15 ppm) and again it must be assumed that the diagenetic minerals formed are not in equilibrium with a silica polymorph except where amorphous silica is present. It is possible that burial depths of one or two kilometers are necessary to effectively stabilize that quartz form. It must be anticipated that the minerals formed under conditions of silica saturation near the earth s surface will be a minority of the examples found in natural rock systems. 

The previous discussions focused on utilizing long-lived radioactive systems, either dissolved in seawater or in authigenic precipitates. Here we discuss its utility in silicate detritus or individual minerals in marine sediments to follow the history of northern hemisphere ice sheets during the last glacial period. This is merely one example of the powerful potential of radiogenic isotopes as provenance tracers wih paleoceanographic implication. 

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