Authigenic precipitation

Carbonates deposited in freshwater lakes exhibit a wide range in isotopic composition, depending upon the isotopic composition of the rainfall in the catchment area, its amount and seasonality, the temperature, the rate of evaporation, the relative humidity, and the biological productivity. Lake carbonates typically consist of a matrix of discrete components, such as detrital components, authigenic precipitates, neritic, and benthic organisms. The separate analysis of such components has the potential to permit investigation of the entire water column. For example, the oxygen isotopic... 

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. 

Today, basin-scale mass transfer of some materials (e.g., helium, water, and petroleum) is unquestioned (e.g.. Hunt, 1996). OAer materials (e.g., titanium and the REEs) are sufficiently mobile to appear within authigenic precipitates, but are likely to be immobile on the scale of a hand specimen. Mobilities of the major elements that make up sandstones and shales (silicon, aluminum, calcium, sodium, potassium) remain controversial. Conflicting notions about processes in rock suites across the wide range of burial conditions and alteration show that fundamental questions remain unanswered about the nature of the volumetrically significant processes within a major segment of the rock cycle. It is very likely that something is wrong, or at least inadequate. 

Recent studies of the level of diagenesis necessary for authigenic precipitation of some trace metals have made it possible to determine the global extent of metal diagenesis by using models of porewater... 

Rasmussen et oL (1998) Rasmussen, B. Buick, R. Taylor, W.R. Removal of oceanic REE by authigenic precipitation of phos-phatic minerals Earth and Planetary Science Letters 164 (1998) 135-149... 

The equations governing the age of secondary carbonate deposits stated above assume that all °Th or Pa present in the mineral is formed in situ by radioactive decay of co-precipitated U. Thorium and Pa content at time of formation can often be considered to be negligible in the pure authigenic phase of calcite or aragonite... 

The isotope composition of biogenic and authigenic mineral precipitates from lake sediments can be used to infer changes in either temperature or the isotope composition of lake water. Knowledge of the factors that may have influenced the isotope composition of the lake water is essential for the interpretation of the precipitated phases (Leng and Marshall 2004). In many lakes the combined analysis of different types of authigenic components (precipitated calcite, ostracodes, bivalves, diatoms, etc.) may offer the possibility of obtaining seasonally specific information. 

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. 

Figure 7. If, at depth, the concentrations of Fe and phosphate are high enough, authigenic vivianite (Fe3(P04)2.8H20) may precipitate out.

Figure 7 Characteristic reactions involving phosphorus in the sediments of Toolik Lake, Alaska. The primary processes controlling porewater phosphorus concentrations are adsorption to and desorption from iron oxyhydroxides and the precipitation of authigenic vivianite...

Rodriguez Navarro, C., Sebastian, E. Rodriguez Gallego, M. (1997). An urban model for dolomite precipitation authigenic dolomite on weathered building stones. 

Pottery is produced by the conversion of sedimentary clay (produced by the weathering of rocks) into hard rocklike objects. The clay minerals, which were formed by the chemical decomposition of certain rock-forming minerals, contain trace elements. The sediments in which these clays are found, however, also contain fragments of the primary minerals from the parent rock (including grains of silica sand). These detrital components, which result from the physical and chemical breakdown of minerals, are often accompanied by authigenic minerals that are chemically precipitated from aqueous solutions. In some ceramics, additional components were added as temper during production. 

The assumption that montmorillonite of reported composition is being produced and is the only authigenic phase is probably only an approximation. The composition may be in error or may vary areally within Rainier Mesa as a result of areal variations in water quality. Although no zeolites or other clay minerals were reported in the bulk of the Paintbrush Tuff, very small amounts may remain undetected by either x-ray diffraction analysis or thin-sectlon petrography and may affect the aqueous composition by precipitation or by ion exchange. 

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