Potassium feldspar formation

The predicted saturation states of the formation minerals (Fig. 23.5), furthermore, no longer identify a unique formation temperature. Whereas the temperatures suggested by albite, quartz, and potassium feldspar are quite close to the 250 °C formation temperature, those predicted by assuming that the fluid was in equilibrium with muscovite and calcite are too low, respectively, by margins of about 25 °C and 100 °C. To avoid error of this sort, we would need to determine the amount of gas lost from the sample and reintroduce it to the equilibrium system before calculating saturation indices. 

To test whether the mixing hypothesis might explain the diagenetic alteration observed, we begin by equilibrating the fresh water, assuming equilibrium with the potassium feldspar ( maximum microcline in the database), quartz, and muscovite (a proxy for illite) in the formation. In REACT, we enter the commands... 

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

A wide variety of zeolites are known to form in saline lakes where the species present is dependent upon the chemistry of the solutions. Rapid zeolite formation is aided by the existence of the volcanic glass and high water salinities. Potassium feldspar occurs with the common alkali zeolites (Hay and Moiola, 1963 Hay, 1964 Hay, 1966 Sheppard and Gude, 1969, 1971), however, albite is not evident as a diagenetic mineral in saline lakes. 

Feldspars are found in nature in the form of mixed crystals of the alkali feldspars orthoclase K[AlSi308], albite Na[AlSi308] and anorthite Ca[Al2Si208]. Unlimited formation of mixed crystals of the alkali feldspars is only possible at high temperatures. During cooling demixing occurs. Potassium feldspar and calcium feldspar are not miscible. 

During the process of weathering, new kinds of minerals, the secondary minerals, are synthesised. As well as precipitates (hydroxides, oxides, sulphates, carbonates and phosphates) there are the clay minerals. For example, potassium feldspar can hydrolyse and lead to the formation of kaolinite ... 

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). 

Thyne G., Boudreau B. P., Ramm M., and Midtbp R. E. (2001) Simulation of potassium feldspar dissolution and ilhtization in the Statljord Formation, North Sea. Am. Assoc. Petrol. Geologists Bull. 85, 621-635. 

The formation of amorphous volatile products during carbothermal reduction processes can cause problems by condensing in the cooler parts of the furnace and altering the composition of the original mixture. Al NMR shows that these amorphous phases all contain Al solely in tetrahedral coordination, with chemical shifts consistent with the potassium feldspars microcline and sanidine (54-58 ppm). Thermodynamic analysis confirmed the feasibility of vapour-phase transport of both Al and alkali metal impurities from the clay which can then condense to form short-range feldspar-like units (Ekstrom et al. 1996). 

Ehrenberg and Nadeau (1989) concluded that extensive illitization occurs in the Gam Formation mainly through reaction between potassium feldspar and early diagenetically formed kaolinite ... 

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.

Oil trapped in inclusions in potassium feldspar overgrowths, the earliest diagenetic phase to host oU inclusions, has a low maturity and different source characteristics than oil in the Ula Formation reservoirs, which was generated from the Mandal Formation... 

The stability fields for merlinoite, analcime, phillipsite, and potassium feldspar has been studied [90D2]. It was concluded that there is no continuous solution-composition path of phillipsite merlinoite analcime — potassium feldspar. Analcime was not stable at the low Na/K ratios required in the solution for merlinoite formation [90D2]. 

Experimental studies of alkali feldspars in Westerley granite with ordered microcline [63,64] and disordered sanidine [65-68] show that (010) [001] shp is active in both. Other dislocations derive from (010)[101] [Fig. 15(a)], (001) [110], and (121) [101] shp. In naturaUy deformed alkali feldspars, subgrain formation was observed, indicative of climb [357-360]. It has been suggested that shear-induced mechanical Albite and Pericline twinning in potassium feldspar may facilitate ordering [361] but this has been disputed [362]. Dislocations have no effect on diffusion in alkah feldspar [363]. 

Olivine, augite, hornblende, and feldspars are all parent minerals that form clays. An example is the formation of kaolinite (Al2Si205(0H)4) from potassium feldspar rock (KAlSisOg) ... 

The final step in the interpretation process is to derive mineralogy from the specific lithology. A list containing some of the common sedimentary minerals used for formation evaluation is provided in Table 5.18. Potentially identifiable minerals are quartz, potassium-feldspar, albite, calcite, dolomite, siderite, anhydrite, iUite/smectite, kaoUnite, glauconite, chlorite, pyrite and others. 

A popular method used to date rocks is the potassium-argon method. Potassium is abundant in rocks such as feldspars, hornblendes, and micas. The K-Ar method has been used to date the Earth and its geologic formations. It has also been applied to determine magnetic reversals that have taken place throughout the Earth s history. Another method used in geologic dating is the rubidium-strontium, Rb-Sr, method. Some of the oldest rocks on Earth have been dated with this method, providing evidence that the Earth is approximately 5 billion years old. The method has also been used to date moon rocks and meteorites. 

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