The diagenetic formation of feldspars
A study was made of the hydrothermal crystallization of dehydrated gels of albite (NaAlSi3O8, Ab100) and K-feldspar (KAlSi3O8, Or100) at 15,000psi (≈1Kbar) and temperatures of 300°C, 250°C and 200°C, and run times of up to 2728 hours. Products were characterised by powder X-ray diffraction, by measuring the angular positions of the 201, 060 and 204 reflections and the angular separation of 131 and 131 reflections. Degree of crystallization was estimated from X-ray diffraction peak quality and optical observations. Variation in Al/Si order, product composition, and time to crystallization were studied as a function of time, temperature, fluid composition and content, added crystalline feldspar and the use of a carbonate matrix. For gel &43 fluid runs, the maximum degree of order achieved was high sanidine for Or_100 gels, and high albite for Ab_100 gels. No increase in ordering was observed with any experimental variable. Hydroxide fluids have shorter times to crystallization than H_2O, and chloride seems to inhibit crystallization. Reduction in temperature increases the time to crystallization, and at 250°C, Ab_100 gels crystallize analcime in preference to albite. Two gels of Or_100 composition showed different crystallization behaviour; one showed a shorter time to crystallization and slightly ordered product with a less anomolous structure. This is due to a property of the gel. Addition of crystalline feldspar to Or_100 runs gives shorter times to crystallization. Carbonate matrix gives product from Or_100 with a structural state close to orthoclase (2t_1≈0.7) after a run time of 917 hours at 250oC. with hydroxide fluid. A synergistic relationship between hydroxide and carbonate is suggested. The morphology, chemistry, distribution and crystallography of authigenic K-feldspar in the New Red Sandstone of Elgin were studied using thin sections, microprobe, scanning electron microscopy and transmission electron microscopy. The overgrowth material is of pure potassium intermediate microcline, and shows that adularia habit. Overgrowth starts as tiny (< 1μm) orientated crystals which enlarge and merge to give increased cover. The composition is consistent with very low temperature growth. Varying levels of triclinicity are shown by the overgrowth, and microtextures probably represent pulsed growth in fluids of varying supersaturation. The overgrowth material also shows depleted Al_2O_3 content. The chemical properties of the detrital feldspars in no way influence the formation of overgrowth, although overgrowth material does show similar unit cell parameters to the grain it has formed on. In the study area, overgrowth forms preferentially on non-twinned cores. It is suggested that the triclinicity and twinning in detrital microclines inhibits overgrowth. Only grain cuticle formation pre-dates K-feldspar overgrowth formation, thus the latter occurs during very early diagenesis. It is suggested that K-feldspar formation occurs in the mixing zone of modified meteoric water and upwelling, high-salinity fluids expelled from the Moray Firth Basin.