No abstract

No abstract

In situ high‐temperature X‐ray diffraction data have been collected for the crystallization of high‐quartz solid solution (hq‐ss) in a Li2O–Al2O3–SiO2 glass–ceramic. The temporal development of the number density, size, and unit cell parameters of hq‐ss crystals indicated a two‐stage crystallization process. At the early stage, lattice parameters of hq‐ss crystals were constant. Their number density increased at a maximum rate of 1.7 × 1012·(mm3·min)−1 up to 1.4 × 1014 mm−3, while hq‐ss grew linear with time at a maximum rate of ≈1 nm/min up to a size of ≈21 nm. In contrast, a decrease of the a‐unit cell parameter, characteristic for increasing Si/Al ratios of the hq‐ss crystals, was evident at the later stage. At this stage, the average crystal size was >21 nm. Their number density was constant or decreased linearly with time. The growth kinetics were very shallow with the exponent ≈1/50, but reached for crystals >30 nm cube root dependence typical for diffusion‐controlled ripening. Avrami analysis of the volume fraction Vf confirmed these findings and resulted in a kinetic exponent in the range from 3.5 to 4.8 for the first (Vf<45%) and <0.3 for the second stage.

Euhedral, post‐depositional albite from the Eastern and Western Alps, the western Carpathians and some Greek islands was examined petrographically and geochemically to gain insights into the nature of feldspar reactions in carbonate rocks. This study focuses on coarsely crystalline, homogeneously nucleated albite in order to avoid problems related to the presence of inseparable detrital material in fine‐grained albite varieties. All albite samples show a very restricted compositional variability and are typically ≥ 99 mol% Ab component. Unit‐cell parameters determined by Rietveld analysis are slightly more variable than previously accepted, but confirm high Al–Si ordering characteristic of low albite. The oxygen isotopic composition of albite ranges from + 19·4‰ to + 28·3‰ VSMOW. There is no direct relationship between the δ18O value and the inferred temperature of albite formation, nor is there one with stoichiometry. The coarse crystal size (up to several millimetres in diameter), petrographic evidence showing albite cross‐cutting stylolites, greater abundance of albite in carbonate rocks subject to high‐grade diagenetic or weak metamorphic overprinting and available fluid inclusion data suggest that albite precipitation is favoured at higher temperatures in carbonates than in sandstones. Pore fluids were invariably brines, as suggested by the inferred high positive δ18Ofluid values, the common association of albite‐bearing carbonates and evaporites and reports of saline fluid inclusions in albite. The presence of authigenic albite may thus be a useful tracer of palaeobrine–carbonate reactions, particularly in deep‐burial and incipient metamorphic settings.

No abstract