SummaryChrysotile fibres, and also single crystals of lizardite, were dehydrated hydrothermally, and the resulting pseudomorphs studied by X-ray rotation photographs. The initial products are normally forsterite and disordered material; talc forms more slowly. At water pressures up to 600 kg/cm2, both forsterite and talc show strong preferred orientation. At higher pressures the talc is oriented but the forsterite tends to be unoriented. The orientation of forsterite formed at 550° C and 500 kg/cm2 differs from that found when chrysotile is dehydrated by heating in air. The orientation relationships suggest that, in the dry reaction, the forsterite orientation is controlled mainly by the octahedral layers of the chrysotile, while in the hydrothermal reaction it is controlled mainly by the tetrahedral layers. This does not support the generally accepted mechanism for the dry reaction. New mechanisms are suggested, for both dry and hydrothermal reactions, similar to that already suggested by us for the dehydration of brucite. The formation of talc in the hydrothermal reaction, and of enstatite in the dry reaction above 1000°, are also discussed. The dehydration of talc, to give enstatite, has also been studied using single crystals, and the mechanism of this process is briefly discussed.
SummaryThis process has been reinvestigated, mainly with X-rays, using an FeO-containing, fibrous variety (nemalite), and also a nearly pure specimen. The results confirm that the transformation is oriented, the c- and a-directions of the brucite becoming the normals to (111) and (11̄0) in the periclase. There is an intermediate, spinel-like stage; the spinel a-axis is parallel to, and twice as long as, that of the periclase. This stage is especially prominent when nemalite is heated in air or nitrogen, but is shown also by the nearly pure material. The nemalite shows also a further intermediate stage under certain conditions. A new hypothesis is proposed for the dehydration mechanism, in which the number of oxygen atoms per unit volume is ahnost unchanged in those parts of the crystal that are converted into periclase. The process is essentially one of cation migration, and the occurrence of the spinel-like phase is readily explained. Similar mechanisms possibly apply to the dehydration of other lamellar hydroxides and oxy-hydroxides (e.g. gibbsite, kaolinite) and of certain carbonates and other oxy-salts.
The kinetics of dehydration of synthetic calcium sulphate dihydrate have been studied between 80" and 152", at partial pressures of water vapour between and 45 mm. The products of reaction are controlled by both temperature and partial pressure. At temperatures greater than 11 0' the dehydration is controlted by a diffusion mechanism, but a t lower temperatures the reaction is complex and both nucleation of product and boundary control are important. Activation energies for some of the processes have been calculated.
The early Devonian, Old Red Sandstone succession at St. Cyrus, NE Scotland, includes the exceptional preservation of a lava field and its associated fluvial drainage system. The lava field developed through punctuated eruptions of low-volume pāhoehoe lava, fed by lava tubes. During periods of rapid effusion, groundwater systems were able to transmit most of the recharge in the subsurface, probably by springs, and where there were areas of localized low permeability, ephemeral lakes developed. Subsequent lava flows underwent significant interaction with the partly consolidated lacustrine sediment, forming peperite deposits. Fluvial conglomerates, with extra-basinal clasts, are interbedded with the lava flows. The maturity of the fluvial system suggests a long-lived catchment that transported sediment westwards. Drainage was periodically deflected away from the lava field by the positive topographic relief created by the lava flows, and was re-established during periods of volcanic dormancy. The lava field developed over periods of decades, but the fluvial systems were longer-lived by three or four orders of magnitude. A key feature of this lava field and its drainage is that the establishment and abandonment of fluvial systems immediately adjacent to the lava field was primarily controlled by the eruption dynamics of the magmatic system.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.