The breakdown of potassium feldspar at high water pressures

The equilibrium position of the reaction between sanidine and water to form “sanidine hydrate” has been determined by reversal experiments on well characterised synthetic starting materials in a piston cylinder apparatus. The reaction was found to lie between four reversed brackets of 2.35 and 2.50 GPa at 450 °C, 2.40 and 2.59 GPa at 550 °C, 2.67 and 2.74 GPa at 650 °C, and 2.70 and 2.72 GPa at 680 °C. Infrared spectroscopy showed that the dominant water species in sanidine hydrate was structural H₂O. The minimum quantity of this structural H₂O, measured by thermogravimetric analysis, varied between 4.42 and 5.85 wt% over the pressure range of 2.7 to 3.2 GPa and the temperature range of 450 to 680 °C. Systematic variation in water content with pressure and temperature was not clearly established. The maximum value was below 6.07 wt%, the equivalent of 1 molecule of H₂O per formula unit. The water could be removed entirely by heating at atmospheric pressure to produce a metastable, anhydrous, hexagonal KAlSi₃O₈ phase (“hexasanidine”) implying that the structural H₂O content of sanidine hydrate can vary. The unit cell parameters for sanidine hydrate, measured by powder X-ray diffraction, were a = 0.53366 (±0.00022) nm and c = 0.77141 (±0.00052) nm, and those for hexasanidine were a = 0.52893 (±0.00016) nm and c = 0.78185 (±0.00036) nm. The behaviour and properties of sanidine hydrate appear to be analogous to those of the hydrate phase cymrite in the equivalent barium system. The occurrence of sanidine hydrate in the Earth would be limited to high pressure but very low temperature conditions and hence it could be a potential reservoir for water in cold subduction zones. However, sanidine hydrate would probably be constrained to granitic rock compositions at these pressures and temperatures. Received: 6 May 1997 / Accepted: 2 October 1997     

A complete infrared Einstein ring in the gravitational lens system B1938 + 666

We report the discovery, using NICMOS on the Hubble Space Telescope , of an arcsec-diameter Einstein ring in the gravitational lens system B1938+666. The lensing galaxy is also detected, and is most likely an early-type galaxy. Modelling of the ring is presented and compared with the radio structure from MERLIN maps. We show that the Einstein ring is consistent with the gravitational lensing of an extended infrared component, centred between the two radio components.     

B0712+472: a new radio four-image gravitational lens

A new four-image gravitational lens system, B0712+472, has been discovered during the Cosmic Lens All-Sky Survey. This system consists of four flat-spectrum radio images that are also seen on a Hubble Space Telescope ( HST ) image, together with the lensing galaxy. We present MERLIN, VLA and VLBA maps and WHT spectra of the system as well as the HST images. The light distribution of the lensing galaxy is highly elongated and so too is the mass distribution deduced from modelling. We suggest a redshift of ∼1.33 for the lensed object; the lens redshift will require further investigation. The discovery of this new system further increases the ratio of four-image to two-image lens systems currently known, exacerbating problems of required ellipticity of matter distributions in lensing galaxies.     

A formula describing both magnetic and isotopic blocking temperatures

The following new theorem is proved: the equation E/RT_B =ln(1.781 k₀τ), where E = activation energy, R = the molar gas constant and T_B = blocking temperature, applies equally well to both the isotopic and the magnetic systems in rocks from a cooling orogen. The isotopic model involves first-order loss of daughter, while the magnetic model applies to an assemblage of single-domain grains with constant E. Dodson supplied the original proof that the equation applied to isotopic systems. The well-known superficial similarity between the concepts of isotopic and magnetic blocking is thus given quantitative expression.     

Resistance of quartz silt to isotopic exchange under burial and intense weathering conditions

Oxygen isotopic compositions of quartz from silty sediments buried to 5400 m depth from two wells in the Gulf of Mexico each showed δ5¹⁸O variations of less than 1.7%. Clay diagenesis has been reported within this depth. The observed variations in the quartz do not appear to be primarly diagenetic effects, but rather are mainly depositional features reflecting variations in the sources of the sediments. Sources may be influenced by the variation of distance from the shore at different depths in a given sampling location and by sediment production by continental glaciations. Stability of the oxygen isotopic composition of quartz in the 10–20 μm size range under long-time humid, temperate weathering conditions was studied by analysis of saprolites formed from Pennsylvanian to Precambrian crystalline rocks. In four of the five cases, the 10–20 μm fraction was found to have only 0.1–0.5%. greater δ¹⁸O than the corresponding 20–50 μm fraction. This increase may be attributable either to a slight oxygen isotopic exchange with ambient ground waters or original differences within the rock since the saprolites were sufficiently coherent to make an influx of extraneous detrital silt unlikely. The amount of oxygen isotopic exchange in silt size quartz over periods of many million years of shallow burial or weathering appears to be small enough to permit the use of the oxygen isotopic ratio of quartz in tracing the origin of eolian and fluvial additions of minerals to continental soils and pelagic sediments.     

Aims and implementation in British urban and regional planning

Failure to fully understand or recognise crucial but culturally-determined British dimensions has led to confusion and error even among the British geographers in understanding and contributing to, urban and regional planning. This point assumes more rather than less significance in the context of a discussion between British and Soviet geographers. This paper therefore examines in part one, three fundamental features of the British system of urban and regional planning and then proceeds in part two to focus on the aims and their implementation. It concludes with a reference to the nature of the relationship between planning and geography.     

A two-dimensional photochemical model of the stratosphere with Rayleigh scattering

Computations of concentrations of minor stratospheric gases using a two-dimensional model of the stratosphere are reported. The model includes Rayleigh scattering. This results in an increase in the photodissociation rate computation time by a factor of 1.5. The effect of ground albedo is assessed by comparing our results with a calculation of photolysis rates for a constant albedo with latitude. Comparison of results with a reference model including only molecular absorption are also made. The largest differences (50%) occur in NO and OH. An approximate expression for including the effects of Rayleigh scattering and surface reflectance in multi-dimensional models is suggested.     

Relations between surface deformation,fault geometry,seismicity, and rupture characteristics during the El Asnam (Algeria) earthquake of 10 October 1980

The El Asnam earthquake of October 10, 1980 (M_s=7.3) produced surface faulting on a northeast-trending thrust fault of 30 km length with displacements of up to 6.5 m, though average displacements were about 3 m. In addition, widespread tensional features were formed, some in clear association with folding above the thrust, and others, in an area beyond the exposure of the thrust at the surface, which may be related to buried reverse faults.The observed thrust fault is split into southern, central and northern segments. Local and teleseismic data are examined to show that the main shock nucleated at the southwest end of the fault, and propagated 12 km northeast where a second rupture of approximately equal moment occurred, continuing the faulting a further 12 km northeast along the central segment. Both ruptures nucleated at about 8–10 km depth. Displacements were largest on the central segment, where they were probably enlarged by aftershocks, including one of m_b=6.1 three hours after the main shock. The northern segment was much shorter than the other two, and showed smaller displacement.The junctions between fault segments are marked by distinct geomorphological characteristics and a change in strike of the faulting, as well as a sudden drop in the observed displacement. It appears that the rupture development is influenced by the changes in fault geometry between segments, and that such junctions or barriers have persisted through much of the late Quaternary.