The thermodynamics of the I \overline{1} –P \overline{1} phase transition in Ca-rich plagioclase from an assessment of the spontaneous strain

In-situ powder diffraction measurements between 90 and 935?K on four anorthite-rich plagioclase samples (An₁₀₀, An₉₆Ab₄, An₈₉Ab₁₁ and An₇₈Ab₂₂) were used to determine the detailed evolution of these samples through the $I \overline{1} $ – $P \overline{1} $ phase transition. The c-type reflections indicative of $P \overline{1} $ symmetry were detected only in An₁₀₀, An₉₆Ab₄, whereas deviations in the evolution of the unit-cell parameters with temperature were observed in all samples, most prominently in the β unit-cell angle. The c-type reflections disappear at ~510 and ~425?K in An₁₀₀ and An₉₆Ab₄ respectively, and their intensity decreases according to a tricritical trend $ I^{2} \propto \left( {T - T_{\text{c}} } \right) $ . The cell parameter changes were used to determine the spontaneous strains arising from the transition which were modelled with Landau theory, allowing for low-temperature quantum saturation, in order to determine the thermodynamic behaviour. In An₁₀₀ tricritical behaviour was observed [T _c?=?512.7(4)?K; θ_s?=?394(4)] in good agreement with previous studies, and the c-type superlattice reflections indicative of $P \overline{1} $ symmetry persist up to the T _c determined from the spontaneous strain, and then disappear. The evolution of the spontaneous strain in An₉₆Ab₄ is tricritical at low temperatures [T _c?=?459(1) K, θ_s?=?396(5)] up to the temperature of disappearance of c-type reflections, but becomes second order beyond ~440?K. In An₈₉Ab₁₁ the strain displays second-order behaviour throughout [T _c?=?500(1) and θ_s?=?212(5)], and the c-type reflections are not detected in the powder diffraction patterns at any temperature. The apparent discrepancy between the absence of c-type reflections in temperature ranges where the cell parameters display significant spontaneous strain is resolved through consideration of the sizes of the anti-phase domains within the crystals. It is deduced that the tricritical phase transition occurs in well-ordered crystals with large domains in which the behavior of individual domains is dominant (i.e. in pure anorthite) or where the $P \overline{1} $ distortions within the domains are large enough to dominate the structural coherency strains between the domains. When both the magnitude of the $P \overline{1} $ pattern of displacements of the tetrahedral framework become smaller and the influence of the structural coherency between anti-phase domains becomes significant, the thermodynamic behavior becomes 2nd-order in character, the c-type reflections disappear, and the orientation of the spontaneous strain changes.     

Thermal expansion of plagioclase feldspars

The volume thermal expansion coefficient and the anisotropy of thermal expansion were determined for nine natural feldspars with compositions, in terms of albite (NaAlSi₃O₈, Ab) and anorthite (CaAl₂Si₂O₈, An), of Ab₁₀₀, An₂₇Ab₇₃, An₃₅Ab₆₅, An₄₆Ab₅₄, An₆₀Ab₄₀, An₇₈Ab₂₂, An₈₉Ab₁₁, An₉₆Ab₄ and An₁₀₀ by high resolution powder diffraction with a synchrotron radiation source. Unit-cell parameters were determined from 124 powder patterns of each sample, collected over the temperature range 298–935 K. The volume thermal expansion coefficient of the samples determined by a linear fit of V/V ₀ = α(T − T ₀) varies with composition (X _An in mol %) as:

Antigorite equation of state and anomalous softening at 6 GPa: an in situ single-crystal X-ray diffraction study

The compressibility of antigorite has been determined up to 8.826(8) GPa, for the first time by single crystal X-ray diffraction in a diamond anvil cell, on a specimen from Cerro del Almirez. Fifteen pressure–volume data, up to 5.910(6) GPa, have been fit by a third-order Birch–Murnaghan equation of state, yielding V ₀ = 2,914.07(23) Å³, K _T0 = 62.9(4) GPa, with K′ = 6.1(2). The compression of antigorite is very anisotropic with axial compressibilities in the ratio 1.11:1.00:3.22 along a, b and c, respectively. The new equation of state leads to an estimation of the upper stability limit of antigorite that is intermediate with respect to existing values, and in better agreement with experiments. At pressures in excess of 6 GPa antigorite displays a significant volume softening that may be relevant for very cold subducting slabs.     

Iron and Trace Metals in Microbial Mats and Underlying Sediments: Results From Guerrero Negro Saltern, Baja California Sur, Mexico

Total trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb, Zn), Al, and pyrite- and reactive-associated metals were measured for the first time in a microbial mat and its underlying anoxic-sulfidic sediment collected in the saltern of Guerrero Negro (GN), Baja California Sur, Mexico. It is postulated that the formation of acid volatile sulfide (AVS) and pyrite in the area of GN could be limited by the availability of reactive Fe, as suggested by its limited abundance (mat and sediment combined average value of only 19 ± 10 ??mol g^?1; n = 22) as well as the low pyrite (0.89?C7.9 ??mol g^?1) and AVS (0.19?C21 ??mol g^?1) concentrations (for anoxic-sulfidic sediments), intermediate degrees of pyritization (12?C50%), high degrees of sulfidization (14?C100%), generally low degrees of trace metal pyritization, and slight impoverishment in total Fe. This is a surprising result considering the large potential reservoir of available Fe in the surrounding desert. Our findings suggest that pyrite formation in the cycling of trace metals in the saltern of GN is not very important and that other sedimentary phases (e.g., organic matter, carbonates) may be more important reservoirs of trace elements. Enrichment factors [EF_Me = (Me/Al)_sample/(Me/Al)_background] of Co, Pb, and Cd were high in the mat (EF_Me = 2.2 ± 0.4, 2.8 ± 1.6 and 34.5 ± 9.8, respectively) and even higher in the underlying sediment (EF_Me = 4.7 ± 1.5, 14.5 ± 6.2 and 89 ± 27, respectively), but Fe was slightly impoverished (average EF_Fe of 0.49 ± 0.13 and 0.50 ± 0.27 in both mat and sediment). Organic carbon to pyrite-sulfur (C/S) molar ratios measured in the mat (2.9 × 10²?C27 × 10²) and sediment (0.81 × 10²?C6.6 × 10²) were, on average, approximately 77 times higher than those typically found in marine sediments (7.5 ± 2.1). These results may indicate that ancient evaporation basins or hypersaline sedimentary environments could be identified on the basis of extremely high C/S ratios (e.g., >100) and low reactive Fe.     

Loggerhead turtles Caretta caretta (Linnaeus) preying on the invading gastropod Rapana venosa (Valenciennes) in the Río de la Plata Estuary

Here we report the first observations of loggerhead turtles Caretta caretta preying on a conspicuous molluscan invader, the rapa whelk Rapana venosa. An average number of 136 opercula were found in stomach contents of five turtles, the curved carapace length ranging in from 51 to 112 cm. No other alimentary items were found in the turtles analyzed. We suggest that the rapa whelk may constitute up to 100% of the diet for immature and mature loggerheads in the Río de la Plata estuary (Uruguay), highlighting the plastic nature of the foraging behavior of loggerheads.     

Hydrologic characteristics of Appalachian loose‐dumped spoil in the Cumberland Plateau of eastern Kentucky

Heavily compacted lands, typical of traditional surface mine reclamation techniques, have been shown to hinder tree growth, increase levels of flooding, and produce suboptimal water quality. Utilizing loose‐dumped spoil, in accordance with the Forestry Reclamation Approach (FRA), has demonstrated success with regards to promoting tree growth and survival; however, additional information is needed to assess the potential of FRA to ameliorate other environmental concerns related to water quantity. To better understand the hydrologic characteristics of loose‐dumped spoil, key hydrograph parameters (discharge volume, peak discharge, discharge duration, lag time, and response time) were monitored for three common spoil types: (1) predominately brown weathered sandstone, (2) predominately gray weathered sandstone, and (3) a mixture of both sandstones and shale. Although spoil types were found to differ hydrologically, these differences were relatively minor. Measured discharge volumes were low (averaging 12% of rainfall for all events and treatments), peak discharge rates were small (between 2·5 × 10^?5 and 3 × 10^?3 m³/s), and the duration of discharge was long (6 days on average). From a hydrologic perspective, the results of this study indicate that mine spoils need not be segregated for reclamation as long as the spoil is placed in accordance with the loose‐dumped techniques as outlined in the FRA. Copyright © 2009 John Wiley & Sons, Ltd.     

On the solution of steady unconfined seepage towards semi-infinite slopes

Analytical solutions for the two-dimensional problem of unconfined seepage towards semi-infinite slopes are described. The analysis employs complex variable techniques. Conformal mappings have been used to reduce the problem to solving an ordinary differential equation. The resulting integrals are presented in terms of special functions. The solutions for the location of the phreatic surface and the exit point have been obtained in parametric form. Some additional results and asymptotic expansions of the solutions are also presented. The numerical results have been calculated and plotted for different values of the slope angle in dimensionless co-ordinate space.