Lewis H. Cohen

High-low cristobalite transitions in SiO2, A1P04 and GaPO4Investigations by differential thermal analysis under hydrostatic pressures ≲ 6 kbar

Abstract High-low cristobalite transition temperatures in SiO2 and A1P04 were deter mined by differential thermal analysis (DTA) under hydrostatic pressures of Ar and of CO2 to 6 kbar, using ‘well-crystallized’ materials with known (at 1 bar) enthalpies and volumes. The high-low GaPO4 cristobalite transitions were also investigated at pressures ∼ 3 kbar, conversion to quartz being unavoidable; these low → high and high → low transition temperatures increase with pressure at ∼34° kbar−1. For the low → high and high → low transitions in SiO2 and A1P04, there are regions of anomalous curvature (- d2T/dp2<0) up to ∼ 1-2 kbar, the pressure-induced variations of transition temperatures then being linear and the hystereses decreasing with increasing pressures. Slopes for the phase transitions are compatible with the thermodynamic data at 1 bar. Since the high-pressure data on the cristobalite transition do not seem to be explained fully by current theories, further theoretical attention is needed. ACKNOWLEDGMENTS

Determination of the β ↔ αL′ transition in Ca2SiO4 to 7 KBAR

Abstract Differential thermal analysis in hydrostatic apparatus to 7 kbar shows that the β → α L ′ transition temperature in Ca 2 SiO 4 linearly increases from 701° ± 2°C at 1 bar at the rate of 10.5 ± 0.5 deg kbar −1 . The α L ′ → β transition temperature is observed some 20°–30° lower in temperature than the β → α L ′ transition and no variation in this hysteresis with pressure is indicated.

Tridymite: Effect of hydrostatic pressure to 6 kbar on temperatures of two rapidly reversible transitions

Trajectories of two reversible phase transitions in a low-Na synthetic tridymite have been determined to 6 kbar by differential thermal analysis (DTA) in hydrostatic apparatus using Ar or CO2. The temperature of the lower transition increases from ≈111 ° C at 1 bar linearly with pressure with slope ∼15 deg kbar−1. Pressure raises the temperature of the upper transition from 157 ±2 ° or 159 ° C (independently determined) at 1 bar witħ a slope of ≈53 deg kbar−1, up to ∼0.7 kbar; for the data above that pressure, the initial slope is ≈64 deg kbar−1. Above 2−1/2 kbar, the variation is linear with slope ≈70 deg kbar−1. No evidence for other transitions was found at any of the apparent changes of slope. Hystereses for both transitions decreased at high pressures compared to 1-bar. Preferred values for the transition enthalpies, together with these slopes and the Clausius-Clapeyron equation, yield estimates for the volume changes at the transitions of ≈0.01 (lower) and 0.15 to 0.25 (upper) cm3 gfw−1. These calculated volume changes are not consonant with many of the high temperature volumetric data on tridymites of varying origins.

Shallow Mesozoic layered gabbros of the Shadow Mountains, San Bernardino County, California

Abstract Jurassic hornblende gabbros intrude as a conspicuous 9 km 2 arcuate complex within Paleozoic metasedimentary pendants of the Shadow Mountains, San Bernardino County, California. This complex is significant because it: (1) displays evidence of processes active in magma chambers which are crystallizing cumulus phases; (2) resembles many mid-Mesozoic plutons described in the western and central Sierra Nevada and western Mojave desert which may all be petrologically related; and (3) may be used as a diagnostic tectonic marker for Mesozoic tectonic reconstructions of the Mojave Desert and western Cordillera. The gabbroic body is unusual for its compact, near-circular plan, conspicuous banding and layering and reverse geochemical zonation. The mafic complex was intruded as a hydrous magma and was emplaced as a concentric epizonal pluton. Oxygen isotopic data indicate the melt was mantle derived but has been contaminated by assimilation of metasediments. The gabbro was likely generated by reaction of olivine with fractionated melt from the melting of a low MgO basalt under water-saturated conditions. The banding and layering is ascribed to the cyclic ascent of felsic rejected solute along the walls of the chamber and its accumulation near the roof of the complex. Cooling was facilitated by conduction and the presence of pendant rocks at uppermost levels. The Shadow Mountains gabbros resemble other gabbroic bodies of similar Jurassic age throughout the Sierra Nevada, Mojave Desert and Transverse Ranges, California (Lahren and Schweickert, 1994; Miller and Glazner, 1995). These intrusions are useful as markers of subsequent large-scale tectonic crustal displacements affecting the western Mojave Desert and Sierra Nevada. In particular, the Shadow Mountains gabbros show age, mineralogie and textural affinities with correlative gabbroic complexes in the central Sierra Nevada 400 km to the north. All these plutonic bodies may represent subvolcanic sources of the Jurassic-Cretaceous Independence dike swarm in the western Mojave Desert and eastern Sierra Nevada.

Temperatures of solid-to-solid transitions in (NH4)SO4, Rb2SO4, Cs2SO4, Cs2SeO4, and Cs2CrO4 under hydrostatic pressures to 0.6 GPa

Abstract Differential thermal analyses (d.t.a.) in hydrostatic apparatus to 0.6 GPa using Ar or CO2 have yielded the following linear variations with pressure, d T tr dp , of the temperatures of rapidly reversible solid-to-solid transitions: [substance, 0.1 MPa transition temperature T tr K , ( d T tr dp ) Pa μ K −1 ] Cs2CrO4, ≈ 1017 to ≈ 1024, ≈ 0.22; Cs2SeO4, (848 ± 4) or (886 ± 4), ≈ 0.24; Cs2SO4, (973 ± 4), (0.173 ± 0.013); Rb2SO4, (915 ± 3), (0.20 ± 0.01); (NH4)2SO4, (629 ± 3), (0.24 ± 0.02). The transition in (NH4)2SO4 apparently has not been encountered previously. Inconsistencies in and lack of measurements on entropy and volume changes thwart attempts to achieve self-consistent thermodynamic quantities for these transitions by using the Clausius-Clapeyron equation.

Solid-solid transition temperatures in SrCl2 from differential thermal analyses to 0.6 GPa

Abstract The reversible phase transition between the high-temperature, cubic C1 and the low-temperature, orthorhombic C23 polymorphs of SrCl 2 has been investigated by differential thermal analysis under hydrostatic pressure to 0.63 GPa. The C23→C1 transition temperature varies linearly with pressure at the rate of 0.42 4 μK(Pa) −1 from the highest pressures down to ca. 0.34 GPa, and also linearly with slope 1.73 μK(Pa) −1 at pressures ≲0.26 GPa; the reverse, C1→C23 transition is not observed or deduced ≲0.21 GPa. The observed curvature for the C1–C23 phase boundary over the range ≲0.26–0.34 GPa, 1000–1050 K can be attributed to intersection with the “diffuse” transition in C1; the latter transition, however, could not be observed unambiguously. Linear extrapolation to 0.1 MPa places the C23→C1 transition near 553 K, which implies that C23 - not C1 - is the stable low-temperature polymorph. The recently-investigated transitions in PbF 2 closely parallel these in in SrCl 2 .