Showing papers in "American Mineralogist in 1994"

The pressure behavior of alpha cristobalite

Abstract: Structural and volume compressibility data for two a cristobalite samples were determined by single-crystal X-ray diffraction methods at pressures up to -1.6 GPa. As with the other silica polymorphs that have been studied at high pressure, the change of the SiO-Si angle is correlated with the volume compressibility. The Si-O bond lengths and the O-Si-O angles remain essentially unchanged. The bulk modulus was determined to be 11.5(7) GPa with a pressure derivative of 9(2). Both crystals underwent reversible structural phase transitions in the pressure interval 1.18-1.60 GPa.

Chlorine, fluid immiscibility, and degassing in peralkaline magmas from pantelleria, italy

Abstract: Ansrnlcr This paper documents immiscibility among vapor, highly saline liquid, and silicate melt during the crystallization of peralkaline rhyolites from Pantelleia, Italy, prior to their eruption. Experiments conducted in a mufre furnace and with a high-temperature heating stage revealed three major types of silicate melt inclusions trapped in quartz phenocrysts. After entrapment in the host phenocryst, type I inclusions contained silicate melt. Type II inclusions contained silicate melt + hydrosaline melt (-60-80 wto/o NaCl equivalent), and type III inclusions contained silicate melt + HrO-CO, vapor. Two inclusions contained all three immiscible fluids: vapor, hydrosaline melt, and silicate melt. Fluid inclusions within outgassed matrix glass, viewed at room temperature, are interpreted as the crystallized equivalents of the hydrosaline melts within type II inclusions. These inclusions, 2-10 pm in size, consist ofa bubble typically surrounded by a spherical shell of halite. The presence of both vapor and hydrosaline melt in the magma indicates that the pantellerite was saturated with subcritical NaCl-HrO fluids. At a given temperature and pressure, the fixed activity of Cl in these two fluids delermines the activity and concentration of Cl in the silicate melt. The high concentrations of Cl in these pantellerites (-9000 ppm) are thus a function of the low activity coefficient for NaCl in pantellerite relative to metaluminous silicate liquids. The Cl contents of Pantellerian rhyolites indicate equilibration at pressures between 50 and 100 MPa. The high Cl contents of outgassed pantellerites may be due to minimal loss of HCI (not NaCl) during eruption, as compared with metaluminous rhyolites, which exsolve more HCI-rich vapors. Discrepancies between the results of heating-stage experiments and longer muffie-furnace experiments indicate that measurements of melting and homogenization temperatures of melt inclusions may not be accurate unless sufficient time (> I h) is allowed for equilibration at magmatic temperatures.

Numerical modeling of crystal shapes in thin sections: Estimation of crystal habit and true size

Abstract: Although the size and shape of crystals in thin sections have been measured in a number of studies, it has not been possible so far to calculate from these data the true, three-dimensional shape and size of the crystals. A numerical model, based on orthogonal solids, has been developed to attack this problem. This model shows that crystal habit can be calculated from width to length ratio distributions for most crystals in massive rocks and for all crystals in laminated or lineated rocks. Variations in the habit of minerals can reveal aspects of the physicochemical conditions of crystallization. The same model has also been used to develop the equations necessary to transform two-dimensional crystal size istributions into true crystal size distributions. Corrections for the cut effect and the intersection probability effect both require a knowledge of the crystal habit.

The behavior of H2O and CO2 in high-temperature lead borate solution calorimetry of volatile-bearing phases

Abstract: High-temperature oxide melt solution calorimetry with molten 2PbO{center_dot} B{sub 2}O{sub 3} as a solvent can be used for determining enthalpies of formation of carbonates and hydrous silicates. Under conditions of gas flow at 1-2 cm{sup 3}/s, all H{sub 2}O and CO{sub 2} is expelled from the solvent, leading to a reproducible final thermodynamic state. Both analytical data and a number of thermodynamic cycles show that, under these conditions, the volatiles neither dissolve in nor interact energetically with the melt. 27 refs., 4 figs., 4 tabs.

Thermal reactions of chrysotile revisited; a 29 Si and 25 Mg MAS NMR study

Abstract: The thermal decomposition sequence of chrysotile samples from New Zealand and Canada has been found by 29 Si and 25 Mg MAS NMR to be more complex than previously indicated by X-ray diffraction and electron optical studies. The greater part of the mineral dehydroxylates, producing a Mg-rich X-ray amorphous phase containing essentially Mg in octahedral coordination and characterized by a broad 29 Si resonance centered at about -73 ppm with respect to tetramethylsilane. This transforms to forsterite, Mg 2 SiO 4 , at 670-700 degrees C, at which temperature the unreacted chrysotile (possibly the deepest layers of the fibrils) forms a different, Si-rich X-ray amorphous dehydroxylate. This phase, called dehydroxylate II, has a 29 Si chemical shift (-97 ppm) not unlike that of talc, but with Mg sites that may be distorted beyond NMR detection, because most of the 25 Mg spectral intensity is lost over this temperature range. Dehydroxylate II forms enstatite (MgSiO 3 ) and free silica at 770-800 degrees C, further enstatite being formed by reaction of the silica with forsterite at 1150 degrees C. The stoichiometry of these reactions is consistent with NMR measurements of the partitioning of Si between the various phases.

Dehydration and hydration of montmorillonite at elevated temperatures and pressures monitored using synchrotron radiation

Abstract: The montmorillonite hydrate with one HrO layer dehydroxylates or melts at 525 "C and 5 + 1.4 kbar, conditions that are much lower than those found at lower pressures. Montmorillonite with basal spacing intermediate between two homogeneous states, which was interpreted as interstratification oftwo hydrates, is also stable at the elevated temperatures and pressures. Results confirm that the dehydration temperature of montmorillonite significantly increases with increasing HrO pressure, consistent with the idea that the density of interlayer HrO in montmorillonite is higher than that of pore HrO. Kinetic data show that the dehydration of interlayer HrO is fast (on the order of minutes) even at high HrO pressure, whereas the rehydration reaction is slower. The rehydration rate significantly increases when the sample cools to temperatures far below equilibrium. The preliminary experiments using micrograms of swelling clay, montmorillonite, affirm that synchrotron radiation can be a very useful tool to quantify the fluid-rock reactions occurring within a diamond-anvil cell.

Crush; a Fortran program for the analysis of the rigid-unit mode spectrum of a framework structure

Abstract: Crush is a program designed to calculate the rigid-unit mode spectrum for any given framework structure. Release version 1.1 is now available and contains some new features compared with the prerelease version described elsewhere.

Rapid-quench hydrothermal experiments at mantle pressures and temperatures

Abstract: New experimental methods for the encapsulation, extraction, and analysis of large volumes of aqueous fluids enable studies of mineral solubilities and dissolution kinetics with the piston-cylinder apparatus. Quench rates and volumes of experimental fluids with these methods are comparable with those used in modern hydrothermal experiments employing cold-seal vessels. Analyses of fluid compositions are reproducible and consistent with independent results. The techniques permit experimental investigations that will better characterize fluid-rock interaction in such environments as subducting slabs, the mantle wedge, Barrovian metamorphic belts, and middle to lower crustal granulites.

The high-pressure crystal chemistry of low albite and the origin of the pressure dependency of Al-Si ordering

Abstract: Structural and volume compressibility data for low albite were obtained by single-crystal X-ray diffraction methods at pressures up to -4 GPa. The bulk modulus was determined to be 54(1) GPa, with a pressure derivative of 6(1). Unit cell compression is anisotropic, as indicated by unit strain tensors. In the softest direction, approximately perpendicular to (100), the structure is three times more compressible than in the stiffest direction. Intensity data were collected, and structures were refined at 0.00, 0.44, 1.22, 2.68, and 3.78 GPa. With increasing pressure, (1) the volumes of the T04 tetrahedra do not vary, (2) the volume of the NaO? polyhedron varies linearly with the volume of the unit cell, and (3) Si-O-Si angles increase or remain constant, but only AI-O-Si angles decrease, which is consistent with the smaller force constant of the AI-O-Si vs. Si-O-Si angle. We conclude that compression is accomplished through the bending of AI-O-Si angles, which squeezes together the chains offour-membered rings that run parallel to [001] and that are separated by zigzag channels containing Na atoms. The feldspar three-dimensional tetrahedral framework can be considered to be made up of these chains, which are linked together by Octype atoms. The average value of the T-Oc T angle correlates with bulk moduli of alkali feldspars. Al-Si disorder tends to stiffen the T -Oc- T angle in high albite, which in turn decreases the compressibility and thus can serve as a mechanism for pressure-dependent ordering of high to low albite.