1,721,003 research outputs found
High-pressure synthesis of mesoporous stishovite. Potential applications in mineral physics
No full textRecently, we have described a successful synthesis route to obtain mesoporous quartz and its high-pressure polymorph coesite by nanocasting at high pressure using periodic mesostructured precursors, such as SBA-16 and FDU-12/carbon composite as starting materials. Periodic mesoporous high-pressure silica polymorphs are of particular interest as they combine transport properties and physical properties such as hardness that potentially enable the industrial use of these materials. In addition, synthesis of mesoporous crystalline silica phases can allow more detailed geology-related studies such as water/mineral interaction, dissolution/crystallization rate and the surface contribution to the associated thermodynamic stability (free energy and enthalpy) of the various polymorphs and their crossover. Here, we present results of synthesis of mesoporous stishovite from cubic large-pore periodic mesoporous silica LP-FDU-12/C composite as precursor with an fcc lattice. We describe the synthesis procedure using multi-anvil apparatus at 9 GPa (about 90,000 atm) and temperature of 500 A degrees C. The synthetic mesoporous stishovite is, then, characterized by wide and small-angle X-ray diffraction, scanning/transmission electron microscopy and gas adsorption. Results show that this new material is characterized by accessible mesopores with wide pore size distribution, surface area of similar to 45 m(2)/g and volume of pores of similar to 0.15 cm(3)/g. Results from gas adsorption indicate that both porosity and permeability are retained at the high pressures of synthesis but with weak periodic order of the pores
Investigation of high-pressure and temperature behavior of surfactant-containing periodic mesostructured silicas
No full textPhase equilibria in the nominally Al65Cu23Fe12system at 3, 5 and 21 GPa. Implications for the quasicrystal-bearing Khatyrka meteorite
No full textTwo of the three natural quasiperiodic crystals found in the Khatyrka meteorite show a composition
within the Al-Cu-Fe system. Icosahedrite, with formula Al63Cu24Fe13, coexists with the new Al62Cu31Fe7
quasicrystal plus additional Al-metallic minerals such as stolperite (AlCu), kryachkoite [(Al,Cu)6(Fe,
Cu)], hollisterite (AlFe3), khatyrkite (Al2Cu) and cupalite (AlCu), associated to high-pressure phases like
ringwoodite/ahrensite, coesite, and stishovite. These high-pressure minerals represent the evidence that
most of the Khatyrka meteoritic fragments formed at least at 5 GPa and 1200 C, if not at more extreme
conditions. On the other hand, experimental studies on phase equilibria within the representative Al-CuFe
system appear mostly limited to ambient pressure conditions, yet. This makes the interpretation of the
coexisting mineral phases in the meteoritic sample quite difficult.
We performed experiments at 3, 5 and 21 GPa and temperatures of 800–1500 C using the multi-anvil
apparatus to investigate the phase equilibria in the Al65Cu23Fe12 system representative of the first natural
quasicrystal, icosahedrite. Our results, supported by single-crystal X-ray diffraction and analyses by scanning
electron microscopy, confirm the stability of icosahedrite at high pressure and temperature along
with additional coexisting Al-bearing phases representative of khatyrkite and stolperite as those found
in the natural meteorite. One reversal experiment performed at 5 GPa and 1200 C shows the formation
of the icosahedral quasicrystal from a pure Al, Cu and Fe mixture, a first experimental synthesis of icosahedrite
under those conditions. Pressure appears to not play a major role in the distribution of Al, Cu and
Fe between the coexisting phases, icosahedrite in particular. Results from this study extend our knowledge
on the stability of icosahedral AlCuFe at higher temperature and pressure than previously examined,
and provide a new constraint on the stability of icosahedrit
The compression behavior of blödite at low and high temperature up to ∼10 GPa. Implications for the stability of hydrous sulfates on icy planetary bodies
No full textRecent satellite inferences of hydrous sulfates as recurrent minerals on the surface of icy planetary bodies
link with the potential mineral composition of their interior. Blödite, a mixed Mg-Na sulfate, is here
taken as representative mineral of icy satellites surface to investigate its crystal structure and stability at
conditions of the interior of icy bodies. To this aim we performed in situ synchrotron angle-dispersive
X-ray powder diffraction experiments on natural blödite at pressures up to ∼10.4 GPa and temperatures
from ∼118.8 K to ∼490.0 K using diamond anvil cell technique to investigate the compression behavior
and establish a low-to-high temperature equation of state that can be used as reference when modeling
the interior of sulfate-rich icy satellites such as Ganymede.
The experimentally determined volume expansivity, α, varies from 7.6 (7) 10−5 K−1 at 0.0001 GPa
(from 118.8 to 413.15 K) to 2.6 (3) 10−5 K−1 at 10 GPa (from 313.0 to 453.0 K) with a δα/δP coefficient = −5.6(9)10−6 GPa−1 K−1.
The bulk modulus calculated from the least squares fitting of P-V data on the isotherm at 413 K using
a second-order Birch - Murnaghan equation of state is 38(5) GPa, which gives the value of δK/δT equal to
0.01(5) GPa K−1. The thermo-baric behavior of blödite appears strongly anisotropic with c lattice parameter being more deformed with respect to a and b.
Thermogravimetric analyses performed at ambient pressure showed three endotherms at 413 K, 533 K
and 973 K with weight losses of approximately 11%, 11% and 43% caused by partial dehydration, full dehydration and sulfate decomposition respectively. Interestingly, no clear evidence of dehydration was observed up to ∼453 K and ∼10.4 GPa, suggesting that pressure acts to stabilize the crystalline structure of
blödite.
The data collected allow to write the following equation of state,
V(P, T)=V0[1+7.6(7)10−5T−0.026(3)P−5.6(9)10−6PT−6.6(9)10−6PT)]
from which the density of blödite can be determined at conditions of the mantle of the large icy
satellites of Jupiter.
Blödite has higher density, bulk modulus and thermal stability than similar hydrous sulfates (e.g.
mirabilite and epsomite) implying, therefore, a different contribution of these minerals to the extent of
deep oceans in icy planets and their distribution over the local geotherm
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Icosahedral AlCuFe quasicrystal at high pressure and temperature and its implications for the stability of icosahedrite
No full textThe first natural-occurring quasicrystal, icosahedrite, was recently discovered in the Khatyrka meteorite, a new CV3 carbonaceous chondrite. Its finding raised fundamental questions regarding the effects of pressure and temperature on the kinetic and thermodynamic stability of the quasicrystal structure relative to possible isochemical crystalline or amorphous phases. Although several studies showed the stability at ambient temperature of synthetic icosahedral AlCuFe up to similar to 35 GPa, the simultaneous effect of temperature and pressure relevant for the formation of icosahedrite has been never investigated so far. Here we present in situ synchrotron X-ray diffraction experiments on synthetic icosahedral AlCuFe using multianvil device to explore possible temperature-induced phase transformations at pressures of 5 GPa and temperature up to 1773 K. Results show the structural stability of i-AlCuFe phase with a negligible effect of pressure on the volumetric thermal expansion properties. In addition, the structural analysis of the recovered sample excludes the transformation of AlCuFe quasicrystalline phase to possible approximant phases, which is in contrast with previous predictions at ambient pressure. Results from this study extend our knowledge on the stability of icosahedral AlCuFe at higher temperature and pressure than previously examined, and provide a new constraint on the stability of icosahedrite
Formation of abiotic hydrocarbon from reduction of carbonate in subduction zones. Constraints from petrological observation and experimental simulation
No full textSubduction is a key process for linking the carbon cycle between the Earth's surface and its interior. Knowing the carbonation and decarbonation processes in the subduction zone is essential for understanding the global deep carbon cycle. In particular, the potential role of hydrocarbon fluids in subduction zones is not well understood and has long been debated. Here we report graphite and light hydrocarbon-bearing inclusions in the carbonated eclogite from the Southwest (S.W.) Tianshan subduction zone, which is estimated to have originated at a depth of at least 80 kilometers. The formation of graphite and light hydrocarbon likely results from the reduction of carbonate under low oxygen fugacity (∼FMQ - 2.5 log units). To better understand the origin of light hydrocarbons, we also investigated the reaction between iron-bearing carbonate and water under conditions relevant to subduction zone environments using large-volume high-pressure apparatus. Our high-pressure experiments provide additional constraints on the formation of abiotic hydrocarbons and graphite/diamond from carbonate-water reduction. In the experimental products, the speciation and concentration of the light hydrocarbons including methane (CH4), ethane (C2H6), and propane (C3H8) were unambiguously determined using gas chromatograph techniques. The formation of these hydrocarbons is accompanied by the formation of graphite and oxidized iron in the form of magnetite (Fe3O4). We observed the identical mineral assemblage (iron-bearing dolomite, magnetite, and graphite) associated with the formation of the hydrocarbons in both naturally carbonated eclogite and the experimental run products, pointing toward the same formation mechanism. The reduction of the carbonates under low oxygen fugacity is, thus, an important mechanism in forming abiotic hydrocarbons and graphite/diamond in the subduction zone settings
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Large-pore periodic mesoporous silicas with crystalline channel walls and exceptional hydrothermal stability synthesized by a general high-pressure nanocasting route
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