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1.
Björn Winkler Mark J. Harris Roger S. Eccleston Karsten Knorr Bernard Hennion 《Physics and Chemistry of Minerals》1997,25(1):79-82
Inelastic magnetic neutron scattering has been used to determine the energy of the 4
A
2→4
T
2 transition in CoAl2O4 spinel and the δ1 transition in Co2[Al4Si5]O18 cordierite. The observed crystal field splitting in Co-spinel is 485 meV (3900 cm−1), which corresponds to a crystal field stabilization energy of 56.2 kJmol−1. The transition energy of the δ1 transition in Co-cordierite has been determined to be 21 meV (170 cm−1). The present data demonstrate that magnetic neutron scattering can be used to measure crystal field transitions at energies
of interest in the study of 3d-containing silicates. It may be used to measure transition energies when the use of optical spectroscopy is inappropriate.
Received: 30 January 1997 / Accepted: 5 July 1997 相似文献
2.
K.-D. Grevel A. Navrotsky W. A. Kahl D. W. Fasshauer J. Majzlan 《Physics and Chemistry of Minerals》2001,28(7):475-487
Calorimetric and P–V–T data for the high-pressure phase Mg5Al5Si6O21(OH)7 (Mg-sursassite) have been obtained. The enthalpy of drop solution of three different samples was measured by high-temperature
oxide melt calorimetry in two laboratories (UC Davis, California, and Ruhr University Bochum, Germany) using lead borate (2PbO·B2O3) at T=700 ∘C as solvent. The resulting values were used to calculate the enthalpy of formation from different thermodynamic datasets;
they range from −221.1 to −259.4 kJ mol−1 (formation from the oxides) respectively −13892.2 to −13927.9 kJ mol−1 (formation from the elements). The heat capacity of Mg5Al5Si6O21(OH)7 has been measured from T=50 ∘C to T=500 ∘C by differential scanning calorimetry in step-scanning mode. A Berman and Brown (1985)-type four-term equation represents
the heat capacity over the entire temperature range to within the experimental uncertainty: C
P
(Mg-sursassite) =(1571.104 −10560.89×T
−0.5−26217890.0 ×T
−2+1798861000.0×T
−3) J K−1 mol−1 (T in K). The P
V
T behaviour of Mg-sursassite has been determined under high pressures and high temperatures up to 8 GPa and 800 ∘C using a MAX 80 cubic anvil high-pressure apparatus. The samples were mixed with Vaseline to ensure hydrostatic pressure-transmitting
conditions, NaCl served as an internal standard for pressure calibration. By fitting a Birch-Murnaghan EOS to the data, the
bulk modulus was determined as 116.0±1.3 GPa, (K
′=4), V
T,0
=446.49 3 exp[∫(0.33±0.05) × 10−4 + (0.65±0.85)×10−8
T dT], (K
T/T)
P
= −0.011± 0.004 GPa K−1. The thermodynamic data obtained for Mg-sursassite are consistent with phase equilibrium data reported recently (Fockenberg
1998); the best agreement was obtained with Δf
H
0
298 (Mg-sursassite) = −13901.33 kJ mol−1, and S
0
298 (Mg-sursassite) = 614.61 J K−1 mol−1.
Received: 21 September 2000 / Accepted: 26 February 2001 相似文献
3.
Detlef W. Fasshauer Bernd Wunder Niranjan D. Chatterjee Günther W. H. Höhne 《Contributions to Mineralogy and Petrology》1998,131(2-3):210-218
The heat capacity of synthetic, stoichiometric wadeite-type K2Si4O9 has been measured by DSC in the 195≤T(K)≤598 range. Near the upper temperature limit of our data, the heat capacity observed by DSC agrees with that reported by
Geisinger et al. (1987) based on a vibrational model of their infrared and Raman spectroscopic data. However, with decreasing
temperature, the Cp observed by DSC is progressively higher than that predicted from the vibrational model, suggesting that
the standard entropy of K2Si4O9 is likely to be larger than 198.9 ± 4.0 J/K · mol computed from the spectroscopic data. A fit to the DSC data gave: Cp(T) = 499.13 (±1.87) − 4.35014 · 103(±3.489 · 101) · T
−0.5, with T in K and average absolute percent deviation of 0.37%. The room-temperature compressibilities of kalsilite and leucite, hitherto
unknown, have been measured as well. The data, fitted to the Murnaghan equation of state, gave K
o = 58.6 GPa, K
o
′ = 0.1 for kalsilite and K
o = 45 GPa, K
o
′ = 5.7 for α-leucite. Apart from the above mentioned data on the properties of the individual phases, we have also obtained
reaction-reversals on four equilibria in the system K2O-Al2O3-SiO2. The Bayesian method has been used simultaneously to process the properties of 13 phases and 15 reactions between them to
derive an internally consistent thermodynamic dataset for the K2O-Al2O3-SiO2 ternary. The enthalpy of formation of K2Si4O9 wadeite is in perfect agreement with its revised calorimetric value, the standard entropy is 232.1 ± 10.4 J/K · mol, ∼15%
higher than that implied by vibrational modeling. The phase diagram, generated from our internally consistent thermodynamic
dataset, shows that for all probable P-T trajectories in the subduction regime, the stable pressure-induced decomposition of K-feldspar will produce coesite + kalsilite rather than coesite + kyanite + K2Si4O9 (cf. Urakawa et al. 1994).
Received: 11 June 1997 / Accepted: 2 December 1997 相似文献
4.
E. J. Palin M. T. Dove S. A. T. Redfern A. Bosenick C. I. Sainz-Diaz M. C. Warren 《Physics and Chemistry of Minerals》2001,28(8):534-544
The nature of Al–Si ordering across the tetrahedral sites in muscovite, K2Al4(Si6Al2O20)(OH)4, was investigated using various computational techniques. Values of the atomic exchange interaction parameters J
l
were obtained. From these parameters, a two-dimensional Al–Si ordering scheme was deduced. The transition temperature T
c for this two-dimensional ordering is 1900 K. There are several possible ordering schemes in three dimensions, based on different
stacking sequences of ordered sheets of tetrahedral sites. Monte Carlo simulations of both two-dimensional and three-dimensional
ordering were performed, but in the three-dimensional simulation only the two-dimensional ordering is seen, implying that
three-dimensional ordering is too slow to be attained during the timescale of the simulation. The effect of the three-dimensional
interactions is to raise the two-dimensional ordering temperature to 2140 K. From the three-dimensional Monte Carlo simulation,
the frequency of occurrence of 4Si0Al, 3Si1Al, 2Si2Al and 1Si3Al clusters was determined, which match those inferred by 29Si MAS–NMR measurements reasonably well. In fact, the match suggests that the cation ordering seen in experiments corresponds
to a configuration with considerable short-range order but no long-range order, similar to a state that is at a temperature
just above an ordering phase transition.
Received: 28 August 2000 / Accepted: 12 March 2001 相似文献
5.
Summary ?Hydrothermal experiments to synthesize pumpellyite group minerals of the pumpellyite–okhotskite series and to investigate
their stability have been carried out at 200, 300 and 400 MPa P
fluid and 250–500 °C by using cold-seal pressure vessels and solid buffers of MnO2–Mn2O3, Cu2O–CuO and Cu2O–Cu buffer assemblages. Okhotskite and pumpellyite rich in the okhotskite component crystallized from an oxide mixture starting
material of Ca4MgMn3+
3Al2Si6O24.5-oxide+excess H2O at P
fluid of 200, 300 and 400 MPa and temperatures of 300 and 400 °C. However, a single phase of okhotskite was not produced, and associated
piemontite, hausmannite, wollastonite, clinopyroxene, corundum, braunite–neltnerite solid solution and alleghanyite also formed.
Mn-pumpellyite of the okhotskite–pumpellyite join occurs as aggregates of needle crystals, rounded grains or flaky crystals.
Chemical compositions are variable and range from pumpellyite-(Mn2+) to okhotskite: 31–36 SiO2, 13–21 Al2O3, 12–25 total Mn2O3, 0.6–4 MgO and 20–24 wt.% CaO. Reconnaissance experiments using a starting material of synthetic Ca2Mn3+Al2Si3O12(OH)-piemontite at 300 MPa and temperatures of 250, 300, 400 and 500 °C indicate that Mn-rich pumpellyite can crystallize
from piemontite at lower temperatures than the stability field of piemontite. The Mn-rich pumpellyite was accompanied by garnet,
wollastonite and alleghanyite. The chemical compositions of the Mn-pumpellyites are 32–36 SiO2, 18–27 Al2O3, 8–18 total Mn2O3 and 20–23 wt.% CaO. This study shows that the stability fields of piemontite, piemontite+Mn-pumpellyite, and Mn-pumpellyite
range in this order with decreasing temperature under high fO2 conditions. The maximum stability temperature of Mn-rich pumpellyite lies between 400 and 500 °C at 200–400 MPa in high fO2 conditions.
Received March 3, 2000; revised version accepted December 28, 2001 相似文献
6.
Magnetic measurement of Fe3−
x
Si
x
O4 spinel solid solutions indicates that their Curie temperatures decrease gradually, but not linearly, from 851 to 12 K with
increasing content of nonmagnetic ions Si4+. Magnetic hysteresis becomes more noticeable in solid solutions having a larger content of Fe2SiO4. Saturation magnetizations of Fe3−
x
Si
x
O4 samples increase up to x=0.357 and they are easily saturated in the field of H=0.1 T. However, magnetization of the sample of x=0.794 does not approach saturation even at high field of H=7.0 T and has a large coercive force. The Si4+ disordered distribution is confirmed to be tetr[Fe3+
1−
x
+
x
t
Si4+
x
(1−
t
)] octa[Fe2+
1+
x
Fe3+
1−
x
−
x
t
Si4+
x
t
] O4 by the spin moment, which is consistent with site occupancy obtained from X-ray crystal structure refinement. Their molecular
magnetizations would be expressed as M
B={4(1+x)+10xt}μB as functions of composition parameter x and Si4+ ordering parameter t of the solid solution. The sample of x=0.794 is antiferromagnetic below the Néel temperature, mainly due to the octahedral cation interaction M
O–M
O, while both M
T–M
O and M
O–M
O interactions induce a ferrimagnetic property. Concerning magnetic spin configuration, in the case of x>0.42, the lowest dɛ level becomes a singlet, resulting in no orbital angular momentum.
Received: 20 April 2000 / Accepted: 11 September 2000 相似文献
7.
G. D. Gwanmesia J. Liu G. Chen S. Kesson S. M. Rigden R. C. Liebermann 《Physics and Chemistry of Minerals》2000,27(7):445-452
Dense isotropic polycrystalline specimens of majorite-rich garnets (Py100, Py62Mj38, Py50Mj50, Py21Mj79 and Mj100) along the pyrope (Mg3Al2Si3O12 = Py100)-majorite (MgSiO3 = Mj100) join were fabricated in a 2000-ton uniaxial split-sphere anvil apparatus (USSA-2000) at pressures from 10 to 18.5 GPa and
temperatures from 1200 to 1850 °C, within their stability fields in runs of 2–4-h duration, using hot-pressing techniques
developed by Gwanmesia et al. (1993). These specimens are single-phased, fine-grained (≤5 mm), free of microcracks, and have
bulk densities greater than 99% of the corresponding single-crystal X-ray density. Elastic compressional (P) and shear (S) wave velocities were determined at room pressure and temperature for these polycrystalline garnet specimens by phase comparison
ultrasonic interferometry. For Mj100, the P and S wave velocities are within 1% of the Hashin-Shtrikman averages calculated from the single crystal elastic moduli measured
by Brillouin spectroscopy. Both the elastic bulk modulus (K) and the shear modulus (G) decrease continuously with increasing majorite content from pyrope garnet (Py100) to pure majorite garnet (Mj100). The compositional dependence of K and G are given by K = 172.3 (40) − 0.085X, and G = 91.6 (10) − 0.038X, where X = mol% majorite), respectively, indicating that substitution of Si for Mg and Al decreases both K and G by about 5% along the solid solution series.
Received: 25 March 1999 / Accepted: 12 July 1999 相似文献
8.
H. Morishima E. Ohtani T. Kato T. Kubo A. Suzuki T. Kikegawa O. Shimomura 《Physics and Chemistry of Minerals》1999,27(1):3-10
The high-pressure and temperature equation of state of majorite solid solution, Mj0.8Py0.2, was determined up to 23 GPa and 773 K with energy-dispersive synchrotron X-ray diffraction at high pressure and high temperature
using the single- and double-stage configurations of the multianvil apparatuses, MAX80 and 90. The X-ray diffraction data
of the majorite sample were analyzed using the WPPD (whole-powder-pattern decomposition) method to obtain the lattice parameters. A least-squares fitting using the third-order Birch-Murnaghan equation
of state yields the isothermal bulk modulus, K
T0
= 156 GPa, its pressure derivative, K′ = 4.4(±0.3), and temperature derivative (∂K
T
/∂T)
P
= −1.9(±0.3)× 10−2 GPa/K, assuming that the thermal expansion coefficient is similar to that of pyrope-almandine solid solution.
Received: 5 October 1998 / Revised, accepted: 24 June 1999 相似文献
9.
Summary. ?Ca-tourmaline has been synthesized hydrothermally in the presence of Ca(OH)2 and CaCl2-bearing solutions of different concentration at T = 300–700 °C at a constant fluid pressure of 200 MPa in the system CaO-MgO-Al2O3-SiO2-B2O3-H2O-HCl. Synthesis of tourmaline was possible at 400 °C, but only above 500 °C considerable amounts of tourmaline formed. Electron
microprobe analysis and X-ray powder data indicate that the synthetic tourmalines are essentially solid solutions between
oxy-uvite, CaMg3- Al6(Si6O18)(BO3)3(OH)3O, and oxy-Mg-foitite, □(MgAl2)Al6(Si6O18)(BO3)3(OH)3O. The amount of Ca ranges from 0.36 to 0.88 Ca pfu and increases with synthesis temperature as well as with bulk Ca-concentration
in the starting mixture. No hydroxy-uvite, CaMg3(MgAl5)(Si6O18)(BO3)3(OH)3(OH), could be synthesized. All tourmalines have < 3 Mg and > 6 Al pfu. The Al/(Al + Mg)-ratio decreases from 0.80 to 0.70
with increasing Ca content. Al is coupled with Mg and Ca via the substitutions Al2□Mg−2Ca−1 and AlMg−1H−1. No single phase tourmaline could be synthesized. Anorthite ( + quartz in most runs) has been found coexisting with tourmaline.
Other phases are chlorite, tremolite, enstatite or cordierite.
Between solid and fluid, Ca is strongly fractionated into tourmaline ( + anorthite). The concentration ratio D = Ca(fluid)/Ca(tur) increases from 0.20 at 500 °C up to 0.31 at 700 °C. For the assemblage turmaline + anorthite + quartz + chlorite or tremolite
or cordierite, the relationship between Ca content in tourmaline and in fluid with temperature can be described by the equation
(whereby T = temperature in °C, Ca(tur) = amount of Ca on the X-site in tourmaline, Ca( fluid) = concentration of Ca2+ in the fluid in mol/l). The investigations may serve as a first guideline to evaluate the possibility to use tourmaline as
an indicator for the fluid composition.
Received July 24, 1998;/revised version accepted October 21, 1999 相似文献
Zusammenfassung. ?Synthese von Ca-Turmelin im System CaO-MgO-Al 2 O 3 -SiO 2 -B 2 O 3 -H 2 O-HCl Im System CaO-MgO-Al2O3-SiO2-B2O3-H2O-HCl wurde Ca-Turmalin hydrothermal aus Ca(OH)2 and CaCl2-haltigen L?sungen bei T = 300–700 °C und einem konstanten Fluiddruck von 200 MPa synthetisiert. Die Synthese von Turmalin war m?glich ab 400 °C, aber nur oberhalb von 500 °C bildeten sich deutliche Mengen an Turmalin. Elektronenstrahl-Mikrosondenanalysen und R?ntgenpulveraufnahmen zeigen, da? Mischkristalle der Reihe Oxy-Uvit, CaMg3Al6(Si6O18)(BO3)3(OH)3O, und Oxy-Mg-Foitit, □(MgAl2)Al6(Si6O18)(BO3)3(OH)3O gebildet wurden. Der Anteil an Ca variiert zwischen 0.36 und 0.88 Ca pfu und nimmt mit zunehmender Synthesetemperatur und zunehmender Ca-Konzentration im System zu. Hydroxy-Uvit, CaMg3(MgAl5) (Si6O18)(BO3)3(OH)3(OH), konnte nicht synthetisiert werden. Alle Turmaline haben < 3 Mg und > 6 Al pfu. Dabei nimmt das Al/(Al + Mg)- Verh?ltnis mit zunehmendem Ca-Gehalt von 0.80 auf 0.70 ab. Al ist gekoppelt mit Mg und Ca über die Substitutionen Al2□Mg−2Ca−1 und AlMg−1H−1. Einphasiger Turmalin konnte nicht synthetisiert werden. Anorthit (+ Quarz in den meisten F?llen) koexistiert mit Turmalin. Andere Phasen sind Chlorit, Tremolit, Enstatit oder Cordierit. Ca zeigt eine deutliche Fraktionierung in den Festk?rpern Turmalin (+ Anorthit). Das Konzentrationsverh?ltnis D = Ca(fluid)/Ca(tur) nimmt von 0.20 bei 500 °C auf 0.31 bei 700 °C zu. Für die Paragenese Turmalin + Anorthit + Quarz mit Chlorit oder Tremolit oder Cordierit gilt folgende Beziehung zwischen Ca-Gehalt in Turmalin und Fluid und der Temperatur: (wobei T = Temperatur in °C, Ca(tur) = Anteil an Ca auf der X-Position in Turmalin, Ca(fluid) = Konzentration von Ca2+ im Fluid in mol/l). Die Untersuchungen dienen zur ersten Absch?tzung, ob Turmalin als Fluidindikator petrologisch nutzbar ist.
Received July 24, 1998;/revised version accepted October 21, 1999 相似文献
10.
Low-temperature heat capacity measurements for MgCr2O4 have only been performed down to 52 K, and the commonly quoted third-law entropy at 298 K (106 J K−1 mol−1) was obtained by empirical extrapolation of these measurements to 0 K without considering the magnetic or electronic ordering
contributions to the entropy. Subsequent magnetic measurements at low temperature reveal that the Néel temperature, at which
magnetic ordering of the Cr3+ ions in MgCr2O4 occurs, is at ∼15 K. Hence a substantial contribution to the entropy of MgCr2O4 has been missed. We have determined the position of the near-univariant reaction MgCr2O4+SiO2=MgSiO3+Cr2O3. The reaction, which has a small positive slope in P-T space, has been bracketed at 100 K intervals between 1273 and 1773 K by reversal experiments. The reaction is extremely sluggish,
and lengthy run times with a flux (H2O, BaO-B2O3 or K2O-B2O3) are needed to produce tight reversal brackets. The results, combined with assessed thermodynamic data for Cr2O3, MgSiO3 and SiO2, give the entropy and enthalpy of formation of MgCr2O4 spinel. As expected, our experimental results are not in good agreement with the presently available thermodynamic data.
We obtain Δ
f
H
∘
298=−1759.2±1.5 kJ mol−1 and S
∘
298=122.1±1.0 J K−1 mol−1 for MgCr2O4. This entropy is some 16 J K−1 mol−1 more than the calorimetrically determined value, and implies a value for the magnetic entropy of MgCr2O4 consistent with an effective spin quantum number (S') for Cr3+ of 1/2 rather than the theoretical 3/2, indicating, as in other spinels, spin quenching.
Received: 9 May 1997 / Accepted: 28 July 1997 相似文献
11.
Wenjun Yong E. Dachs A. C. Withers E. J. Essene 《Contributions to Mineralogy and Petrology》2008,155(2):137-146
The low-temperature heat capacity (C
p) of Si-wadeite (K2Si4O9) synthesized with a piston cylinder device was measured over the range of 5–303 K using the heat capacity option of a physical
properties measurement system. The entropy of Si-wadeite at standard temperature and pressure calculated from the measured
heat capacity data is 253.8 ± 0.6 J mol−1 K−1, which is considerably larger than some of the previous estimated values. The calculated phase transition boundaries in the
system K2O–Al2O3–SiO2 are generally consistent with previous experimental results. Together with our calculated phase boundaries, seven multi-anvil
experiments at 1,400 K and 6.0–7.7 GPa suggest that no equilibrium stability field of kalsilite + coesite intervenes between
the stability field of sanidine and that of coesite + kyanite + Si-wadeite, in contrast to previous predictions. First-order
approximations were undertaken to calculate the phase diagram in the system K2Si4O9 at lower pressure and temperature. Large discrepancies were shown between the calculated diagram compared with previously
published versions, suggesting that further experimental or/and calorimetric work is needed to better constrain the low-pressure
phase relations of the K2Si4O9 polymorphs.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
12.
The spinel solid solution was found to exist in the whole range between Fe3O4 and γ-Fe2SiO4 at over 10 GPa. The resistivity of Fe3−
x
Si
x
O4 (0.0<x<0.288) was measured in the temperature range of 80∼300 K by the AC impedance method. Electron hopping between Fe3+ and Fe2+ in the octahedral site of iron-rich phases gives a large electric conductivity at room temperature. The activation energy
of the electron hopping becomes larger with increasing γ-Fe2SiO4 component. A nonlinear change in electric conductivity is not simply caused by the statistical probability of Fe3+–Fe2+ electron hopping with increasing the total Si content. This is probably because a large number of Si4+ ions occupies the octahedral site and the adjacent Fe2+ keeping the local electric neutrality around Si4+ makes a cluster, which generates a local deformation by Si substitution.
The temperature dependence of the conductivity of solid solutions indicates the Verwey transition temperature, which decreases
from 124(±2) K at x=0 (Fe3O4) to 102(±5) K at x=0.288, and the electric conductivity gap at the transition temperature decreases with Si4+ substitution.
Received: 15 March 2000 / Accepted: 4 September 2000 相似文献
13.
G. Diego Gatta Ilaria Adamo Martin Meven Erica Lambruschi 《Physics and Chemistry of Minerals》2012,39(10):829-840
The chemical composition and the crystal structure of pezzottaite [ideal composition Cs(Be2Li)Al2Si6O18; space group: ${\it{R}} \overline{\text{3}} $ c, a?=?15.9615(6) ?, c?=?27.8568(9) ?] from the type locality in Ambatovita (central Madagascar) were investigated by electron microprobe analysis in wavelength dispersive mode, thermo-gravimetric analysis, Fourier-transform infrared spectroscopy, single-crystal X-ray (at 298?K) and neutron (at 2.3?K) diffraction. The average chemical formula of the sample of pezzottaite resulted Cs1,Cs2(Cs0.565Rb0.027K0.017)Σ0.600 Na1,Na2(Na0.101Ca0.024)Σ0.125Be2.078Li0.922 Al1,Al2(Mg0.002Mn0.002Fe0.003Al1.978)Σ1.985 Si1,Si2,Si3(Al0.056Si5.944)Σ6O18·0.27H2O. The (unpolarized) IR spectrum over the region 3,800–600?cm?1 was collected and a comparison with the absorption bands found in beryl carried out. In particular, two-weak absorption bands ascribable to the fundamental H2O stretching vibrations (i.e. 3,591 and 3,545?cm?1) were observed, despite the mineral being nominally anhydrous. The X-ray and neutron structure refinements showed: (a) a non-significant presence of aluminium, beryllium or lithium at the Si1, Si2 and Si3 sites, (b) the absence (at a significant level) of lithium at the octahedral Al1, Al2 and Al3 sites and (c) a partial lithium/beryllium disordering between tetrahedral Be and Li sites. 相似文献
14.
《Geochimica et cosmochimica acta》1986,50(6):1261-1265
The viscosity-temperature relationships of five melts on the join Na2Si2O2-Na4Al2O5 (5, 10, 20, 30 and 40 mole percent Na4Al2O5) have been measured in air, at 1 atm and 1000–1350°C with a concentric cylinder viscometer. All the melts on this join of constant bulk polymerization behave as Newtonian fluids, in the range of shear rates investigated, and the melts exhibit Arrhenian viscosity-temperature relationships.Isothermal viscosities on this join initially decrease and then increase with increasing mole percent Na4Al2O5. The minimum viscosity occurs near 20 mole percent Na4Al2O5 at 1000°C and moves to higher Na4Al2O5 content with increasing temperature.The observation of a viscosity minimum along the join Na2Si2-O5-Na4Al2O5 is not predicted based on earlier viscosity data for the system Na2O-Al2O3-SiO2 (RlEBLlNG, 1966) or based on calculation methods derived from this and other data (Bottinga and Weill, 1972). This unexpected behavior in melt viscosity-temperature relations emphasizes the need for a more complete data set in simple silicate systems.Previous spectroscopic investigation of melts on the join Na22Si2O5-Na4Al2O5 offer a structural explanation for the observed viscosity data in terms of a disproportionation reaction involving polyanionic units. Macroscopically, the viscosity data may be qualitatively reconciled with the configurational entropy model for viscous flow (Richet, 1984). 相似文献
15.
I. Gavrieli Alan Matthews J. B. Holland 《Contributions to Mineralogy and Petrology》1996,125(2-3):251-262
The hydrothermal reaction between grossular and 1 molar manganese chloride solution was studied at 2 kbar and 600 °C at various
bulk Ca/(Ca+Mn) compositions:
Ca3Al2Si3O12+3Mn2+(aq) ⇔ Mn3Al2Si3O12+3Ca2+(aq)
The reaction products are garnets of the spessartine-grossular solid-solution series which discontinuously armour the dissolving
grossular grains. The first garnet to crystallize is spessartine rich (X
gt
Mn≥0.95), reflecting the high Mn content of the solution, but as the reaction proceeds more calcium-rich garnets progressively
overgrow the initial products. The armouring product layer is detached from the dissolving grossular, which allows the progressive
overgrowth to occur on both its external and internal surfaces and results in the development of a two directional Ca/(Ca+Mn)
zoning pattern in the product grains. The compositional changes in the run products are consistent with attainment of heterogeneous
equilibrium between the external rims of the spessartine-grossular garnets and the bulk solutions in runs of duration ≥24
hours. Plots of ln KD versus X
gt
Ca maxima show linear variations that are not consistent with the ideal mixing that has been proposed for spessartine-grossular
garnets at temperatures of 900 to 1200 °C. The data rather fit a regular solution model with the parameters ΔG° (600 °C, 2 kbar)=−8.0±0.8 kJ/mol and w
gt
CaMn=2.6±2.0 kJ/mol. Existing solubility measurements and thermodynamic data from other Ca and Mn silicates support the calculated
data. Grossular activities calculated using the w
gt
CaMn parameter indicate that even in manganese-rich metapelites pressure estimates calculated using the garnet-plagioclase-Al2SiO5-quartz barometer will not be increased by more than 0.2 kbar.
Received: 18 January 1995/Accepted: 4 June 1996 相似文献
16.
Geometrical changes induced by cation substitutions {Si4+/Al3+}[Mg2+/Al3+], {2Si4+/2Al3+} [2Mg2+/2Al3+], {Si4+/Fe3+} [Mg2+/Al3+] or [Mg2+/Fe3+], where {} and [] indicate tetrahedral and octahedral sheet in lizardite 1T, are studied by ab-initio quantum chemistry calculations. The majority of the models are based on the chemical compositions
reported for various lizardite polytypes with the amount of Al in the tetrahedral sheets reported to vary from 3.5% to 8%
in the 1T and 2H
1, up to ~30% in the 2H
2 polytype. Si4+ by Fe3+ substitution in the tetrahedral sheet with an Al3+ (Fe3+) in the role of a charge compensating cation in the octahedral sheet is also examined. The cation substitutions result in
the geometrical changes in the tetrahedral sheets, while the octahedral sheets remain almost untouched. Substituted tetrahedra
are tilted and their basal oxygens pushed down from the plane of basal oxygens. Ditrigonal deformation of tetrahedral sheets
depends on the substituting cation and the degree of substitution. 相似文献
17.
E. L. Belokoneva Yu. K. Gubina J. B. Forsyth P. J. Brown 《Physics and Chemistry of Minerals》2002,29(6):430-438
The chemical bonding in the ring silicate mineral dioptase is investigated on the basis of accurate single-crystal X-ray
diffraction data. A multipole model is used in the refinements. Static deformation electron density is mapped for the silicon
tetrahedron, Cu-octahedron and water molecule in different sections. The silicon tetrahedron exhibits peaks resulting from
σ-bonds between Si–sp3 hybrid orbitals and O–p orbitals. The excess density is located on bonds between the Si atom and bridge (in ring) O(1)-,
O(1′)-oxygens and across the interior of the Si–O–Si angle. In the Jahn-Teller distorted Cu octahedron, in addition to peaks
which result from single Cu–O σ-bonds, there are peaks which are due to 3d electrons. The analysis of crystal-field influence on the Cu charge distribution is made using the tetragonal D
4
d
approximation for the low-symmetry (C1) Cu octahedron. The calculation of the occupancies of the 3d atomic orbitals shows that the Cu non-bonding orbitals are most populated (˜20%) and the bonding orbitals least populated
(14%), as is typical for the Jahn-Teller octahedron. The effective atomic charge on the Cu atom in dioptase determined from
the multipoles is +1.23e: closer to the Cu+1 than to the Cu+2 state. The charge on the Si atom has a value +1.17e, which is in the range typical for Si atoms already determined by this
method. The accumulation of density on bridge oxygens and across the interior of the Si–O–Si angle may be explained by additional
strain in the bond with the decrease of the Si–O–Si angle in dioptase to 132°. The same effect was found earlier in coesite.
A single-crystal neutron diffraction study shows that dioptase becomes antiferromagnetic below a Néel temperature of 15.9(1)
K, in contrast to the previously reported specific heat anomaly at 21 K. The magnetic propagation vector is (0, 0, 3/2) on
the hexagonal triple cell or (1/2, 1/2, 1/2) in rhombohedral indices. The relation between the antiferromagnetic and the charge-density
models for dioptase is discussed. The less occupied Cu d
x2−y2
orbitals are responsible for the magnetic properties. These lie in the Cu–O squares, which are approximately perpendicular
to c
hex, but which are alternately inclined to it by a small angle. The magnetic moments of 0.59(1)μ
B
on the Cu ions in the same level are ordered ferromagnetically, but between ions in alternate levels the coupling is antiferromagnet.
Within experimental error the magnetic moments are perpendicular to the square planes, which make an angle ±13(3)° to the
triad axis.
Received: 8 June 2001 / Accepted: 10 January 2002 相似文献
18.
N. V. Zubkova D. Yu. Pushcharovsky G. Giester E. Tillmanns I. V. Pekov D. A. Kleimenov 《Mineralogy and Petrology》2002,75(1-2):79-88
Summary
The crystal structure of arsentsumebite, ideally, Pb2Cu[(As, S)O4]2(OH), monoclinic, space group P21/m, a = 7.804(8), b = 5.890(6), c = 8.964(8) ?, β = 112.29(6)°, V = 381.2 ?3, Z = 2, dcalc. = 6.481 has been refined to R = 0.053 for 898 unique reflections with I> 2σ(I). Arsentsumebite belongs to the brackebuschite group of lead minerals with the general formula Pb2
Me(XO4)2(Z) where Me = Cu2+, Mn2+, Zn2+, Fe2+, Fe3+; X = S, Cr, V, As, P; Z = OH, H2O. Members of this group include tsumebite, Pb2Cu(SO4)(PO4)(OH), vauquelinite, Pb2Cu(CrO4)(PO4)(OH), brackebuschite, Pb2 (Mn, Fe)(VO4)2(OH), arsenbracke buschite, Pb2(Fe, Zn)(AsO4)2(OH, H2O), fornacite, Pb2Cu(AsO4)(CrO4)(OH), and feinglosite, Pb2(Zn, Fe)[(As, S)O4]2(H2O). Arsentsumebite and all other group members contain M = M–T chains where M = M means edge-sharing between MO6 octahedra and M–T represents corner sharing between octahedra and XO4 tetrahedra. A structural relationship exists to tsumcorite, Pb(Zn, Fe)2(AsO4)2 (OH, H2O)2 and tsumcorite-group minerals Me(1)Me(2)2(XO4)2(OH, H2O)2.
Received June 24, 2000; revised version accepted February 8, 2001 相似文献
19.
The relative stabilities of orthozoisite, Ca2Al3[O|OH|Si2O7|SiO4], space group Pnma, and the monoclinic polymorph, clinozoisite, space group P21/m, have been investigated using calculations based on density functional theory. It is found that orthozoisite is more stable
than clinozoisite by about 1 kJ mol−1 at zero pressure in the athermal limit. The bulk moduli of the two polymorphs have been calculated to be Bortho=117.5(1.7) GPa and Bclino=136(4) GPa.
Received: 20 March 2000 / Accepted: 26 February 2001 相似文献
20.
The equilibrium water content of cordierite has been measured for 31 samples synthesized at pressures of 1000 and 2000 bars
and temperatures from 600 to 750° C using the cold-seal hydrothermal technique. Ten data points are presented for pure magnesian
cordierite, 11 data points for intermediate iron/magnesium ratios from 0.25 to 0.65 and 10 data points for pure iron cordierite.
By representing the contribution of H2O to the heat capacity of cordierite as steam at the same temperature and pressure, it is possible to calculate a standard
enthalpy and entropy of reaction at 298.18° K and 1 bar for,
(Mg,Fe)2Al4Si5O18+H2O ⇄ (Fe,Mg)2Al4Si5O18.H2O
Combining the 31 new data points with 89 previously published experimental measurements gives: ΔH
°
r
=–37141±3520 J and ΔS
°
r
=–99.2±4 J/degree. This enthalpy of reaction is within experimental uncertainty of calorimetric data. The enthalpy and entropy
of hydration derived separately for magnesian cordierite (–34400±3016 J, –96.5±3.4 J/degree) and iron cordierite (–39613±2475,
–99.5±2.5 J/degree) cannot be distinguished within the present experimental uncertainty. The water content as a function of
temperature, T(K), and water fugacity, f(bars), is given by n
H2O=1/[1+1/(K ⋅ f
H2O)] where the equilibrium constant for the hydration reaction as written above is, ln K=4466.4/T–11.906 with the standard state for H2O as the gas at 1 bar and T, and for cordierite components, the hydrous and anhydrous endmembers at P and T.
Received: 2 August 1994/Accepted: 7 February 1996 相似文献