摘要:

AbstractWe present29Si MAS-NMR spectroscopic data for a series of synthetic feldspar samples along the join CaAl2Si2O8-SrAl2Si2O8, from which the composition dependence and cou­pling of order parameters describing Si-Al order and the triclinic-monoclinic displacive transition were determined. Spectra of SrAl2Si2O8contain narrow peaks for the two crys­tallographic Si sites, plus additional peaks for Si having three and two Al nearest neighbors, indicating the presence of approximately 0.14 Al-O-Al linkages per formula unit and a value of σ = 0.93 for the short-range order parameter. For the triclinic feldspar samples, short-range Si-Al order increases continuously with Sr content from σ = 0.89(3) for CaAl2Si2O8to 0.97(1) for Sr0.85Ca0.20Al2Si2O8but then decreases discontinuously to 0.93(2) for the monoclinic samples, Sr0.85Ca0.15Al2Si2O8to SrAl2Si2O8. The variation of peak posi­tions with composition is consistent with a structural phase transition near Sr0.85Ca0.15Al2Si2O8from I1̄ to I2/c. The order parameter for this displacive transition is reflected by the chemical shift of the Tlmz crystallographic site, and its composition dependence gives an order-parameter critical exponent of β = 0.49(2), indicating classical second-order behavior.

ISSN:0003-004X    

DOI:10.2138/am-1997-1-201

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractLattice-energy minimization calculations have been performed on the feldspar systems (Ca,Sr)Al2Si2O8and disordered (Na,K)AlSi3O8as functions of composition to simulate the ferroelastic phase transitions I2/c-I1̄ and C2/m-C1̄, respectively. In both cases the phase transition occurs as a function of composition and is driven by the vanishing of the quantity C44C66- C246, without any of the individual elastic constants being strongly dependent on composition and without softening of an optic mode. In both cases, the strains Ɛ4and Ɛ6are proportional to each other for small values of strain, but nonlinear coupling becomes dominant when |Ɛ4| becomes larger than about 0.02. The results are consistent with exper­imental data and explain the nature of coupling of the displacive transition to Al-Si or­dering in Al:Si 2:2 feldspars.

ISSN:0003-004X    

DOI:10.2138/am-1997-1-202

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractHigh-resolution neutron powder diffraction was used to study phase transitions in the leucite phases of KAlSi2O6, RbAlSi2O6, CsAlSi2O6, and KFeSi2O6. The temperature-depen­dent structural behavior involves two mechanisms: relaxation of the tetrahedral framework about channel cations, and slowly changing T-O bond lengths. The high-temperature cubic phase is characterized by a fully-extended tetrahedral framework; thermal expansion occurs by an increase in mean T-O bond lengths. On decreasing temperature, a displacive phase transition to tetragonal symmetry is manifested by an optic instability; twisting of tetrag­onal prisms of comer-linked (Al,Si)O4tetrahedra about [001] leads to collapse of the 〈111〉 structural channels and concomitant volume reduction.

ISSN:0003-004X    

DOI:10.2138/am-1997-1-203

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractWe report the direct experimental observation of a structural anomaly near 825 K in synthetic and natural titanite samples by high-temperature, hard-mode infrared spectros­copy. The anomaly in titanite, CaTiSiO5, is characterized by a break of the temperature dependence of the 562 cm-1Si-O bending mode, the 675 cm-1Ti-O band, and the 900 cm-1Si-O stretching modes, and the rapid decrease of the IR signal at 873 cm-1. The order parameter, as determined from the temperature evolution of the frequencies of the absorption bands in the middle infrared (MIR) region follows a second-order Landau be­havior with an order-parameter exponent β = ½. At T>825 K, the Ti-O band shows further softening, whereas the Si-O bands at 562 and 900 cm-1show hardening with increasing temperature. In natural samples, the effects of impurities on the high-tempera­ture transition are weak. For pure titanite, the transition temperature, Tc, is about 825 K and increases with increasing impurity concentration.

ISSN:0003-004X    

DOI:10.2138/am-1997-1-204

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractThe coordination environment of Ti in eight Ti-bearing glasses of the Na2Si4O9-Na2Ti4O9join (NTS) and in six ATY2 glasses (A2O·TiO2·2YO2, with A = Na, K, or Rb and Y = Si or Ge) was determined using high-resolution, X-ray absorption near-edge structure (XANES) spectroscopy at the Ti K edge in ambient conditions.Fivefold-coordinated Ti ([5]Ti) is the dominant Ti species (≥50 ± 10% of the total Ti) in all the glasses studied. Sixfold-coordinated Ti was detected mostly in sodic glasses (NTS, NTS2, NTG2), and it increases with TiO2content (as high as 40 ± 10% of the total of Ti in the most TiO2-rich NTS glasses) and in the order Si

ISSN:0003-004X    

DOI:10.2138/am-1997-1-205

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractThe coordination environments of Ti in two fully metamict zirconolite samples and two partially metamict titanite samples were determined using high-resolution. X-ray absorp­tion near-edge structure (XANES) spectroscopy at the Ti K edge. Fivefold-coordinated Ti is the dominant Ti species in the zirconolite samples (~-80 ± 10% of the total Ti atoms). This unusual Ti coordination is also possible in the titanite samples. No significant evidence for[4]Ti was found in any of the samples studied.Comparison with other amorphous materials, such as other metamict minerals (aes- chynite and pyrochlore) and titanosilicate glasses and melts, suggests that fivefold coor­dination is rather common for Ti4+in aperiodic structures. However, the metamict state is characterized by the presence of unusual trigonal bipyramids around|5|Ti4+.

ISSN:0003-004X    

DOI:10.2138/am-1997-1-206

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractThe structural and elastic properties of MgO periclase were studied up to 150 GPa with the first-principles pseudopotential method within the local density approximation. The calculated lattice constant of the B1 phase over the pressure range studied is within 1% of experimental values. The observed B1 phase of MgO was found to be stable up to 450 GPa, precluding the B1-B2 phase transition within the lower mantle. The calculated tran­sition pressure is less than one-half of the previous pseudopotential prediction but is very close to the linearized augmented plane-wave result. All three independent elastic con­stants, c11, c12, and c44, for the B1 phase are calculated from direct computation of stresses generated by small strains. The calculated zero-pressure values of the elastic moduli and wave velocities and their initial pressure dependence are in excellent agreement with ex­periments. MgO was found to be highly anisotropic in its elastic properties, with the magnitude of the anisotropy first decreasing between 0 and 15 GPa and then increasing from 15 to 150 GPa. Longitudinal and shear-wave velocities were found to vary by 23 and 59%, respectively, with propagation direction at 150 GPa. The character of the an­isotropy changes qualitatively with pressure. At zero pressure longitudinal and shear-wave propagations are fastest along [111] and [100], respectively, whereas above 15 GPa, the corresponding fast directions are [100] and [110]. The Cauchy condition was found to be strongly violated in MgO, reflecting the importance of noncentral many-body forces.

ISSN:0003-004X    

DOI:10.2138/am-1997-1-207

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractCompressibility data of clinozoisite and zoisite were measured by single-crystal X-ray diffraction in a diamond-anvil cell up to a pressure of about 50 kbar. In both polymorphs, the unit cell parameters varied linearly with pressure but in an anisotropic pattern: βa=2.1(1) × 10-4, βb= 2.8(1) × 10-4, βc= 3.3(1) × 10-4kbar-1for clinozoisite, and βa=2.3(2) × 10-4, βb= 2.9(1) × 10-4, βc= 3.7(2) × 10-4kbar-1for zoisite. The principal coefficients of the strain ellipsoid of clinozoisite are β1= 2.0 × 10-4, β2= 2.7 × 10-4, β3= 3.3 × 10-4kbar-1; β1and β3were symmetrically oriented in the (010) plane with an angle of about 12° between β1and the a axis, whereas β2coincides with the b axis. Bulk moduli calculated as the reciprocal of cell-volume compressibility were 1300(20) kbar for the monoclinic and 1140(20) for the orthorhombic polymorph. K0, determined by fitting the unit-cell parameters with a third-order Birch-Murnaghan equation of state, was 1270(45) kbar, with K' = 0.5(2) for clinozoisite and 1020(65) kbar with K' = 4.8(4) for zoisite.Structural refinements of clinozoisite performed at 0.5, 19.4, and 42 kbar, and also under ambient conditions, showed that the compression mechanism included both shrinking of the polyhedra (i.e., octahedra and Ca polyhedra) and tilting of the Si2O7group, with re­duction of the Si-O-Si angle. The different effect of these mechanisms explains the aniso­tropic compressional pattern in clinozoisite and the similar behavior observed in the two polymorphs.Comparison of high-pressure and high-temperature data for clinozoisite showed that a given increase in pressure produced structural effects very similar to those seen after a proportional decrease in temperature. The calculated volume-expansivity-to-compressibility ratio of 38 bar/K indicates that the cell volume of clinozoisite remains unchanged with geothermal gradients of about 10 °C/km. The crystallographic data support the results of experimental petrology in indicating that epidote is a good candidate for transporting H2O in down-going subduction slabs.

ISSN:0003-004X    

DOI:10.2138/am-1997-1-208

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractDehydration of Ca- and Mg-exchanged montmorillonite was studied along H2O iso­chores in the hydrothermal diamond-anvil cell by in situ X-ray diffraction using a syn­chrotron radiation source. At pressures between the H2O liquid-vapor (L-V) boundary and ∼ 10 kbar, the dehydration temperature for the conversion from the 19 Å hydration state to the 15 Å hydration state occurred over the temperature range 260-350 °C for Ca-ex- changed montmorillonite and 200-250 °C for Mg-exchanged montmorillonite, with a slight increase with increasing pressure. For both materials, the rehydration from the 15 Å to 19 Å states occurred at the same temperature as dehydration at pressures along the H2O L-V boundary, thus showing no hysteresis. The rehydration hysteresis increased to nearly 75 °C at 6 kbar for Ca-exchanged montmorillonite and to the same amount at 2.5 kbar for Mg-exchanged montmorillonite. Dehydration experiments on Mg-exchanged montmoril­lonite along the isochores of 1.024 and 0.75 g/cm3showed conversion from the 15 Å hydrate to the 12.5 Å hydrate at 590-605 °C. The 12.5 Å hydrate only partially rehydrated after cooling to room temperature along those two isochores. In an experiment started from the 15 Å state, in which the pressure was below the H2O L-V curve, dehydration occurred at 400-500 °C and rehydration at 430-350 °C. When our previous results on Na-exchanged montmorillonite are combined with the current experimental data, systematic trends can be found in the effect of pressure and interlayer-cation species on the dehydration tem­perature and rehydration hysteresis.

ISSN:0003-004X    

DOI:10.2138/am-1997-1-209

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractA new X-ray diffraction method for the determination of the amount and distribution of fixed NH+4in illite-smectite has been developed. Illite-smectite was saturated with K+and heated at 150 °C. The 002 and 005 reflections were recorded with steps of 0.01° 2θ, and the experimental d values and the values for full-width at half-height (FWHH) were determined using a peak-profile-fitting procedure. Peak profiles were calculated with the NEWMOD program for illite structures having different amounts of NH+4and different patterns for the distribution of NH+4in interlayers. For Upper Jurassic illite-smectite from North Sea oil source rocks, the amount and the distribution of NH+4in illite interlayers were determined by comparing the experimental values for d005and FWHH with the values calculated for the selected illite structures. The amounts of NH+4determined in this manner correlate well with the amounts determined by an isotopic dilution method. The results demonstrate that these illite-smectite samples have K end-member illite and NH4end- member illite (tobelite) layers and that the illite layers formed during diagenesis and oil generation actually are tobelite layers.

ISSN:0003-004X    

DOI:10.2138/am-1997-1-210

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter