摘要:

AbstractA lattice-dynamical treatment of displacive phase transitions leads naturally to the softmode model, in which the phase-transition mechanism involves a phonon frequency that falls to zero at the transition temperature. The basic ideas of this approach are reviewed in relation to displacive phase transitions in silicates. A simple free-energy model is used to demonstrate that Landau theory gives a good approximation to the free energy of the transition, provided that the entropy is primarily produced by the phonons rather than any configurational disorder. The ‘‘rigid unit mode’’ model provides a physical link between the theory and the chemical bonds in silicates and this allows us to understand the origin of the transition temperature and also validates the application of the soft-mode model. The model is also used to reappraise the nature of the structures of high-temperature phases. Several issues that remain open, such as the origin of first-order phase transitions and the thermodynamics of pressure-induced phase transitions, are discussed.

ISSN:0003-004X    

DOI:10.2138/am-1997-3-401

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractThe hydrostatic compression of synthetic single crystals of diopside, CaMgSi2O6, and hedenbergite, CaFeSi2O6, was studied at 33 pressures up to 10 GPa by X-ray diffraction. In addition, intensity data for hedenbergite were collected at 12 pressures up to 10 GPa. For determination of the elasticity two crystals were loaded together in a diamond cell. The axial compressibilities βa, βb, and βcof diopside and hedenbergite are 2.36(4), 3.17(4), and 2.50(4) × 10-3GPa-1, and 1.93(5), 3.38(6), and 2.42(8) × 10-3GPa-1, respectively. The bulk moduli (KT₀) and their pressure derivatives (K′T₀) were determined simultaneously from a weighted linear fit of a third order Birch-Murnaghan equation of state to the volume data at elevated pressures. KT₀and K′T₀are 104.1(9) GPa and 6.2(3) for diopside and 117(1) GPa and 4.3(4) for hedenbergite, respectively.The unit-cell parameters decrease continuously with pressure. The larger polyhedra show more compression than the smaller ones. Between 0.1 MPa and 10 GPa the polyhedral volumes of CaO8, FeO6, and SiO4decrease by 8.4, 6.6, and 2.9%, respectively. The longest bonds of CaO8and FeO6show most compression. Significant compression in the two shortest Si-O1 and Si-O2 bond lengths of the SiO4tetrahedra was observed at relatively low pressures, resulting in a tetrahedral volume compression of 1.6% between 0.1 GPa and 4 GPa and 1.3% between 4 and 10 GPa. The compression of the unit cell can be described by the volume compression of the individual CaO8and FeO6polyhedra, with the SiO4tetrahedron playing a minor role. Diopside is more compressible than hedenbergite as shown by their axial and volume compressibilities because the FeO6octahedron is significantly more rigid than MgO6at high pressures. This observation implies that octahedrally coordinated Fe2+behaves differently from Mg at high pressures, in contrast to their behavior at ambient conditions.

ISSN:0003-004X    

DOI:10.2138/am-1997-3-402

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractClinopyroxene (Cpx) is a principal upper-mantle phase for concentrating large cations but has not been viewed as a major crystal-chemical reservoir for K because K+is considered too large to enter the largest site, M2, in the pyroxene structure. Accumulating data from high-pressure conditions indicate this inference is incorrect, so multianvil experiments have been performed to evaluate maximal K solubility in Cpx at high pressure. End members and mixtures of diopside, jadeite, and kosmochlor have been mixed with K2CO3, KHCO3or both in welded platinum capsules and heated typically for 24 h in the range of 5 to 14 GPa and 1200 to 1700 °C. These experiments produced K-rich Cpx in solid solutions by means of a fictive Kcpx component (KCrSi2O6or KAlSi2O6). The maximum K2O content obtained is 4.7 wt% in a Cpx (Di38Ko39Kcpx22En1) formed from a 50:50 Di 1 Ko mixture at 10 GPa, 1400 °C. K uptake and partitioning is dependent positively on P, complexly on Cpx composition but not demonstrably on T.Cpx/liqDK₂Ois in the range of 0.03-0.1 andCpx/liqDNa₂Ovaries from 0.5 to 5, although the variations for each with Cpx composition are different. In diopside, Kcpx increases are always accompanied by increases in Nacpx, and cooperative Nacpx solution is necessary for Kcpx solution in the compositional systems examined. K appears to be accommodated in the M2 site of the Cpx structure by two types of spatial averaging: a large average M2 site, as in the case of Di, ameliorates the fit, but local accommodation by size averaging with a smaller M2 occupant, presumably Na, appears necessary, suggesting that the polyhedral compressibility of Na and K are large in comparison with Ca. In application to Cpx inclusions in diamond, the data here imply that a chromium diopside with ∼1 wt% K2O forms in the presence of a C-rich melt with 15-28 wt% K2O.

ISSN:0003-004X    

DOI:10.2138/am-1997-3-403

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractWadsleyite [b-(Mg,Fe)2SiO4] is known to accept variable amounts of H and may be an important reservoir of H in the transition zone of the mantle. The crystal structure of Fo94.6hydrous wadsleyite (Mg1.730Fe0.098Al0.008Si0.991H0.355O4) synthesized at 1400 °C and 17 GPa was refined in space group I2/m from 1784 measured intensities of which 830 were unique, and, of these, 650 were of intensity greater than 4s. Unit cell parameters are a 5 5.6715(7), b 5 11.582(2), c 5 8.258(1) Å , and β = 90.397(9)°. The final R(F) for all reflections was 0.026 (Rw= 0.024); goodness of fit was 1.68. An H position was located on the nonsilicate O atom (O1). Partial occupancy of a normally vacant tetrahedral site adjacent to M3 was observed. This is postulated to be the result of Si moving from M3 to the adjacent tetrahedral void on decompression. Deviation from orthorhombic symmetry appears to result from ordering of H, (Mg, Fe), and possibly Si, within the two nonequivalent M3 sites.

ISSN:0003-004X    

DOI:10.2138/am-1997-3-404

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractA new KAlSiO4polymorph was found in a granulite facies gneiss from the Punalur district, southern India. The structure was solved and refined on a twinned crystal to an R index value of 1.98% for 265 independent reflections. Metamorphic kalsilite is trigonal, space group P31c with a 5.157 (1) Å , c = 8.706 (3) Å , V = 200.52 (9) Å3, Z = 2. The overall diffraction symmetry 6/mmm exhibited from all the crystals examined arises from a {0001} twinning, related to a mistake in the ordered Al-Si-Al-Si sequence along the c axis. The crystal structure is a stuffed derivate of tridymite, and is characterized by sixmembered tetrahedral rings with ditrigonal shape. Individual layers of this structure are the same as those of P63kalsilite, but are stacked along the c axis in an eclipsed manner rather than in the staggered manner of the P63structure.

ISSN:0003-004X    

DOI:10.2138/am-1997-3-405

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractA series of aluminous tremolites was synthesized at 800-850 °C and 6-13 kbar along the join Ca1.8Mg5.2Si8O22(OH)2-Ca1.8(Mg4.2Al)(AlSi7)O22(OH)2by means of the Mg-Tschermaks substitution. Although the amphibole yields were high (97-99 wt%), traces of orthopyroxene, clinopyroxene, amorphous silica, and, for the most aluminous mixture, corundum, were identified in some synthesis products. The samples were characterized by electron microprobe (EMP), X-ray diffraction Rietveld structure refinement (XRD), Fourier- transform infrared (FTIR) spectroscopy, and magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy for the purpose of identifying the extent of Al, Mg, and Si order-disorder in the octahedral and tetrahedral sites. Both EMP and27Al NMR verify that the Mg-Tschermaks substitution, with even distribution of Al between tetrahedral and octahedral sites, is obeyed in these amphiboles up to a limit of 1.9 total Al atoms per formula unit (apfu).29Si MAS NMR spectra indicate that Al substitutes essentially equally into both the T1 and T2 sites. For octahedral sites, the FTIR spectra indicate the presence of Al on the M1, M3, or both sites (presently indistinguishable) by the presence of an (MgMgAl)-OH band at 3652 cm-1, which increases steadily with increasing Al content. The FTIR and27Al MAS NMR spectra show that Al is always present in the M1-M3 sites as well as the M2 site but partitioned among these sites in ways that are not clearly resolved at present. It is clear in this study that Al is more widely distributed over the octahedral and tetrahedral sites in synthetic aluminous tremolite with only a moderate Al content (<2.0 Al apfu) than has been traditionally thought.

ISSN:0003-004X    

DOI:10.2138/am-1997-3-406

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractAmphiboles were synthesized at 750 °C, 1 kbar (H2O) on the binary joins (nickel, magnesium)-richterite and (magnesium, cobalt)-richterite. Structural variations and site occupancies were characterized by Rietveld structure refinement, with final RBraggindices in the range 4-9%, and by powder infrared spectroscopy in the principal OH-stretching region. Site-occupancy refinement of Ni-Mg and Mg-Co distributions give the partition coefficients over M1,3 and M2 where KM2+= (M2+/Mg)M1,3/(M2+/Mg)M2, and M2+= Ni2+or Co2+, KdNi= 2.98 ± 0.37 and KdCo= 1.34 ± 0.31. Both Kdvalues are greater than 1.0, whereas[6]r(Ni2+)<[6]r(Mg)<[6]r(Co2+); this indicates that cation size is not the primary factor affecting the ordering of Ni-Mg and Mg-Co over the octahedral sites. The infrared spectra of intermediate binary compositions show fine structure caused by ordering of Ni- Mg or Mg-Co over the M1,3 sites and by ordering of Na and ⃞ (vacancy) at the A site; thus intermediate compositions show an eight-band spectrum in the principal OH stretching region. Precise band intensities were derived by nonlinear least-squares fitting of Gaussian band shapes to the observed spectra. The relative observed intensities of the combinations of bands3I₀A+2I₀B+ I₀Cand I₀B+2I₀C+3I₀Dare in accord with the equations of Burns and Strens (1966), indicating that there is no significant variation in molar absorptivity with frequency (energy) for individual bands within a single sample (spectrum). Combined with the results of Skogby and Rossman (1991) on polarized single-crystal infrared spectra of amphiboles, this result suggests that different local configurations of M1,3 cations in amphiboles couple such that the transition probabilities of the associated OH groups are equal.

ISSN:0003-004X    

DOI:10.2138/am-1997-3-407

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractSynthetic mackinawite (tetragonal FeS) has been found to transform rapidly to greigite (Fe3S4) above ∼373 K during heating experiments, as observed by in situ X-ray diffraction. Using monochromatic synchrotron radiation (λ = 0.60233 Å), we measured the unit-cell parameters of both synthetic mackinawite between 293 and 453 K and of greigite formed from this mackinawite between 293 and 593 K.The coefficients of thermal expansion for mackinawite are α1= α2= (1.36 ± 6 0.11) × 10-5, α3= (2.98 ± 0.12) × 10-5, and αvol= (5.67 ± 0.19) × 10-5between 293 and 453 K. The coefficients of thermal expansion for greigite are α1= α2= α3= (1.63 ± 0.15) × 10-5, and αvol= (4.86 ± 0.25) x 10-5between 293 and 593 K. On further heating in situ, we observed the reaction greigite → pyrrhotite + magnetite.Partial transformation of mackinawite to greigite was also observed using transmission electron microscopy (TEM) following in situ heating. Electron diffraction patterns show that (001) of mackinawite is parallel to (001) of greigite, and [110] of mackinawite is parallel to [100]of greigite. This orientation relationship confirms that the cubic closepacked S array in mackinawite is retained in greigite and implies that oxidation of some Fe2+in mackinawite drives rearrangement of Fe to form the new phase. Small regions of the crystallites show Moiré fringes resulting from the lattice mismatch between mackinawite and greigite. Electron diffraction patterns of mackinawite subjected to prolonged exposure to the atmosphere also show faint spots corresponding to greigite.We propose that in these experiments surplus Fe is accommodated by reaction with either adsorbed O2or H2O to form amorphous nanophase Fe-O(H). Because greigite is so easily formed by oxidation from mackinawite, greigite should be an important precursor for pyrite nucleation, although any orientation relationship between greigite and pyrite remains to be determined.

ISSN:0003-004X    

DOI:10.2138/am-1997-3-408

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractCathodoluminescence (CL) emission spectra of forsterite from the Allende (CV3) and Murchison (C2) meteorites and from Cr-doped and pure synthetic forsterite samples have been obtained at room (296 K) and liquid nitrogen (77 K) temperatures using a transmission electron microscope. At room temperature, three broad CL emissions centered near 420, 640, and 800 nm occur in the meteoritic forsterite, and one peak at 800 nm occurs in one Cr3+-doped forsterite sample. Only the 420 nm peak is present in pure synthetic forsterite. Relative to room temperature, at liquid-nitrogen temperature there is a general increase in overall CL intensity, whereas a broad peak located between 700 and 800 nm changes to a series of sharp emissions centered near 700 nm and a narrower but still broad peak centered near 800 nm.The portion of the broad peak near 700 nm at room temperature and equivalent sharp peaks at low temperature are assigned to Cr3+in octahedral coordination, whereas the broad peak near 800 nm is attributed to Cr associated with structural defects. The peak at 640 nm is consistent with a similar peak in Mn2+-doped forsterite. The peak at 420 nm results from an unknown structural effect that can be eliminated in synthetic forsterite by mechanical deformation. The variation of relative intensities at room temperature for the two broad peaks at 700 and 800 nm in meteoritic forsterite can be correlated with meteorite type and may reflect different processes in forsterite formation or subsequent processing.

ISSN:0003-004X    

DOI:10.2138/am-1997-3-409

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter

摘要:

AbstractAmphibole from alkali granites in the Ramnes cauldron in the Oslo rift was altered hydrothermally by corrosion and growth through multiple events of fluid circulation. The alteration developed successive zones of amphibole at the crystal margins, resulting in a (1) brownish ferro-edenitic hornblende core (FE), (2) deep bluish-green hastingsitic hornblende zone (HH), and (3) light-greenish Fe-rich actinolite rim (FA). The edenitic core preserves the original igneous amphibole composition. The Cl content of amphibole strongly increases from the FE (0.78-0.82 wt%) to the HH zone (2.07-2.96 wt%) and abruptly decreases in the FA rim (0.01-0.36 wt%). In the Cl-rich HH amphibole zone, amphibole has characteristically high Cl content [2.96 wt%, 0.82 atoms per formula unit (apfu)] and high concentrations in both[4]Al (1.73 apfu) and A-site occupancy (0.86 apfu) with a large K/(Na + K) value of 0.47.Both[4]Al and A-site occupancy increase systematically with positive correlation with Cl content throughout the three amphibole zones. On the other hand, Fe2+content is not so simply correlated to the Cl content. Based on crystal structure considerations on Cl-rich amphiboles, the cation substitutions are illustrated by structural (geometrical) constraints for[4]Al and by a chemical constraint for Fe2+. These contributions for Cl incorporation are expressed empirically by ln(Cl/OH)amp= ln(Cl/OH)fluid+ A·[4]Al·Fe2+/RT + B/RT, where A and B are constant.[4]Al·Fe2+vs. ln(Cl/OH) plots of the three distinct amphibole zones suggest different fluid conditions in chemistry and temperature for the three zones. The zoning was developed through two stages of hydrothermal alteration. In the early hydrothermal event, a saline and high-temperature fluid altered the original hornblende (FE) to the Cl-rich HH zone. Late stage alteration by a high Fe/Cl and relatively low-temperature fluid partially over-printed the FA zone at the crystal margin.

ISSN:0003-004X    

DOI:10.2138/am-1997-3-410

出版商:Mineralogical Society of America    

年代:1997

数据来源: Degruyter