Thermal Expansion of Periclase (MgO) and Tungsten (W) to Melting Temperatures

 In situ x-ray data on molar volumes of periclase and tungsten have been collected over the temperature range from 300 K to melting. We determine the temperature by combining the technique of spectroradiometry and electrical resistance wire heating. The thermal expansion (α) of periclase between 300 and 3100 K is given by α=2.6025 10⁻⁵+1.3535 10⁻⁸ T+6.5687 10⁻³ T⁻¹−1.8281 T⁻². For tungsten, we have (300 to 3600 K) α=7.862 10⁻⁶+6.392 10⁻⁹ T. The data at 298 K for periclase is: molar volume 11.246 (0.031) cm³, α=3.15 (0.07) 10⁻⁵ K⁻¹, and for tungsten: molar volume 9.55 cm³, α=9.77 (10.08) 10⁻⁶ K⁻¹. Received: July 18, 1996 / Revised, accepted: February 14, 1997     

Computer modelling of a pressure induced phase change in clinoenstatite pyroxenes

 Using lattice dynamic modelling of pure MgSiO₃ clinopyroxenes, we have be able to simulate the properties of both the low-clino (P2₁/c) and a high-density-clino (C2/c) phases and our results are comparable with the high pressure (HP) X-ray study of these phases (Angel et al. 1992). The transition between the two phases is predicted to occur at 6GPa. The volume variation with pressure for both phases is described by a third-order Birch-Murnaghan equation of state with the parameters V _{0  low}=31.122 cm³·mol⁻¹, K _{T0  low}= 107.42 GPa, K′ _{T0  low}=5.96, V _{0  high}=30.142 cm³·mol^–1, K _{T0  high}102.54 GPa and K′ _{T0  high}=8.21. The change in entropy between the two modelled phases at 6GPa is ΔS _{6 Gpa}=−1.335 J·mol⁻¹·K⁻¹ and the equivalent change in volume is ΔV _{6 GPa}=−0.92 cm³·mol⁻¹, from which the gradient of the phase boundary δP/δT is 0.0014 GPa·K⁻¹. The variation of the bulk modulus with pressure was also determined from the modelled elastic constants and compares very well with the EOS data. The reported Lehmann discontinuity, ∼220 km depth and pressure of 7.11Gpa, has an increase in the seismic compressional wave velocity, v _p , of 7.14% using the data given for PREM (Anderson 1989). At a pressure of 7GPa any phase transition of MgSiO₃ pyroxene would be between ortho (Pbca) and high-clino. We find the value of v _p at 7GPa, for modelled orthoenstatite (Pbca), is 8.41 km·sec⁻¹ and that for the modelled high-clino phase at 7GPa is 8.93 km·sec⁻¹, giving a dv _p /v _p of 6.18%. Received: July 26, 1996 / Revised, accepted: September 27, 1996     

Transport properties of low-sanidine single-crystals, glasses and melts at high temperature

Thermal diffusivity (D) was measured using laser-flash analysis from oriented single-crystal low-sanidine (K_0.92Na_0.08Al_0.99Fe³⁺ _0.005Si_2.95O₈), and three glasses near KAlSi₃O₈. Viscosity measurements of the three supercooled liquids, in the range 10^6.8 to 10^12.3 Pa s, confirm near-Arrhenian behavior, varying subtly with composition. For crystal and glass, D decreases with T, approaching a constant near 1,000 K: D _sat ∼ 0.65 ± 0.3 mm² s⁻¹ for bulk crystal and ∼0.53 ± 0.03 mm² s⁻¹ for the glass. A rapid decrease near 1,400 K is consistent with crossing the glass transition. Melt behavior is approximated by D = 0.475 ± 0.01 mm² s⁻¹. Thermal conductivity (k _lat) of glass, calculated using previous heat capacity (C _P) and new density data, increases with T because C _P strongly increases with T. For melt, k _lat reaches a plateau near 1.45 W m⁻¹ K⁻¹, and is always below k _lat of the crystal. Melting of potassium feldspars impedes heat transport, providing positive thermal feedback that may promote further melting in continental crust.     

X-ray diffraction study of magnesite at high pressure and high temperature

 P–V–T measurements on magnesite MgCO₃ have been carried out at high pressure and high temperature up to 8.6 GPa and 1285 K, using a DIA-type, cubic-anvil apparatus (SAM-85) in conjunction with in situ synchrotron X-ray powder diffraction. Precise volumes are obtained by the use of data collected above 873 K on heating and in the entire cooling cycle to minimize non-hydrostatic stress. From these data, the equation-of-state parameters are derived from various approaches based on the Birch-Murnaghan equation of state and on the relevant thermodynamic relations. With K′₀ fixed at 4, we obtain K₀=103(1) GPa, α(K⁻¹)=3.15(17)×10⁻⁵ +2.32(28)×10⁻⁸ T, (∂K_T/∂T)_P=−0.021(2) GPaK⁻¹, (dα/∂P)_T=−1.81×10⁻⁶ GPa⁻¹K⁻¹ and (∂K_T/∂T)_V= −0.007(1) GPaK⁻¹; whereas the third-order Birch-Murnaghan equation of state with K′₀ as an adjustable parameter yields the following values: K₀=108(3) GPa, K′₀=2.33(94), α(K⁻¹)=3.08(16)×10⁻⁵+2.05(27) ×10⁻⁸ T, (∂K_T/∂T)_P=−0.017(1) GPaK⁻¹, (dα/∂P)_T= −1.41×10⁻⁶ GPa⁻¹K⁻¹ and (∂K_T/∂T)_V=−0.008(1) GPaK⁻¹. Within the investigated P–T range, thermal pressure for magnesite increases linearly with temperature and is pressure (or volume) dependent. The present measurements of room-temperature bulk modulus, of its pressure derivative, and of the extrapolated zero-pressure volumes at high temperatures, are in agreement with previous single-crystal study and ultrasonic measurements, whereas (∂K_T/∂T)_P, (∂α/∂P)_T and (∂K_T/∂T)_V are determined for the first time in this compound. Using this new equation of state, thermodynamic calculations for the reactions (1) magnesite=periclase+CO₂ and (2) magnesite+enstatite=forsterite+CO₂ are consistent with existing experimental phase equilibrium data. Received September 28, 1995/Revised, accepted May 22, 1996     

Electronic absorption and luminescence spectroscopic studies of kyanite single crystals: differentiation between excitation of FeTi charge transfer and Cr3+ dd transitions

A selected set of five different kyanite samples was analysed by electron microprobe and found to contain chromium between <0.001 and 0.055 per formula unit (pfu). Polarized electronic absorption spectroscopy on oriented single crystals, R₁, R₂-sharp line luminescence and spectra of excitation of λ₃- and λ₄-components of R₁-line of Cr³⁺-emission had the following results: (1) The Fe²⁺–Ti⁴⁺ charge transfer in c-parallel chains of edge connected M(1) and M(2) octahedra shows up in the electronic absorption spectra as an almost exclusively c(||Z′)-polarized, very strong and broad band at 16000 cm⁻¹ if <, in this case the only band in the spectrum, and at an invariably lower energy of 15400 cm⁻¹ in crystals with  ≥ . The energy difference is explained by an expansion of the O_f–O_k, and O_b–O_m edges, by which the M(1) and M(2) octahedra are interconnected (Burnham 1963), when Cr³⁺ substitutes for Al compared to the chromium-free case. (2) The Cr³⁺ is proven in two greatly differing crystal fields a and b, giving rise to two sets of bands, derived from the well known dd transitions of Cr^{3+ 4}A_2g→⁴T_2g(F)(I), →⁴T_1g(F)(II), and →⁴T_1g(P)(III). Band energies in the two sets a and b, as obtained by absorption, A, and excitation, E, agree well: I: 17300(a, A), 17200(a, E), 16000(b, A), 16200(b, E); II: 24800(a, A), 24400(a, E); 22300(b, A), 22200(b, E); III: 28800(b,A) cm⁻¹. Evaluation of crystal field parameters from the bands in the electronic spectra yield Dq(a)=1730 cm⁻¹, Dq(b)=1600 cm⁻¹, B(a)=790 cm⁻¹, B(b)=620 cm⁻¹ (errors ca. ±10 cm⁻¹), again in agreement with values extracted from the λ³, λ⁴ excitation spectra. The CF-values of set a are close to those typical of Cr³⁺ substituting for Al in octahedra of other silicate minerals without constitutional OH⁻ as for sapphirine, mantle garnets or beryl, and are, therefore, interpreted as caused by Cr³⁺ substituting for Al in some or all of the M(1) to M(4) octaheda of the kyanite structure, which are crystallographically different but close in their mean Al–O distances, ranging from 1.896 to 1.919 A (Burnham 1963), and slight degrees of distortion. Hence, band set a originates from substitutive Cr³⁺ in the kyanite structural matrix. The CF-data of Cr³⁺ type b, expecially B, resemble those of Cr³⁺ in oxides, especially of corundum type solid solutions or eskolaite. This may be interpreted by the assumption that a fraction of the total chromium contents might be allocated in a precursor of a corundum type exsolution. Received: 3 January 1997 / Revised, accepted: 2 May 1997     

Manganese olivine II: point defect relaxation

To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn₂SiO₄, the point defect relaxation time (τ) characterizing the rate of re-equilibration of electrical conductivity following a change in oxygen fugacity was measured for single crystals oriented for electrical conduction along the [010] direction. The experiments were carried out at temperatures T = 1173–1473 K and oxygen fugacities with the MnSiO₃ activity controlled at unity. The value of τ, which ranges from 130 to 1463 s, increases with decreasing temperature. At 1273 K, the value of τ in the regime is a factor of ∼1.8 smaller than that in the regime. The point defect relaxation time was used to calculate the chemical diffusivity (). Values of lie in the range 2.2× 10^{− 10} − 2.5×10^{− 9} m²/s. For the regime, a semi-log plot of vs 1/T yields a concave downward curve. Based on these results combined with those from Part I of this work for the point defect structure and electrical conductivity in Mn-olivine, it is proposed that the relaxation rate of electrical conduction is determined by the coupled diffusion of manganese vacancies and electron holes which rate-control the relaxation process at low and high temperatures, respectively. Deconvolution of the curve into two straight-line segments yields values for the mobilities and the diffusivities for Mn vacancies and electron holes. These results, combined with the measured electrical conductivity data, were used to calculate the concentrations of Mn vacancies and electron holes. These results in conjunction with those published for other transition-metal silicate olivines reveal the following: The mobility of electron holes in Mn-olivine is about two orders of magnitude smaller than that in Fe-olivine and is somewhat larger than that in (Mg_0.9Fe_0.1)-olivine. The mobilities of metal vacancies in these three olivines are similar both in magnitude and in temperature dependence. The concentrations of the majority point defects are the highest in Fe-olivine crystals and the lowest in Co-olivine crystals, while those in Mn-olivine and (Mg_0.9Fe_0.1)-olivine crystals lie in between. The deviation from stoichiometry in the cation sublattice for a transition-metal silicate olivine is about two orders of magnitude lower than that for the corresponding transition-metal oxide. Received: 10 January 1996 / Revised, accepted: 29 May 1997     

Heat capacities of Tschermak substituted Fe-biotite

Six members of the annite–siderophyllite join were synthesized in a three step process – crystallization of biotite from gels, decomposition of the fine-grained biotite under oxidizing conditions and resynthesis of Fe-Al biotite with planned compositions from these products – producing biotite crystals with thicknesses of up to 10 μm. The biotite was characterized by microprobe, electron microscopy and X-ray diffraction. Heat capacities of these biotites were measured with a DSC (differential scanning calorimeter) over the temperature range 143 to 623 K. Using a least-squares technique, the data were fitted to a c_p-polynomial, c _p =k ₀+k ₁ T ^−0.5+ k ₂ T ⁻²+k ₃ T ⁻³. In the temperature range 143 to 250 K, heat capacities of the different annite–siderophyllite members decrease linearly with increasing Al content. At higher temperatures, however, the c_p-polynomial of biotites with intermediate composition (except Ann₇₉Sid₂₁) exhibit a steeper slope than those of other biotites. This produces positive excess heat capacities in the annite–siderophyllite join at higher temperatures. The activity-composition data of the same binary derived from phase equilibrium experiments (Benisek et al. 1996) and the data of this study suggest two compositional regions along this join, with different extent of deviation from ideality. One at X _Sid < 0.3, characterized by a small deviation, one at X _Sid > 0.3 showing a higher nonideality, resulting in a discontinuity visible at this composition. Powder IR-spectra of these solid solutions were measured with a FTIR-spectrometer and used to calculate heat capacities according to the vibrational model of Kieffer (1979). The comparison of the vibrational function with the c_p-polynomials shows that the vibrational function reproduces well the DSC-data of the siderophyllite-poor and -rich members, but deviates for intermediate compositions, where the excess heats of mixing occur. With increasing Tschermak vector, the tetrahedral rotation angle α increases from 0 to 13° for annite to siderophyllite, respectively. At the composition of the discontinuity, this rotation angle α reaches a value of ∼8^∘. The processing of ∼300 chemical data of natural biotites indicates that over 90% of them have a tetrahedral rotation angle that lies between 7 and 9°. It would appear that biotites with these structural characteristics are most stable. Received: 27 August 1998 / Accepted: 10 November 1998     

Crystal field transitions in Co2[Al4Si5]O18 cordierite and CoAl2O4 spinel determined by neutron spectroscopy

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     

Metamorphic reactions between fluid inclusions and mineral hosts. I. Progress of the reaction calcite+quartz=wollastonite+CO2 in natural wollastonite-hosted fluid inclusions

 At the Bufa del Diente contact-metamorphic aureole, brine infiltration through metachert layers embedded in limestones produced thick wollastonite rims, according to Cc+Qz=Wo+CO₂. Fluid inclusions trapped in recrystallized quartz hosts include: (1) high salinity four phase inclusions [Th(V-L)=460–573° C; Td(salts)=350–400° C; (Na+K)_Cleq=64–73 wt%; X _{CO 2}≤0.02]; (2) low density vapour-rich CO₂-bearing inclusions [Th(L-V)≈500±100° C; X _{CO 2}=0.22–0.44; X _NaCl≤0.01], corresponding to densities of 0.27± 0.05 gcm⁻³. Petrographical observations, phase compositions and densities show that the two fluids were simultaneously trapped in the solvus of the H₂O-CO₂-salts system at 500–600° C and 700±200 bars. The low density fluid was generated during brine infiltration at the solvus via the wollastonite producing reaction. Identical fluid types were also trapped as inclusion populations in wollastonite hosts 3 cm adjacent to quartz crystals. At room temperature, both fluid types additionally contain one quartz and one calcite crystal, generated by the back-reaction Wo+CO₂=Cc+Qz of the host with the CO₂-proportion of the fluid during retrogression. All of the CO₂ was removed from the fluid. On heating in the microstage, the reaction progress of the prograde reaction was estimated via volume loss of the calcites. In vapour-rich fluids, 50% progress is reached at 490–530° C; 80% at 520–560° C; and 100% at 540–590° C, the latter representing the trapping temperatures of the original fluid at the two fluid solvus. The progress is volume controlled. With knowledge of compositions and densities from unmodified inclusions in quartz and using the equation of state of Duan et al. (1995) for H₂O-CO₂-NaCl, along with f _{CO 2}-values extracted from it, the reaction progress curve was recalculated in the P-T-X-space. The calculated progress curve passes through the two fluid solvus up to 380° C/210 bars, continues in the one fluid field and meets the solvus again at trapping conditions. The P-T slope is steep, most of the reaction occurs above 450° C and there is high correspondence between calculated and measured reaction progress. We emphasize that with the exception of quartz, back-reactions between inclusion fluids and mineral hosts is a common process. For almost any prograde metamorphic mineral that was formed by a devolatilization reaction and that trapped the equilibrium fluid or any peak metamorphic fluid as an inclusion, a fluid-host back-reaction exists which must occur somewhere along the retrograde path. Such retrograde reactions may cause drastic changes in density and composition of the fluid. In most cases, however, evidence of the evolving mineral assemblages is not given for they might form submicroscopical layers at the inclusion walls. Received: 15 March 1995 / Accepted: 1 June 1995     

Phase equilibrium and calorimetric study of the transition of MnTiO3 from the ilmenite to the lithium niobate structure and implications for the stability field of perovskite

The phase boundary between MnTiO₃ I (ilmenite structure) and MnTiO₃ II (lithium niobate structure) has been determined by analysis of quench products from reversal experiments in a cubic anvil apparatus at 1073–1673 K and 43–75 kbar using mixtures of MnTiO₃ I and II as starting materials. Tight brackets of the boundary give P(kbar)=121.2−0.045 T(K). Thermodynamic analysis of this boundary gives ΔH^o=5300±1000 J·mol⁻¹, ΔS^o = 1.98 ±1J·K⁻¹· mol⁻¹. The enthalpy of transformation obtained directly by transposed-temperature-drop calorimetry is 8359 ±2575 J·mol⁻¹. Possible topologies of the phase relations among the ilmenite, lithium niobate, and perovskite polymorphs are constrained using the above data and the observed (reversible with hysteresis) transformation of II to III at 298 K and 20–30 kbar (Ross et al. 1989). The observed II–III transition is likely to lie on a metastable extension of the II–III boundary into the ilmenite field. However the reversed I–II boundary, with its negative dP/ dT does represent stable equilibrium between ilmenite and lithium niobate, as opposed to the lithium niobate being a quench product of perovskite. We suggest a topology in which the perovskite occurs stably at low T and high P with a triple point (I, II, III) at or below 1073 K near 70 kbar. The I–II boundary would have a negative P-T slope while the II–III and I–III boundaries would be positive, implying that entropy decreases in the order lithium niobate, ilmenite, perovskite. The inferred positive slope of the ilmenite-perovskite transition in MnTiO₃ is different from the negative slopes in silicates and germanates. These thermochemical parameters are discussed in terms of crystal structure and lattice vibrations.     

Thermal equations of state of dioctahedral micas on the join muscovite–paragonite

 Powder diffraction measurements at simultaneous high pressure and temperature on samples of 2M1 polytype of muscovite (Ms) and paragonite (Pg) were performed at the beamline ID30 of ESRF (Grenoble), using the Paris-Edinburgh cell. The bulk moduli of Ms, calculated from the least-squares fitting of V–P data on each isotherm using a second-order Birch–Murnaghan EoS, were: 57.0(6), 55.1(7), 51.1(7) and 48.9(5) GPa on the isotherms at 298, 573, 723 and 873 K, respectively. The value of (∂K _T /∂T) was −0.0146(2) GPa K⁻¹. The thermal expansion coefficient α varied from 35.7(3) × 10⁻⁶ K⁻¹ at P ambient to 20.1(3) × 10⁻⁶ K⁻¹ at P = 4 GPa [(∂α/∂P) _T = −3.9(1) × 10⁻⁶ GPa⁻¹ K⁻¹]. The corresponding values for Pg on the isotherms at 298, 723 and 823 K were: bulk moduli 59.9(5), 55.7(6) and 53.8(7) GPa, (∂K _T /∂T) −0.0109(1) GPa K⁻¹. The thermal expansion coefficient α varied from 44.1(2) × 10⁻⁶ K⁻¹ at P ambient to 32.5(2) × 10⁻⁶ K⁻¹ at P = 4 GPa [(∂α/∂P) _T = −2.9(1) × 10⁻⁶ GPa⁻¹ K⁻¹]. Thermoelastic coefficients showed that Pg is stiffer than Ms; Ms softens more rapidly than Pg upon heating; thermal expansion is greater and its variation with pressure is smaller in Pg than in Ms. Received: 28 January 2002 / Accepted: 5 April 2002     

A theoretical study of the absorption spectra of Pb+ and Pb3+ in site K+ of microcline: application to the color of amazonite

The blue-green color of amazonite has been assigned by various authors to ions Pb⁺ (6 s)² (6 p) and/or Pb³⁺ (6 s) in site of K⁺ of microcline. Owing to the complex which forms between the ion Pb³⁺ and the lone pairs of the oxygen atoms surrounding it, the peripheral electron of Pb³⁺ passes on the levels (6 p) of the latter, which results in a great similarity of the spectra of Pb⁺ and Pb³⁺ in amazonite (the transition energies are multiplied by a factor greater than 1), whereas, in the isolated state, these spectra are completely different from one another. An analytical development of the crystal field around a site K⁺ is established. Under the effect of the crystal field, the transition ² P _1/2→² P _3/2 (6 p) is split into two double transitions. The lower transition only falls in the visible domain (1.6–1.8 eV for Pb⁺), the second in U−V. The green color would arise from the ion Pb⁺, whereas the blue one would be attributed to the ion Pb³⁺. Received: 23 January 1997 / Revised, accepted: 10 September 1997     

Low-temperature thermodynamic properties of disordered zeolites of the natrolite group

 The heat capacity of paranatrolite and tetranatrolite with a disordered distribution of Al and Si atoms has been measured in the temperature range of 6–309 K using the adiabatic calorimetry technique. The composition of the samples is represented with the formula (Na_1.90K_0.22Ca_0.06)[Al_2.24Si_2.76O₁₀]·nH₂O, where n=3.10 for paranatrolite and n=2.31 for tetranatrolite. For both zeolites, thermodynamic functions (vibrational entropy, enthalpy, and free energy function) have been calculated. At T=298.15 K, the values of the heat capacity and entropy are 425.1 ± 0.8 and 419.1 ±0.8 J K⁻¹ mol⁻¹ for paranatrolite and 381.0 ± 0.7 and 383.2 ± 0.7 J K⁻¹ mol⁻¹ for tetranatrolite. Thermodynamic functions for tetranatrolite and paranatrolite with compositions corrected for the amount of extraframework cations and water molecules have also been calculated. The calculation for tetranatrolite with two water molecules and two extraframework cations per formula yields: C _p (298.15)=359.1 J K⁻¹ mol⁻¹, S(298.15) −S(0)=362.8 J K⁻¹ mol⁻¹. Comparing these values with the literature data for the (Al,Si)-ordered natrolite, we can conclude that the order in tetrahedral atoms does not affect the heat capacity. The analysis of derivatives dC/dT for natrolite, paranatrolite, and tetranatrolite has indicated that the water- cations subsystem within the highly hydrated zeolite may become unstable at temperatures above 200 K. Received: 30 July 2001 / Accepted: 15 November 2001     

Electrical properties of opal-CT

 The electrical properties of opal-CT are validated at temperatures from 600 to 840 °C and frequencies from 5 Hz to 10 MHz. The opals are hydrothermal, containing less than 11270 ppm total of Al, Fe, Ca, Na, and K, and from 1.17 to 17.63 wt% H₂O interstitial and structural. Opal-CT shows fine crystallites, measuring from 19.4 to 22.7 μm, of an ordered tridymite-M stratification with high-cristobalite, embedded in a non-crystalline matrix. When heated to 600 °C, the non-crystalline phase devitrifies to the same stacked high-cristobalite-tridymite-M crystals. Opals containing less than 2070 ppm of cationic impurities are characterized by one single high-frequency complex impedance arc corresponding to the bulk polarization of the crystalline phase, of capacitances between 25 and 30×10⁻¹² F and resistances from 132 to 890 ohm. Opals having over 6300 ppm of cationic impurities show two superimposed high- and low-frequency complex impedance arcs. The high-frequency arc corresponds to the bulk polarization of the crystalline phase, of capacitance between 8 and 15.7×10⁻¹² F and resistance from 14 to 236 ohm, less than the capacitance of 0.25 to 0.53×10⁻⁹ F and resistance from 26 to 360 ohm of the non-crystalline minor intergranular material represented by the low-frequency impedance arc. The electric module shows one single vertex, ascribed to the bulk polarization of the crystalline phase. The conductivities are from 10⁻⁷ to 10⁻⁴ ohm⁻¹cm⁻¹, in the range of poor ionic conductors, essentially constant below 1.8 kHz, rapidly increasing at higher frequencies, due to ionic and electronic charge carriers. The activation energy changes between 0.905 and 1.003 eV for the conduction mechanism in the crystalline phase and from 0.924 to 1.087 eV in the non-crystaline phase. X-ray diffraction and impedance spectroscopy confirm that opal-CT is a crystalline stacked sequence of tridymite-M and cristobalite-high, in a non-crystalline matrix. Received October 20, 1995/Revised, accepted June 15, 1996     

Solubility of water in the α, β and γ phases of (Mg,Fe)2SiO4

 The solubility of hydroxyl in the α, β and γ phases of (Mg,Fe)₂SiO₄ was investigated by hydrothermally annealing single crystals of San Carlos olivine. Experiments were performed at a temperature of 1000° or 1100 °C under a confining pressure of 2.5 to 19.5 GPa in a multianvil apparatus with the oxygen fugacity buffered by the Ni:NiO solid-state reaction. Hydroxyl solubilities were determined from infrared spectra obtained of polished thin sections in crack-free regions ≤100 μm in diameter. In the α-stability field, hydroxyl solubility increases systematically with increasing confining pressure, reaching a value of ∼20,000 H/10⁶Si (1200 wt ppm H₂O) at the α-β phase boundary near 13 GPa and 1100 °C. In the β field, the hydroxyl content is ∼400,000 H/10⁶Si (24,000 wt ppm H₂O) at 14–15 GPa and 1100 °C. In the γ field, the solubility is ∼450,000 H/10⁶Si (27,000 wt ppm H₂O) at 19.5 GPa and 1100 °C. The observed dependence of hydroxyl solubility with increasing confining pressure in the α phase reflects an increase in water fugacity with increasing pressure moderated by a molar volume term associated with the incorporation of hydroxyl ions into the olivine structure. Combined with published results on the dependence of hydroxyl solubility on water fugacity, the present results for the α phase can be summarized by the relation C _OH = A(T)f nH₂Oexp(−PΔV/RT), where A(T) = 1.1 H/10⁶Si/MPa at 1100 °C, n = 1, and ΔV = 10.6×10^–6 m³/mol. These data demonstrate that the entire present-day water content of the upper mantle could be incorporated in the mineral olivine alone; therefore, a free hydrous fluid phase cannot be stable in those regions of the upper mantle with a normal concentration of hydrogen. Free hydrous fluids are restricted to special tectonic environments, such as the mantle wedge above a subduction zone. Received: 10 February 1995 / Accepted: 23 October 1995     

Excess heat capacity and entropy of mixing in the high-structural state (K,Ca)-feldspar binary

Low- and high-temperature heat capacities were measured for a series of synthetic high-structural state (K,Ca)-feldspars (Or–An) using both a relaxation and a differential scanning calorimeter. The data were collected at temperatures between 5 and 800 K on polycrystalline samples that had been synthesised and characterised in a previous study. Below T = 300 K, Or₉₀An₁₀, and Or₈₀An₂₀ showed excess heat capacities of mixing with maximum values of ~3 J mol⁻¹ K⁻¹. The other members of this binary (An > 20 mol%) had lower excess heat capacity values of up to ~1 J mol⁻¹ K⁻¹. Above T = 300 K, some compositions exhibited negative excess heat capacities of mixing (with maximum values of −2 J mol⁻¹ K⁻¹). The vibrational entropy at 298.15 K for Or₉₀An₁₀ and Or₈₀An₂₀ deviated strongly from the behaviour of a mechanical mixture, with excess entropy values of ~3.5 J mol⁻¹ K⁻¹. More An-rich members had only small excess vibrational entropies at T = 298.15 K. The difference in behaviour between members with An > 20 mol% and those with An ≤ 20 mol% is probably a consequence of the structural state of the (K,Ca)-feldspars, i.e., (K,Ca)-feldspars with An ≤ 20 mol% have monoclinic symmetry, whereas those with An > 20 mol% are triclinic. At T = 800 K, the vibrational entropy values were found to scatter around the values expected for a mechanical mixture and, thus, correspond to a quasi-ideal behaviour. The solvus for the (K,Ca)-feldspar binary was calculated based on the entropy data from this study in combination with enthalpy and volume of mixing data from a previous study.     

Thermal expansion of gehlenite, Ca2Al[AlSiO7], and the related aluminates LnCaAl[Al2O7] with Ln=Tb, Sm

The thermal expansion of gehlenite, Ca₂Al[AlSiO₇], (up to T=830 K), TbCaAl[Al₂O₇] (up to T=1,100 K) and SmCaAl[Al₂O₇] (up to T=1,024 K) has been determined. All compounds are of the melilite structure type with space group Thermal expansion data was obtained from in situ X-ray powder diffraction experiments in-house and at HASYLAB at the Deutsches Elektronen Synchrotron (DESY) in Hamburg (Germany). The thermal expansion coefficients for gehlenite were found to be: α₁=7.2(4)×10⁻⁶ K⁻¹+3.6(7)×10⁻⁹ΔT K⁻² and α₃=15.0(1)×10⁻⁶ K⁻¹. For TbCaAl[Al₂O₇] the respective values are: α₁=7.0(2)×10⁻⁶ K⁻¹+2.0(2)×10⁻⁹ΔT K⁻² and α₃=8.5(2)×10⁻⁶ K⁻¹+2.0(3)×10⁻⁹ΔT K⁻², and the thermal expansion coefficients for SmCaAl[Al₂O₇] are: α₁=6.9(2)× 10⁻⁶ K⁻¹+1.7(2)×10⁻⁹ΔT K⁻² and α₃=9.344(5)×10⁻⁶ K⁻¹. The expansion-mechanisms of the three compounds are explained in terms of structural trends obtained from Rietveld refinements of the crystal structures of the compounds against the powder diffraction patterns. No structural phase transitions have been observed. While gehlenite behaves like a ’proper’ layer structure, the aluminates show increased framework structure behaviour. This is most probably explained by stronger coulombic interactions between the tetrahedral conformation and the layer-bridging cations due to the coupled substitution (Ca²⁺+Si⁴⁺)-(Ln ³⁺+Al³⁺) in the melilite-type structure. Electronic Supplementary Material Supplementary material is available for this article at     

Thermal expansion of gehlenite, Ca2Al[AlSiO7], and the related aluminates LnCaAl[Al2O7] with Ln = Tb, Sm

The thermal expansion of gehlenite, Ca₂Al[AlSiO₇], (up to T=830 K), TbCaAl[Al₂O₇] (up to T=1100 K) and SmCaAl[Al₂O₇] (up to T=1024 K) has been determined. All compounds are of the melilite structure type with space group Thermal expansion data were obtained from in situ X-ray powder diffraction experiments in-house and at HASYLAB at the Deutsches Elektronen Synchrotron (DESY) in Hamburg (Germany). The thermal expansion coefficients for gehlenite were found to be: α₁=7.2(4)×10⁻⁶×K⁻¹+3.6(7)×10⁻⁹ΔT×K⁻² and α₃=15.0(1)×10⁻⁶×K⁻¹. For TbCaAl[Al₂O₇] the respective values are: α₁=7.0(2)×10⁻⁶×K⁻¹+2.0(2)×10⁻⁹ΔT×K⁻² and α₃=8.5(2)×10⁻⁶×K⁻¹+2.0(3)×10⁻⁹ΔT×K⁻², and the thermal expansion coefficients for SmCaAl[Al₂O₇] are: α₁=6.9(2)×10⁻⁶×K⁻¹+1.7(2)×10⁻⁹ΔT×K⁻² and α₃=9.344(5)×10⁻⁶×K⁻¹. The expansion mechanisms of the three compounds are explained in terms of structural trends obtained from Rietveld refinements of the crystal structures of the compounds against the powder diffraction patterns. No structural phase transitions have been observed. While gehlenite behaves like a ‘proper’ layer structure, the aluminates show increased framework structure behavior. This is most probably explained by stronger coulombic interactions between the tetrahedral conformation and the layer-bridging cations due to the coupled substitution (Ca²⁺+Si⁴⁺)–(Ln ³⁺+Al³⁺) in the melilite-type structure. This article has been mistakenly published twice. The first and original version of it is available at .     

Viscoelastic behaviour of mica-based glass-ceramic aggregate

The present study deals with the small strain torsion deformation of MACOR glass-ceramic samples at high temperatures (450–850 °C) and over a range of low frequencies (20 Hz–5 mHz). The samples of MACOR ceramic consist of 55 vol% randomly oriented, sheet-like fluorophlogopite mica crystals (∼100–20 μm in planar size, 1–2 μm in thickness) and 45 vol% of isotropic alumino-borosilicate glass matrix. Measurements of the complex shear modulus show that the sample does not possess the relaxed shear viscosity even at temperatures above the glass transition temperature of the glass matrix. The maximum of the imaginary component G ^′′() of the shear modulus is ∼0.15 of the unrelaxed value G _∞, the relaxation strength Δ≈0.9. The activation energy of the peak of G ^′′() is ∼245 kJ mol⁻¹. Using this value of E _a , the data obtained at various frequencies and temperatures have been reduced to a master curve using the dimensionless variable ωτ, where ∼₀ exp(−E _a /RT). The internal friction Q⁻¹(ωτ) is ∝1/()^0.35−0.4 in the low-temperature high-frequency range (1); passes through a maximum at ∼1 and trends asymptotically to a value Q⁻¹∼0.25–0.30 at ≪1. The behaviour of Q ⁻¹(ωτ) differs from that of a Caputo body by the presence of the resolved peak which may be attributed to the slow mechanical relaxation of mica crystals due to rotation as well as flexing and bending modes of crystal deformation. Received: 26 June 1998 / Revised, accepted: 13 January 1999     

Source and tectonic implications of tonalite-trondhjemite magmatism in the Klamath Mountains

 In the Klamath Mountains, voluminous tonalite-trondhjemite magmatism was characteristic of a short period of time from about 144 to 136 Ma (Early Cretaceous). It occurred about 5 to 10 m.y. after the ∼165 to 159 Ma Josephine ophiolite was thrust beneath older parts of the province during the Nevadan orogeny (thrusting from ∼155 to 148 Ma). The magmatism also corresponds to a period of slow or no subduction. Most of the plutons crop out in the south-central Klamath Mountains in California, but one occurs in Oregon at the northern end of the province. Compositionally extended members of the suite consist of precursor gabbroic to dioritic rocks followed by later, more voluminous tonalitic and trondhjemitic intrusions. Most plutons consist almost entirely of tonalite and trondhjemite. Poorly-defined concentric zoning is common. Tonalitic rocks are typically of the low-Al type but trondhjemites are generally of the high-Al type, even those that occur in the same pluton as low-Al tonalite. The suite is characterized by low abundances of K₂O, Rb, Zr, and heavy rare earth elements. Sr contents are generally moderate (∼450 ppm) by comparison with Sr-rich arc lavas interpreted to be slab melts (up to 2000 ppm). Initial ⁸⁷Sr/⁸⁶Sr, δ ¹⁸O, and ɛ _Nd are typical of mantle-derived magmas or of crustally-derived magmas with a metabasic source. Compositional variation within plutons can be modeled by variable degrees of partial melting of a heterogeneous metabasaltic source (transitional mid-ocean ridge to island arc basalt), but not by fractional crystallyzation of a basaltic parent. Melting models require a residual assemblage of clinopyroxene+garnet±plagioclase±amphibole; residual plagioclase suggests a deep crustal origin rather than melting of a subducted slab. Such models are consistent with the metabasic part of the Josephine ophiolite as the source. Because the Josephine ophiolite was at low T during Nevadan thrusting, an external heat source was probably necessary to achieve significant degrees of melting; heat was probably extracted from mantle-derived basaltic melts, which were parental to the mafic precursors of the tonalite-trondhjemite suite. Thus, under appropriate tectonic and thermal conditions, heterogeneous mafic crustal rocks can melt to form both low- and high-Al tonalitic and trondhjemitic magmas; slab melting is not necessary. Received: 1 September 1994 / Accepted: 28 August 1995