Thermodynamic stability of various alkylated, dealkylated and rearranged 17α- and 17β-hopane isomers using molecular mechanics calculations

The thermodynamic stability of selected alkylated, dealkylated and rearranged 17α- and 17β-hopane isomers in the C₂₇, C₂₈, C₂₉, C₃₀ and C₃₁ families were calculated using molecular mechanics (MM2) methods and, where possible, calculated equilibrium ratios of certain isomers were compared with observed ratios of isomers in thermally mature crude oil samples. Those calculated and observed ratios having similar values include: (1) the relative distributions among 17β(H)/17α(H) and 21β(H)/21α(H)-hopanes including the absence of the 17β(H),21β(H)- and 17α(H),21α(H)-hopanes; (2) the 22R/22S ratios in 30-methyl-17α-hopane and 30-methyl-17β-moretane; (3) the relative distributions among 17α(H)/17β(H)- and 21α(H)/21β(H)-28,30-bisnorhopanes and among 25,28,30-trisnorhopanes, including the relatively greater stability of 17β(H) isomers in contrast to the regular hopane series; and (4) the ratios of 28(18−17S)abeo hopanes with respect to their unrearranged counterparts including the C₂₇ compounds, Ts/Tm.     

C-S-Fe relationships and the isotopic composition of pyrite in the New Albany Shale of the Illinois Basin,U.S.A.

The relationship between pyritic sulfur content (S_pyr) and organic carbon content (C_org) of shales analyzed from the New Albany Group depends upon C_org. For samples of <6 wt.% C_org, S_pyt, and C_org are strongly correlated (r = 0.85). For C_org-“rich” shales (>6 wt.%), no S_pty-C_org, correlation is apparent. The degree of Fe pyritization (DOP) shows similar relationships to C_org. These C-S-Fe relationships suggest that pyrite formation was limited by the availability of metabolizable organic carbon in samples where C_org < 6 wt.% and by the availability of reactive Fe for samples where C_org > 6 wt.%. Apparent sulfur isotope fractionations relative to contemporaneous seawater sulfate (Δ³⁴S) for pyrite formation average −40% for non-calcareous shales and −25%. for calcareous shales. Δ³⁴S values become smaller with increasing C_org, S_pyt, and DOP for all C_org-“poar” (<6 wt%) and some C_org-“nch” (<6 wt.%) shales. These trends suggest that pyrite formation occurred in a closed system or that instantaneous bacterial fractionation for sulfate reduction decreased in magnitude with increasing organic carbon content. The isotopic trends observed in the New Albany Group are not necessarily representative of other shales having a comparable range of organic carbon content, e.g. Cretaceous shales and mudstones from the western interior of North America (GAUTIER, 1986). Δ³⁴S values in the remainder of the C_org-rich New Albany Group shales are relatively large (−38 to −47%.) and independent of C_org, S_pyr, and DOP, which suggests that pyrite in these shales formed mostly at or above the sediment-water interface by precipitation from an isotopically uniform reservoir of dissolved H₂S.     

Late Quaternary Mediterranean sapropels. III: Assessment of source of input and palaeotemperature as derived from biological markers

Sources of input contributing to the organic matter of four different Mediterranean sapropels (S₁, S₅, S₆, S₇) are inferred from the relative distributions of terrigenous and marine biological markers in these Quaternary deposits. The relative terrigenous contribution does not vary significantly. Within the marine contribution there is a significant compositional variation. A contribution from dinoflagellates is relatively important in the S₁ sapropel, whereas sapropels S₆ and S₇ are characterized by a relatively large contribution from prymnesiophyte algae and planktonic cyanobacteria. The abundance of diatoms in sapropel S₅, as deduced from microscopic observations, is probably reflected by a high concentration of loliolide.Variations in sea-surface water temperatures can be deduced from the relative abundance of di- and triunsaturated C₃₇ ketones (U^k₃₇ index) and from the relative abundance of esterified 27-nor-24-methylcholesta-5,22E-dien-3β-ol and cholesta-5,22E-dien-3β-ol (S_TI index). These two molecular temperature indices are consistent with the δ¹⁸O record and with the pollen record of the sapropels investigated.     

Elastic constants of chrome-diopside: application of resonant ultrasound spectroscopy to monoclinic single-crystals

The right-rectangular parallelepiped resonance (RPR) form of resonant ultrasound spectroscopy (RUS) is applied to monoclinic crystal symmetry to determine the 13 adiabatic elastic constants, C_ij, of single-crystal chrome-diopside Di_0.93Hd_0.03Ur_0.02X_0.02 (Di, diopside; Hd, hedenbergite; Ur, ureyite; X, other or unknown). These data represent the first published values of the elastic tensor for a monoclinic single-crystal determined by the RPR method, thus demonstrating the feasibility of this method for studying elasticity of low-symmetry crystals. The lowest 62 modal frequencies of a gem-quality specimen were measured and identified at ambient temperature and pressure. Inverting the modal frequencies results in, respectively C₁₁, C₂₂, C₃₃, C₄₄, C₅₅, C₆₆, C₁₂, C₁₃, C₂₃, C₁₅, C₂₅, C₃₅, C₄₆ values (GPa) of 228.1(1.0), 181.1(0.6), 245.4(1.3), 78.9(0.3), 68.2(0.2), 78.1(0.2), 78.8(0.5), 70.2(0.7), 61.1(0.7), 7.9(0.5), 5.9(0.5), 39.7(0.4), and 6.4(0.2), where numbers in parentheses indicate experimental uncertainty. The corresponding isotropic bulk, K_S, and shear, G, moduli are 116.5 (0.9) and 72.8(0.4) GPa, respectively. When comparing these results with other pyroxene data, we find systematic correlations between M2 site composition and C_ij values for most, but not all, moduli. Unusual compositional dependence near end-member diopside is seen for a few moduli, and is propagated to the K_S and G moduli, which should prompt a reexamination of some C_ij values of end-member diopside.     

Elasticity of TiO2 rutile to 1800 K

The ambient pressure elastic properties of single-crystal TiO₂ rutile are reported from room temperature (RT) to 1800 K, extending by more than 1200 oK the maximum temperature for which rutile elasticity data are available. The magnitudes of the temperature derivatives decrease with increasing temperature for five of the six adiabatic elastic moduli (C _ij ). At RT, we find (units, GPa): C ₁₁=268(1); C ₃₃=484(2); C ₄₄=123.8(2); C ₆₆=190.2(5); C ₂₃=147(1); and C ₁₂=175(1). The temperature derivatives (units, GPa K⁻¹) at RT are: (∂C ₁₁/∂T) _P =−0.042(5); (∂C ₃₃/∂T) _P =−0.087(6); (∂C ₄₄/∂T) _P =−0.0187(2); (∂C ₆₆/∂T) _P =−0.067(2); (∂C ₂₃/∂T) _P =−0.025; and (∂C ₁₂/∂T) _P −0.048(5). The values for K _S (adiabatic bulk modulus) and μ (isotropic shear modulus) and their temperature derivatives are K _S =212(1) GPa; μ=113(1) GPa; (∂K _S /∂T) _P =−0.040(4) GPa K⁻¹; and (∂μ/∂T) _P =−0.018(1) GPa K⁻¹. We calculate several dimensionless parameters over a large temperature range using our new data. The unusually high values for the Anderson-Gròneisen parameters at room temperature decrease with increasing temperature. At high T, however, these parameters are still well above those for most other oxides. We also find that for TiO₂, anharmonicity, as evidenced by a non-zero value of [∂ln (K _T )/∂lnV] _T , is insignificant at high T, implying that for the TiO₂ analogue of stishovite, thermal pressure is independent of volume (or pressure). Systematic relations indicate that ∂² K _S /∂T∂P is as high as 7×10⁻⁴ K⁻¹ for rutile, whereas ∂²μ/∂T∂P is an order of magnitude less. Received: 19 September 1997 / Revised, accepted: 27 February 1998     

Direct observation of anti-phase domain structure in omphacite

Single crystal electron diffraction patterns of an omphacite (CaNa, Mg²⁺, Fe²⁺, Al³⁺, Fe³⁺)Si₂O₆ from a glaucophane schist facies (type C) eclogite, Tiburon Peninsula, California show weak reflections violating the C2/c space group. Transmission electron microscopy with dark field imaging, using h+k = even reflections reveal no special features, while dark field viewing through certain superstructure reflections violating the C2/c space group (h k l, with h+k = odd and h 0 l with h and/or l odd) reveal anti-phase domains, 100 to 3000 Å in width. Diffraction contrast experiments show that these domains are related by a displacement vector R=1/2[110]. This displacement vector explains the origin of the domains in terms of ordering of various cations in the octahedral (M1 type) and polyhedral (M2 type) sites without any changes in the tetrahedral silicate chains.     

The elastic constants of monoclinic single-crystal chrome-diopside to 1,300 K

Values of the complete adiabatic elastic tensor for single-crystal chrome-diopside (a monoclinic pyroxene mineral) are presented from 298 to 1,300 K. The data were obtained using resonant ultrasound spectroscopy (RUS). They are the first published results for the temperature T dependences of the 13 individual elastic constants C _ij of any clinopyroxene mineral. Each C _ij is appropriately described by a linear function in T throughout the range of T. Values for each (∂C _ij /∂T) _P in GPa K⁻¹ are as follows: C ₁₁, −0.0291; C ₂₂, −0.0248; C ₃₃, −0.0179; C ₄₄, −0.0103; C ₅₅, −0.0077; C ₆₆, −0.0152; C ₁₂, −0.0119; C ₁₃, −0.0064; C ₂₃, 0.0000; C ₁₅, 0.0025; C ₂₅, 0.0022; C ₃₅, −0.0046; and C ₄₆, 0.0026. Values of (∂M/∂T) _P in GPa K⁻¹, where M represents an isotropic bulk property calculated from the C _ij data, are as follows: adiabatic bulk modulus K _S , −0.0123; isothermal bulk modulus K _T , −0.0178; and shear modulus G, −0.00998. Some diopside derivatives, notably (∂K _S /∂T) _P , (∂K _T /∂T) _P , and (∂V _P /∂T) _P , where V _P is the compressional wave velocity, have smaller magnitudes than all other minerals of importance in Earth’s mantle, thus, confirming predictions from systematics studies. We find several dimensionless quantities for this monoclinic mineral have normal values compared to other mantle minerals. Further, αK _T (α is the volume coefficient of thermal expansion) for diopside is approximately independent of both T and volume V at elevated temperature, so its equation of state is accurately expressed in simplified form.     

Die Kristallstruktur des Finnemanits,Pb5Cl(AsO3)3, mit einem Vergleich zum Strukturtyp des Chlorapatits,Ca5Cl(PO4)3

Zusammenfassung Die Neubestimmung der Struktur des Finnemanits, Pb₅Cl(AsO₃)₃, (a ₀=10,322(7) Å,c ₀=7,055(6) Å RaumgruppeC _6h ² -P6₃/m) ergab nach einer Verfeinerung der Atomkoordinaten und der anisotropen Temperaturparameter von Pb, As und Cl bzw. der isotropen für die Sauerstoffe für 463 beobachtete Reflexe einen konventionellenR-Wert von 0,076. Die Struktur stellt ein aus Pb(1)–O und Pb(2)–O–Cl Polyeder sowie [AsO₃]-Pyramiden dreidimensional verknüpftes Gerüst dar. Dieses Gerüst steht in enger Beziehung zur Struktur des Chlorapatits.

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The crystal structure of finnemanite, Pb₅Cl(AsO₃)₃, with a comparison to the structure-type of chlorapatite, Ca₅Cl(PO₄)₃

Summary The redetermination of the crystal structure of finnemanite, Pb₅Cl(AsO₃)₃, (a ₀=10.322(7) Å,c ₀=7.055(6) Å; space groupC _6h ² -P6₃/m) converged for the refinement of the atomic coordinates, the anisotropic temperature parameters for Pb, As and Cl and isotropic for the O-atoms to a final conventionalR-value of 0.076 for 463 observed reflections. The crystal structure consists of a three-dimensional network built up of Pb(1)–O and Pb(2)–O–Cl polyhedra as well as [AsO₃]-pyramides. This network is closely related to the structure of chlorapatite.

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Mit 3 Abbildungen     

Volatile C1C8 organic compounds in sediments from the Peru upwelling region

Volatile C₁C₈ organic compounds were analyzed in seven cores recovered from a transect across the Peru Shelf Upwelling Region. Compounds detected in ng/g dry weight quantities included methane, ethane, propane and other C₄C₇ alkanes as well as functionalized compounds including alkenes, furans and aldehydes. Strong correlations were observed between some groups of compounds with similar structures. Maxima in two groups of compounds were observed: (1) C₁, C₂, C₃, furans, and butanals (plus sporadic alkanes) as nitrate levels decreased below about 2–4 μM in the subsurface, and (2) C₄C₆ alkenes together with predominantly branched and cycloalkanes at the sediment-water interface in cores recovered from oxygenated bottom waters. The levels of C₁C₇ alkanes in these sediments are comparable to levels found in DSDP cores buried to depths of less than 600 m (or not exposed to geothermal temperatures exceeding about 30°C).     

Magnetic properties and neutron diffraction study of two manganese sulfosalts: monoclinic MnSb<Subscript>2</Subscript>S<Subscript>4</Subscript> and benavidesite (MnPb<Subscript>4</Subscript>Sb<Subscript>6</Subscript>S<Subscript>14</Subscript>)

Mn²⁺Sb₂S₄, a monoclinic dimorph of clerite, and benavidesite (Mn²⁺Pb₄Sb₆S₁₄) show well-individualized single chains of manganese atoms in octahedral coordination. Their magnetic structures are presented and compared with those of iron derivatives, berthierite (Fe²⁺Sb₂S₄) and jamesonite (Fe²⁺Pb₄Sb₆S₁₄). Within chains, interactions are antiferromagnetic. Like berthierite, MnSb₂S₄ shows a spiral magnetic structure with an incommensurate 1D propagation vector [0, 0.369, 0], unchanged with temperature. In berthierite, the interactions between identical chains are antiferromagnetic, whereas in MnSb₂S₄ interactions between chains are ferromagnetic along c-axis. Below 6 K, jamesonite and benavidesite have commensurate magnetic structures with the same propagation vector [0.5, 0, 0]: jamesonite is a canted ferromagnet and iron magnetic moments are mainly oriented along the a-axis, whereas for benavidesite, no angle of canting is detected, and manganese magnetic moments are oriented along b-axis. Below 30 K, for both compounds, one-dimensional magnetic ordering or correlations are visible in the neutron diagrams and persist down to 1.4 K.     

Temperature variation of elastic constants of quartz across the α - β transition

The α − β transition of quartz was successfully observed with using a single sample by means of the rectangular parallelepiped resonance (RPR) method. An oriented rectangular parallelepiped of α-quartz single crystal was prepared and the resonant frequencies of 30–11 vibrational modes were measured from room temperature to 700°C. The softening of quartz crystal was observed as the significant reduction of resonant frequencies near the α–β transition. The present study is the first application of the RPR method to the study of phase transition. The complete set of elastic constants of α- and β-quartz were determined as a function of temperature by the least-squares inversion of the measured frequency data obtained by a single run. This is a merit yielded by the RPR method. It is shown near the α − β transition in both α- and β-quartz that the elastic parameters decrease proportionally to |T−T ₀|⁻ⁿ , where T is temperature and T ₀ is the transition temperature, 573.0°C for α-quartz and 574.3°C for β-quartz. It was also seen that linear incompressibilities K ₁ = (C ₁₁ +C ₁₂ +C ₁₃)/3 and K ₃ = (C ₃₃ +2C ₁₃)/3 decrease rapidly toward the transition, whereas, shear moduli C ₄₄, C _S1 = (C ₁₁ +C ₃₃ -2C ₁₃)/4 and C _S3 = (C ₁₁ -C ₁₂)/2 = C ₆₆ decrease only slightly. The shear modulus C _S3 = C ₆₆ increased slightly in α-quartz. The elastic properties of isotropic aggregate of quartz were calculated, and it is shown that the longitudinal wave velocity significantly decreases at the α − β transition, whereas, the shear wave velocity decreases only slightly.     

Elektronenstrahlmikroanalyse der graphit-troilitphase des cañon diablo-eisenmeteorits

Zusammenfassung Der vom American Meteorite Laboratory, Denver (Colorado), erworbene Eisenmeteorit Cañon Diablo mit drei sphärischen Graphit-Troiliteinschlüssen wurde von uns mit einer Elektronenstrahlmikrosonde JEOL JXA-3 A untersucht. Dabei wurde in den Graphit-Troiliteinschlüssen eine bisher noch nicht festgestellte Phase der Zusammensetzung (C_0,250,Ni_0,321,Fe_0,429)S_1,013 und (Co_0,144,Ni_0,391,Fe_0,465)S_1,016 aufgefunden. Diese Phase ist in ihrer Zusammensetzung von den bereits bekannten Mineralen Pentlandit (Fe,Ni)₉S₈ und Kobaltpentlandit (Co,Fe,Ni)₉S₈ deutlich verschieden. Sie bildet in der Troilit-Graphitmatrix Einsprenglinge von 3–6 m Durchmesser. Aus diesem Grunde war es bisher nicht möglich, ihre Kristallstruktur zu ermitteln.

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Electron-probe analysis of the graphite-troilite phase in the cañon diablo iron-meteorite

Summary The Cañon Diablo iron-meteorite, containing three spherical graphitetroilite inclusions has been investigated by means of an electronmicroprobe JEOL JXA-3 A. Within these graphite-troilite inclusions a phase, not known so far, has been noted, whose composition ranges from (Co_0.250,Ni_0.321,Fe_0.429)S_1.013 to (Co_0.144,Ni_0.391,Fe_0.465)S_1.016. This phase is decisively different from pentlandite (Fe,Ni)₉S₈ and Co-pentlandite (Co,Fe,Ni)₉S₈. It forms phenocrysts of 3–6 m in diameter. For this reason it was not possible so far to determine its crystal-structure.

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Herrn Prof. DDr.H. Wieseneder zum 65. Geburtstag gewidmet.     

Triclinic elastic constants for low albite

The full set of elastic constants for plagioclase end-member phase albite (NaAlSi₃O₈) is reported for the first time. Velocities of surface acoustic waves (both Rayleigh and pseudo-surface waves) were measured using impulsively stimulated light scattering on polished surfaces having six different orientations (three normal to the Cartesian axes and three lying on diagonals). Data were inverted and results tested using several non-linear optimization techniques. Compliance moduli determined under hydrostatic compression provided additional constraints and reduced covariance in the reported constants. The Cartesian coordinate system associated with the constants (using the unit cell) has the y-axis parallel to the crystal b axis, the x-axis parallel to a* (perpendicular to b and c) and the z-axis consistent with a right-handed coordinate system. The values of the moduli C₁₁, C₁₂, C₁₃, C₁₄, C₁₅, C₁₆, C₂₂, C₂₃, C₂₄, C₂₅, C₂₆, C₃₃, C₃₄, C₃₅, C₃₆, C₄₄, C₄₅, C₄₆, C₅₅, C₅₆, C₆₆ and their 2σ uncertainties (in parentheses) are, respectively, 69.1(0.6), 34.0(0.7), 30.8(0.5), 5.1(0.1), −2.4(0.1), −0.9(0.1), 183.5(2.7), 5.5(2.2), −3.9(0.5), −7.7(0.7), −5.8(0.7), 179.5(2.3), −8.7(0.4), 7.1(0.6), −9.8(0.6), 24.9(0.1), −2.4(0.1), −7.2(0.1), 26.8 (0.2), 0.5(0.1), 33.5(0.2). These constants differ significantly from the previously reported pseudo-monoclinic constants that were based on velocity measurements on polysynthetic twinned crystal aggregates. Differences are consistent with systematic errors in the earlier study associated with sparse data and the presence of cracks and other imperfections.     

Elastic properties of the incommensurate phase of akermanite,Ca2MgSi2O7

The crystal structure of akermanite, Ca₂Mg-Si₂O₇, consists of mixed tetrahedral sheets formed by [MgO₄] tetrahedra and [Si₂O₇] groups interleaved along the c axis with Ca²⁺ ions in eight-fold coordination. Above 358 K, the structure is tetragonal , and below it is incommensurate with modulations parallel to [110] and . The elastic stiffness moduli, C ^ij of the incommensurate phase at room temperature were measured from wave velocities in the 20–75 MHz carrier frequency range by the ultrasonic phase comparison method using optically clear synthetic single crystal plates (3×3×2 mm) oriented parallel to (100), (001), (110) and (101) planes. The C _ij values (GPa) are: C ₁₁ 159.40, C ₃₃ 149.43, C ₄₄ 30.26, C ₆₆ 58.10, C ₁₂ 76.58 and C ₁₃ 57.80. In (010) and (001) planes, the compressional modulus, V ²(L) from the longitudinal wave, L is considerably larger than the shear moduli, V ²(T₁, T ₂) both from the in-plane and perpendicular-to-plane shear waves, T ₁ and T ₂. The relatively small values of the shear moduli indicate the ease of tetrahedral rotations in response to in-plane and perpendicular-to-plane shears and may provide preconditions for structural changes involving shear-type atomic movements.     

Kyanoxalite,a new cancrinite-group mineral species with extraframework oxalate anion from the Lovozero alkaline pluton,Kola Peninsula

Kyanoxalite, a new member of the cancrinite group, has been identified in hydrothermally altered hyperalkaline rocks and pegmatites of the Lovozero alkaline pluton, Kola Peninsula, Russia. It was found at Mount Karnasurt (holotype) in association with nepheline, aegirine, sodalite, nosean, albite, lomonosovite, murmanite, fluorapatite, loparite, and natrolite and at Mt. Alluaiv. Kyanoxalite is transparent, ranging in color from bright light blue, greenish light blue and grayish light blue to colorless. The new mineral is brittle, with a perfect cleavage parallel to (100). Mohs hardness is 5–5.5. The measured and calculated densitiesare 2.30(1) and 2.327 g/cm³, respectively. Kyanoxalite is uniaxial, negative, ω = 1.794(1), ɛ = 1.491(1). It is pleochroic from colorless along E to light blue along O. The IR spectrum indicates the presence of oxalate anions C₂O₄²⁻ and water molecules in the absence of CO₃²⁻ Oxalate ions are confirmed by anion chromatography. The chemical composition (electron microprobe; water was determined by a modified Penfield method and carbon was determined by selective sorption from annealing products) is as follows, wt %: 19.70 Na₂O, 1.92 K₂O, 0.17 CaO, 27.41 Al₂O₃, 38.68 SiO₂, 0.64 P₂O₅, 1.05 SO₃, 3.23 C₂O₃, 8.42 H₂O; the total is 101.18. The empirical formula (Z = 1) is (Na_6.45K_0.41Ca_0.03)_Σ6.89(Si_6.53Al_5.46O₂₄)[(C₂O₄)_0.455(SO₄)_0.13(PO₄)_0.09(OH)_0.01]_Σ0.68 · 4.74H₂O. The idealized formula is Na₇(Al₅₋₆Si₆₋₇O₂₄)(C₂O₄)_0.5−1 · 5H₂O. Kyanoxalite is hexagonal, the space group is P6₃, a = 12.744(8), c = 5.213(6) -ray powder diffraction pattern are as follows, [d, [A] (I, %)(hkl)]: 6.39(44) (110), 4.73 (92) (101), 3.679 (72) (300), 3.264 (100) (211, 121), 2.760 (29) (400), 2.618 (36) (002), 2.216, (29) (302, 330). According to the X-ray single crystal study (R = 0.033), two independent C₂O₄ groups statistically occupy the sites on the axis 6₃. The new mineral is the first natural silicate with an additional organic anion and is the most hydrated member of the cancrinite group. Its name reflects the color (κɛανgoΣς is light blue in Greek) and the species-forming role of oxalate anions. The holotype is deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, registration no. 3735/1.     

Thermodynamic properties,low-temperature heat-capacity anomalies,and single-crystal X-ray refinement of hydronium jarosite, (H<Subscript>3</Subscript>O)Fe<Subscript>3</Subscript>(SO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>6</Subscript>

Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO₄ solutions. Single-crystal XRD refinement with R₁=0.0232 for the unique observed reflections (|F_o| > 4_F) and wR₂=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, V^o=160.11(4) cm³, and fractional positions for all atoms except the H in the H₃O groups. The chemical composition of this sample is described by the formula (H₃O)_0.91Fe_2.91(SO₄)₂[(OH)_5.64(H₂O)_0.18]. The enthalpy of formation (H^o_f) is –3694.5 ± 4.6 kJ mol^–1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, -FeOOH, MgO, H₂O, and -MgSO₄. The entropy at standard temperature and pressure (S^o) is 438.9±0.7 J mol^–1 K^–1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (C_p) data between 273 and 400 K were fitted to a Maier-Kelley polynomial C_p(T in K)=280.6 + 0.6149T–3199700T^–2. The Gibbs free energy of formation is –3162.2 ± 4.6 kJ mol^–1. Speciation and activity calculations for Fe(III)–SO₄ solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the C_p data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in C_p, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H₃O)Fe₃(SO₄)₂(OH)₆ was estimated: G^o_f= –3226.4 ± 4.6 kJ mol^–1, H^o_f=–3770.2 ± 4.6 kJ mol^–1, S^o=448.2 ± 0.7 J mol^–1 K^–1, C_p (T in K)=287.2 + 0.6281T–3286000T^–2 (between 273 and 400 K).     

Continental rifting as a function of lithosphere mantle strength

The role of the uppermost mantle strength in the pattern of lithosphere rifting is investigated using a thermo-mechanical finite-element code. In the lithosphere, the mantle/crust strength ratio (S_M/S_C) that decreases with increasing Moho temperature T_M allows two strength regimes to be defined: mantle dominated (S_M > S_C) and crust dominated (S_M < S_C). The transition between the two regimes corresponds to the disappearance of a high strength uppermost mantle for T_M > 700 °C. 2D numerical simulations for different values of S_M/S_C show how the uppermost mantle strength controls the style of continental rifting. A high strength mantle leads to strain localisation at lithosphere scale, with two main patterns of narrow rifting: “coupled crust–mantle” at the lowest T_M values and “deep crustal décollement” for increasing T_M values, typical of some continental rifts and non-volcanic passive margins. The absence of a high strength mantle leads to distributed deformations and wide rifting in the upper crust. These numerical results are compared and discussed in relation with series of classical rift examples.     

四川南桐地区二叠系龙潭煤层生物标记物及其地质意义

四川南桐地区二叠系龙潭煤层具有良好的生油潜力。其生物标记物包括正构和异构烷烃,类异戊二烯烷烃,倍半萜烷,二萜烷,三环萜烷,藿烷,一种未知结构的五环三萜烷(C₃₀),甾烷,4-甲基甾烷等。煤层生物标记物特征表明在其形成过程中有藻类和细菌等微生物物质的加入。     

Terpenoid biomarker distributions in low maturity crude oils

Twenty-seven heavy crude oils of diverse origin were geochemically assessed with respect to both bulk and mlecular composition for the purpose of identifying and quanttfying valid biomarker parameters for low maturity oils. The low thermal maturity level of many of these oils is evident from the bulk and alipathic chromatographic data, and oil sourced from both marine and terrigenous organic matter are represented. Selective metastable ion monitoring (SMIM) was employed to measure separately the distribution of C₂₇, C₂₈, and C₂₉ sterane isomers. The useful maturity indicators include the C₂₉ 5α(H) 20S/20R ratio, the relative quantity of the biological sterane configuration in each of the total normal C₂₇, C₂₈, and C₂₉ steranes, and the rearranged to normal sterane ratio. In addition, C₂₇ rearranged steran es appear to form at a faster rate than C₂₈ or C₂₉ rearranged steranes. However, the isomerization of the C₂₇ biological component appears to occur at a slower rate than the C₂₉ counterpart suggesting that the former may be used as a maturity parameter at higher levels of thermal maturation. In the triterpane distributions, the C₂₇ trisnorhopane isomers and the moretane to hopane ratios appear to be both source and maturity related and cannot be used as successful maturity parameters in oils unless they share a common source. The C₃₁₊ hopane 22S/22R equilibrium ratio appears to increase with increasing molecular weight (C₃₁–C₃₄).     

Displacive phase transitions in C-centred clinopyroxenes: spodumene, LiScSi2O6 and ZnSiO3

The clinopyroxenes spodumene (LiAlSi₂O₆), LiScSi₂O₆ and ZnSiO₃, all with space group C2/c at ambient conditions, were studied under high pressures by single-crystal X-ray diffraction in a diamond-anvil cell. Changes in the evolution of the unit-cell parameters, optical properties and the appearance of h + k odd reflections characteristic of a primitive lattice, indicate that all three pyroxenes undergo phase transitions. The transitions are mostly displacive in character, and are non-quenchable. Transition pressures are 3.19 GPa in spodumene, ∼0.6 GPa in LiScSi₂O₆ and 1.92 GPa in ZnSiO₃. The space group of all three high-pressure phases was determined to be P2₁/c by structure refinement to single-crystal X-ray intensity data collected in the DAC. In the ZnSiO₃ clinopyroxene the intermediate P2₁/c phase further transforms to a second C2/c phase (HP-C2/c) at 4.9 GPa (confirmed by structure refinement). The volume change at this transition is about 2.6%, three times larger than in the first phase transition, and typical of the P2₁/c→ HP-C2/c phase transitions found previously in MgSiO₃, FeSiO₃, etc. These results therefore provide the first direct evidence that the HP-C2/c and the HT-C2/c structures of pyroxenes are distinct polymorphs with the same space group. The phase transition from C2/c to P2₁/c symmetry in spodumene and LiScSi₂O₆ therefore occurs because the polymorphs stable at ambient conditions are isotypic to the high-temperature C2/c phases of clinopyroxenes such as pigeonite and clinoenstatite. Received: 22 December 1999 / Accepted: 7 June 2000