Calibration of Al/Si order variations in anorthite

New single crystal diffraction data for natural and heat-treated anorthite crystals (Angel et al. 1990) allow the determination of their states of Al/Si order in terms of a macroscopic order parameter,Q _OD , for the transition. Numerical values ofQ _OD obtained from estimates of site occupancies are shown to vary with the scalar spontaneous strain, _s , as _s Q _OD ² , and with the ratio of the sums of typeb (superlattice) reflections and typea (sublattice) reflections asI _b/I _a Q _OD ² . An empirical calibration for pure anorthite is obtained giving varies between 0.92 and 0.87 in samples equilibrated at T1300° C, but then falls off relatively rapidly with increasing temperature, reaching 0.7 near the melting point ( 1557° C). The observed temperature dependence does not conform to the predictions of the simplest single order parameter models; coupling ofQ _OD withQ of the transition is suspeeted.     

β(Mg0.9, Fe0.1)2SiO4: Single crystal structure,cation distribution,and properties of coordination polyhedra

The synthesis boundaries of the phase transformation; ++ in (Mg_0.9, Fe_0.1)SiO₄, have been clarified at temperatures to 2000° C and pressures up to 20 GPa in order to synthesize single crystals of high quality. A single crystal of (Mg_0.9, Fe_0.1)₂SiO₄ was grown successfully to a size of 500 m. The crystal structure has been refined from single-crystal X-ray intensities. The ferrous ions prefer M1 and M3 sites to over the larger M2 site. The volume change of all the occupied polyhedra does not contribute to the decrease of total volume in the transformation; rather it tends to increase the bulk volume through the expansion of occupied tetrahedra. The volume reduction in the phase transformations is accounted for by unoccupied polyhedra, with the octahedra contributory 60% and the tetrahedra 40% to the V of the transition. The volume change in the transformation is caused also partly by the volume decrease of MO ₆ (25%), partly the unoccupied tetrahedra (45%) and octahedra (30%).     

Thermal expansion,heat capacity,and entropy of MgO at mantle pressures

The pressure dependence of the Raman spectrum of forsterite was measured over its entire frequency range to over 200 kbar. The shifts of the Raman modes were used to calculate the pressure dependence of the heat capacity, C _v, and entropy, S, by using statistical thermodynamics of the lattice vibrations. Using the pressure dependence of C _v and other previously measured thermodynamic parameters, the thermal expansion coefficient, , at room temperature was calculated from = K _S (T/P) _S C _V/TVK _T, which yields a constant value of ( ln / ln V)_T= 6.1(5) for forsterite to 10% compression. This value is in agreement with ( ln / ln V)_T for a large variety of materials.At 91 kbar, the compression mechanism of the forsterite lattice abruptly changes causing a strong decrease of the pressure derivative of 6 Raman modes accompanied by large reductions in the intensities of all of the modes. This observation is in agreement with single crystal x-ray diffraction studies to 150 kbar and is interpreted as a second order phase transition.     

Diamagnetic anisotropies of oxide minerals

The diamagnetic anisotropy of oxide minerals is analyzed in terms of a new model, in which the anisotropy is assigned to the individual chemical bond in the [MO₆] octahedral unit of the crystal. The diamagnetic principal axis of the individual M-O bond is assumed to be parallel to the direction of the bond. The calculated anisotropy based on this model shows a good correlation with the measured diamagnetic anisotropy, , for various minerals such as talc, sericite, kaolinite of the sheetsilicate group, forsterite of the orthosilicate group, and corundum of the hematite group. The values of many diamagnetic minerals are still unknown since the measurement is difficult to perform by means of conventional methods. The magnetic grain orientation recently observed in the mineral suspensions is effective for estimating the value, when the single crystal of the mineral cannot be obtained. The observation of fieldinduced crystal oscillation in the high magnetic fields can be applied for measuring the minerals with small values of less than 5 × 10^–10 emu/cc. The chemical bond model on the diamagnetic anisotropy can be confirmed, when the compiling of data on various mineral is made by means of the above two methods.     

31P solid-state nuclear magnetic resonance spectroscopy of aluminum phosphate minerals

This study examines the links between ³¹P solidstate NMR studies of aluminum phosphate minerals and their crystallographic structures. We found that ³¹P isotropic chemical shift values, _iso, carry little information about mineral structures. There seems to be no relation between the chemical shift anisotropy, =₃₃–₁₁ (₃₃>₂₂> ₁₁), and indicies of phosphate-tetrahedra distortion. ³¹P¹H heteronuclear magnetic dipole interactions, on the other hand, carry important information about hydrous phosphate mineral structures, information that should prove to be quite valuable in studies of phosphate adsorbed on mineral surfaces. This interaction can be measured through a variety of qualitative and quantitative experiments. It appears that spin diffusion is so rapid that subtle differences in hydrogen-bonding environments cannot be resolved.     

Titania precipitation in sapphire containing iron and titanium

Titania, TiO₂, precipitation in natural blue sapphire (Fe, Ti: -Al₂O₃) has been investigated using high resolution and analytical transmission electron microscopy. The structure and habit of the TiO₂ precipitate depends on both the Ti⁴⁺ concentration and the temperature at which the precipitate formed. Tetragonal TiO₂ (Rutile) grows at 1350° C but at 1150° C an orthorhombic non-equilibrium TiO₂ polymorph precipitates. Both TiO₂ polymorphs nucleate in the (0001)s plane as lens shaped discs twinned along their diameter. The crystallographic alignment of each type of TiO₂ precipitate with respect to the -Al₂O₃ host matrix provides a high degree of structural coherency with minimal lattice mismatch. Electron diffraction analysis established the following precipitate/host orientation relationships: tetragonal TiO₂: {011}r {11 07B;100}r(0001)s and 01 r10 0s twinned along the (011)r planeand orthorhombic TiO₂: {021}{11 0}s, {100}(0001)s and 0 2 10 0s twinned along the (021) plane.     

Phase relations in the system NaAlSi3O8-KAlSi3O8-CaAl2Si2O8-SiO2 at 1 kilobar water vapour pressure

Crystal-melt relations at a water vapour pressure of 1 kilobar have been determined for planes at 3, 5, 7.5, and 10 weight per cent anorthite in the system NaAlSi₃O₈KAlSi₃O₈-CaAl₂Si₂O₈-SiO₂. The ratio of the silicate components in the liquids which are in univariant equilibrium with plagioclase, alkali feldspar, quartz and gas are Ab₃₁Or₂₈Q₃₈An₃ (weight per cent) at 730°±5–10° C, Ab₂₁Or₃₄Q₄₀An₅ at 745°±5–10° C and Ab₁₀Or₃₉ Q_43.5An_7.5 at 780°±10° C. The univariant curve on which the above compositions lieoriginates on the H₂O-saturated Or-An-Q plane at a composition containing less than 10 weight per cent An and terminates within 1.5 weight per cent An of the H₂O-saturated Or-Ab-Q plane. Experimental data for the synthetic system have been used to illustrate a discussion on the partial melting of metasediments and the possible significance of such a process with respect to the genesis of granitic rocks. Data taken from the literature (Winkler and v. Platen, 1960, 1961a) have been used to illustrate that the normative salic composition of a sediment has a strong influence on the composition of any melt which form when such a rock is subjected to high temperatures and pressures.     

The α-β phase transition in cristobalite,SiO2

Cristobalite, a high temperature phase of silica, SiO₂, undergoes a (metastable) first-order phase transition from a cubic, , to a tetragonal, P4₃2₁2 (or P4₁2₁2), structure at around 220° C. The cubic C9-type structure for -cristobalite (Wyckoff 1925) is improbable because of two stereochemically unfavorable features: a 180° Si-O-Si angle and an Si-O bond length of 1.54 Å, whereas the corresponding values in tetragonal -cristobalite are 146° and 1.609 Å respectively. The structure of the -phase is still controversial. To resolve this problem, a symmetry analysis of the (or P4₁2₁2) transition in cristobalite has been carried out based on the Landau formalism and projection operator methods. The starting point is the ideal cubic ( ) C9-type structure with the unit cell dimension a (7.432 Å) slightly larger than the known a dimension (7.195 Å at 205° C) of -cristobalite, such that the Si-O-Si angle is still 180°, but the Si-O bond length is 1.609 Å. The six-component order parameter driving the phase transition transforms according to the X₄ representation. The transition mechanism essentially involves a simultaneous translation and rotation of the silicate tetrahedra coupled along 110. A Landau free-energy expression is given as well as a listing of the three types of domains expected in -cristobalite from the transition. These domains are: (i) transformation twins from a loss of 3-fold axes, (ii) enantiomorphous twins from a loss of the inversion center, and (iii) antiphase domains from a loss of translation vectors 1/2 110 (FP). These domains are macroscopic and static in -cristobalite, and microscopic and dynamic in -cristobalite. The order parameter , couples with the strain components as ², which initiates the structural fluctuations, thereby causing the domain configurations to dynamically interchange in the -phase. Hence, the - cristobalite transition is a fluctuation-induced first-order transition and the -phase is a dynamic average of -type domains.     

Chemistry of some Pb-(Cu,Fe)-(Sb,Bi sulfosalts from France and Portugal. Implications for the crystal chemistry of lead sulfosalts in the Cu-poor part of the Pb2S2-Cu2S-Sb2S3-Bi2-Bi2S3 system

Electron microprobe analysis of Pb-Cu(Fe)-Sb-Bi sulfosalts from Bazoges and Les Chalanches (France), and Pedra Luz (Portugal), give new data about (Bi, Sb) solid-solution and incorporation of the minor elements Cu, Fe or Ag in jaskolskiite, and in izoklakeite-giessenite and kobellite-tintinaite series. Jaskolskiite from Pedra Luz has high Sb contents (from 17.9 to 20.7 wt.%), leading to the extended general formula: Cu _x Pb_2+x (Sb_1–y Bi _y )_2–x S₅, with 0.10 x 0.22 and 0.19 y 0.41. Fe-free, Bi-rich izoklakeite from Bazoges has high Ag contents (up to 2.2 wt. %), leading to the simplified formula Cu₂Pb₂₂Ag₂(Bi, Sb)₂₂S₅₇; in Les Chalanches it contains less Ag content (1.2 wt.%), but has an excess of Cu that gives the formula: Cu_2.00 (Cu_0.49Ag_1.18)_=1.67Pb_22.70(Bi_12.63Sb_8.99)_=21.62S_57.27.In tintinaite from Pedra Luz, the variation of the Fe/Cu ratio can be explained by the substitution: Cu + (Bi, Sb) Fe + Pb; Fe-free kobellite from Les Chalanches has a Cu-excess, corresponding to the formula Cu_2.81Ag_0.54Pb_9.88(Bi_10.37Sb_5.21)_=15.38S_35.09. Eclarite from the type locality, structurally related to kobellite, shows a Cu excess too. In natural samples of the kobellite homologous series, Fe is positively correlated with Pb, and its contents never exceed that of Cu. Ag substitutes for Pb, together with (Bi, Sb). Taking into account the possibility of Cu excess, but excluding formal Cu²⁺ and Fe³⁺, general formulae can be written:     

Der Strukturtyp von Emmonsit, {Fe2[TeO3]3·H2O}·xH2O (x=0−1)

Zusammenfassung Emmonsit kristallisiert triklin, RaumgruppeP , Gitterkonstanten:a ₀=7,90 Å,b ₀=8,00 Å,c ₀=7,62 Å, =96^o44, =95^o 0, =84^o 28,Z=2. Der Strukturtyp wurde aus 3-dimensionalen photographischen Röntgendaten ermittelt. Die Eisenatome werden je von 6 Sauerstoffen verzerrt oktaedrisch koordiniert. Jedes Telluratom wird von 3 Sauerstoffen in einem Abstand <2,0 Å umgeben. Ein vierter Sauerstoff hat bezüglich dieser drei einen um etwa 25–35% größeren Abstand, so daß jedes Telluratom im weiteren Sinne eine (3+1)-Koordination aufweist.

---

The structure type of emmonsite, {Fe₂[TeO₃]₃·H₂O}·xxH₂O (x=0–1)

Summary Emmonsite is triclinic with space groupP , and lattice constantsa ₀=7.90 Å,b ₀=8.00 Å,c ₀=7.62 Å, =96^o 44, =95^o 0, =84⁰ 28,Z=2. The structure type is derived from 3-dimensional photographic X-ray data. The iron atoms are coordinated by six oxygens in the form of a distorted octahedron. Each tellurium atom is coordinated to 3 oxygens at a distance <2.0 Å. Compared with these 3 Te–O distance the distance of a fourth oxygen is only 25 to 35% greater; therefore each tellurium atom has a (3+1)-coordination of oxygens.

---

---

Mit 2 Abbildungen     

Oxygen isotope geochemistry of hydrothermally-altered synmetamorphic granitic rocks from South-Central Maine,USA

Hydrothermally-altered mesozonal synmetamorphic granitic rocks from Maine have whole-rock ¹⁸O (SMOW) values 10.7 to 13.8. Constituent quartz, feldspar, and muscovite have ¹⁸O in the range 12.4 to 15.2, 10.0 to 13.2, and 11.1 to 12.0, respectively. Mean values of _Q–F ( ¹⁸O_quartz– ¹⁸O_feldspar)=2.4 and _Q–M ( ¹⁸O_quartz– ¹⁸O_muscovite)=3.3 are remarkably uniform (standard deviations of both are 0.2). Measured _Q–F and _Q–M values demonstrate that the isotopic compositions of the minerals are altered from primary magmatic ¹⁸O values but that the minerals closely approached oxygen isotope exchange equilibrium at subsolidus temperatures. Analyzed muscovites have D (SMOW) values in the range –65 to –82.Feldspars in the granitic rocks are mineralogically altered to either (a) muscovite+calcite, (b) muscovite+calcite+epidote, (c) muscovite+epidote, or (d) muscovite only. A consistent relation exists between the assemblage of secondary minerals and the oxygen isotope composition of whole rocks, quartz, and feldspar. Rocks with assemblage (a) have whole-rock ¹⁸O>12.1 and contain quartz and feldspar with ¹⁸O>13.8 and >11.4, respectively. Rocks with assemblages (b), (c), and (d) have whole-rock ¹⁸O<11.4 and contain quartz and feldspar with ¹⁸O< 13.1 and <11.0, respectively. The correlation suggests that the mineralogical alteration of the rocks was closely coupled to their isotopic alteration.Three mineral thermometers in altered granite suggest that the hydrothermal event occurred in the temperature range 400°–150° C, 100°–150° C below the peak metamorphic temperature inferred for country rocks immediately adjacent to the plutons. Calculations of mineral-fluid equilibria indicate that samples with assemblage (a) coexisted during the event with CO₂-H₂O fluids of and ¹⁸O=10.8 to 12.2 while samples with assemblages (b), (c), or (d) coexisted with fluids of and ¹⁸O=9.4 to 10.1. Compositional variations of the hydrothermal fluids were highly correlated: fluids enriched in CO₂ were also enriched in ¹⁸O. Because CO₂ was added to the granites during hydrothermal alteration and because fluids enriched in CO₂ were enriched in ¹⁸O, some or all of the variation in ¹⁸O of altered granites may have been caused by addition of ¹⁸O to the rocks during the hydrothermal event. The source of both the CO₂ and ¹⁸O could have been high-¹⁸O metasedimentary country rocks. The inferred change in isotopic composition of the granites is consistent with depletion of the metacarbonate rocks in ¹⁸O close to the plutons and with large volumes of fluid that were inferred from petrologic data to have infiltrated the metacarbonate rocks during metamorphism.A close approach of minerals to oxygen isotope exchange equilibrium in altered mesozonal rocks from Maine is in marked contrast to hydrothermally-altered epizonal granites whose mineral commonly show large departures from oxygen isotope exchange equilibrium. The difference in oxygen isotope systematics between altered epizonal granites and altered mesozonal granites closely parallels a differences between their mineralogical systematics. Both differences demonstrate the important control that depth exerts on the products of hydrothermal alteration. Deeper hydrothermal events occur at higher temperature and are longer-lived. Minerals and fluid have sufficient time to closely approach both isotope exchange and heterogeneous chemical equilibrium. Shallower hydrothermal events occur at lower temperatures and are shorter-lived. Generally there is insufficient time for fluid to closely approach equilibrium with all minerals.     

Mechanisms of the transformations between the α, β and γ polymorphs of Mg2SiO4 at 15 GPa

Data on the mechanisms of mantle phase transformations have come primarily from studies of analogue systems reacted experimentally at low pressures. In order to study transformation mechanisms in Mg₂SiO₄ at mantle pressures, forsterite () has been reacted in the stability field of -phase, at 15 GPa and temperatures up to 900° C, using a multianvil split-sphere apparatus. Transmission electron microscope studies of samples reacted for times ranging from 0.25–5.0 h show that forsterite transforms to -phase by an incoherent nucleation and growth mechanism involving nucleation on olivine grain boundaries. This mechanism and the resultant microstructures are very similar to those observed at much lower pressures in analogue systems (Mg₂GeO₄ and Ni₂SiO₄) as the result of the olivine to spinel () transformation. Metastable spinel () also forms from Mg₂SiO₄ olivine at 15 GPa, in addition to -phase, by the incoherent nucleation and growth mechanism. With time, the spinel progressively transforms to the stable -phase. After 1 h, spinels exhibit a highly striated microstructure along {110} and electron diffraction patterns show streaking parallel to [110] which indicates a high degree of structural disorder. High resolution imaging shows that the streaking results from thin lamellae of -phase intergrown with the spinel. The two phases have the orientation relationship [001]//[001] and [010]//[110] so that the quasi cubic-close-packed oxygen sublattices are continuous between both phases. These microstructures are similar to those observed in shocked meteorites and show that spinel transforms to -phase by a martensitic (shear) mechanism. There is also evidence that the mechanism changes to one involving diffusion-controlled growth at conditions close to equilibrium.     

A dynamical model for the P1 — I1 phase transition in anorthite,CaAl2Si2O8

Electron diffraction and electron microscopic evidence is presented for a dynamical and reversible phase transition in anorthite at T _c=516 K. Antiphase boundaries with a displacement vector, R=1/2[111] become unstable at T _c, while other antiphase boundary loops with the same displacement vector are formed. These interfaces are very mobile and vibrate with a frequency which increases strongly with temperature. At temperatures considerably above T _c, a shimmering effect is observed on imaging in dark field using diffuse c reflections. These observations are in agreement with the interpretation of the high temperature body-centered phase as a statistical dynamical average of very small c type antiphase domains of primitive anorthite. We propose that the c type antiphase domains in primitive anorthite originate from ordered and anti-ordered configurations around Ca²⁺ ions at (ooo) and (oio) [likewise (zoo) and (zio)] positions. The dynamical model for the transition involves a two-stage mechanism: a softmode mechanism causing the aluminosilicate framework to approach body-centered symmetry, followed by an orderdisorder of the Ca²⁺ ion configurations. Close to T _c, statistical fluctuations set in and breathing motion type lattice vibrations of the aluminosilicate framework cause the configurations around Ca (ooo) and Ca(oio) [likewise Ca(zoo) and Ca(zio)] in the configuration to dynamically interchange through an intermediate configuration. The dynamical nature of the phase transition in anorthite is comparable to the — phase transition in quartz.     

Pressure-volume-temperature behavior of γ-Fe2SiO4 (spinel) based on static compression measurements at 400° C

Thirteen energy-dispersive x-ray diffraction spectra for -Fe₂SiO₄ (spinel) collected in situ at 400° C and pressures to 24 GPa constitute the basis for an elevated-temperature static compression isotherm for this important high-pressure phase. A Murnaghan regression of these molar volume measurements yields 177.3 (±17.4) GPa and 5.4(±2.5) for the 400° C, room pressure values of the isothermal bulk modulus (K _{P 0}) and its first pressure derivative (K _{P 0}), respectively. When compared to the room-Tdeterminations of K _{P 0} available in the literature, our 400° C K _{P 0} yields -4.1 (±6.2)×10^-2 GPa/degree for the average value of (K/T) _{P 0} over the temperature interval 25° C<><400°>A five-parameter V(P, T) equation for -Fe₂SiO₄ based on simultaneous regression of our data combined with the elevated P-Tdata of Yagi et al. (1987) and the extrapolated thermal expansion values from Suzuki et al. (1979) yields isochores which have very little curvature [(² T/P ²) _v 0], in marked contrast to the isochores for fayalite (Plymate and Stout 1990) which exhibit pronounced negative curvature [(T/P ²) _v <0]. along=" the=">-Fe₂SiO₄ reaction boundary V_Rvaries from a minimum of approximately 8.3% at approximately 450° C to approximately 8.9% at 1200° C. Extrapolation of the fayalite and -Fe₂SiO₄ V(P, T) relationships to the temperature and pressure of the 400 km discontinuity suggests a V _R of approximately 8.4% at that depth, approximately 10% less than the 9.3% V _R at ambient conditions.     

Stress reorientation during folding

Intermediate principal stress, ₂, is, for mechanical reasons, taken to be parallel to the statistical direction of fold axes and traces of thrust faults during evolution of fold and thrust belts. Regionally, maximum principal stress, ₁, and least, ₃, may be taken to be the directions of maximum shortening and maximum thickening of the section, respectively. Where crystalline basement is not involved in the deformation, maximum shortening is manifestly parallel to the top of the basement, or subhorizontal, and ₃ is, therefore, subvertical. While this stress system is grossly adequate on the scale of the fold and thrust belt, it fails locally, particularly in late stages of deformation. Sinuosity develops on all scales within the belt as deformation progresses. Individual fold axes tend to be straight lines in incipient stages of folding, as shown by unrolling folds, but commonly develop with increasing curvature during deformation. The curvature resulting during deformation is a measure of extension parallel to the axial direction, if the ends of the fold are fixed points. Thus, ₂ progressively decreases. With marked sinuosity, stress parellel to the axial direction can be reduced at a given depth below the magnitude of the weight of the overburden, originally ₃. Intermediate and least principal stresses switch position, and strike-slip faulting is favored where the deformational response is failure by shear fracture. The percent axial extension can be expressed in an equation that compares the final arcuate length with the original length. With a knowledge of the physical properties of the rock, the time in the evolution of the structure at which he stresses switch can be predicted, as well as the subsequent structural response.

---

Zusammenfassung Die Richtung des intermediären Hauptstresses, ₂, wird aus mechanischen Gründen als parallel zu der statistischen Richtung der Faltenachsen und der Spuren der Überschiebungsflächen angenommen während der Entwicklung von Faltungs- und Überschiebungszonen. Regional können die Richtungen des maximalen Haupstresses, ₁, und des minimalen Hauptstresses, ₃, als Richtungen der größten Verkürzung, respektive der größten Verdickung des Querschnittes betrachtet werden. Wo der kristalline Untergrund nicht in den Deformationsvorgang einbezogen wird, ist die Richtung der maximalen Verkürzung offenbar parallel zur Kristallinoberfläche oder subhorizontal und ₃ somit subvertikal orientiert. Währenddessen im großen Maßstab diese Zuordnung der Hauptstreßrichtungen zu einer ganzen Faltungs- und Überschiebungszone vorgenommen werden kann, versagt sie im lokalen Bereich, vor allem in späten Phasen der Deformation. Bei fortschreitender Deformation entwickeln sich im Deformationsgürtel Bogenformen in verschiedenem Maßstab. Individuelle Faltenachsen neigen dazu, sich an geraden Linien auszubilden in frühen Stadien der Faltung, wie dies die Abwicklung von Falten zeigt. Sie entwickeln sich aber im allgemeinen während der weiteren Deformation mit zunehmend gebogener Achsenrichtung. Die resultierende Kurvatur ist ein Maß der Dehnung parallel zur Achsenrichtung, wenn die seitlichen Endpunkte der Falte Fixpunkte darstellen. In dieser Weise nimmt der Betrag von ₂ fortschreitend ab. Bei ausgeprägter Bogenform kann der Streß parallel zur Richtung der Faltenachse in einer bestimmten Tiefe reduziert werden bis zu einem Betrag, der unterhalb des Ausmaßes der Überlast, also ursprünglich ₃, liegt. Die Richtungen des intermediären und des kleinsten Hauptstresses wechseln ihre Positionen, und Blattverschiebungen werden begünstigt, wo die Deformation Scherbrüche erzeugt. Das Ausmaß der axialen Dehnung kann durch eine Gleichung ausgedrückt werden, welche die Länge des endgültigen Faltenbogens mit der ursprünglichen Länge der Falte verknüpft. Mit der Kenntnis der physikalischen Eigenschaften des Gesteins können sowohl der Zeitpunkt in der Entwicklung der Faltenstruktur, zu welchem die Hauptstreßrichtungen ihre Positionen wechseln, als auch die nachfolgende strukturelle Entwicklung bestimmt werden.

---

Résumé La contrainte principale intermédiaire, ₂, est, pour des raisons mécaniques, considérée comme étant parallèle à la direction statistique des axes du pli et des traces de chevauchement durant l'évolution du pli et des zones de chevauchement. Régionalement, on peut supposer que les contraintes principales maximum, ₁, et minimum, ₃, suivent respectivement les directions du raccourcissement maximum et de l'épaississement maximum du profil. Là, où le soubassement cristallin n'est pas entrainé dans la déformation, le raccourcissement maximum est manifestement parallèle à la surface du soubassement, ou subhorizontal, et ₃ est, par conséquent, subverticale. Tant que l'échelle de ce système de contraintes correspond à peu près à celle du pli et de la zone de chevauchement, il change de direction localement, spécialement dans les derniers stades de la déformation. Une sinuosité se développe à toutes les échelles à l'intérieur de la zone, tandis que la déformation progresse. Les axes individuels du pli ont tendance à devenir des lignes droites dans les stades embryonnaires de la déformation, comme le montre le déroulement des plis, mais ordinairement ils se développent avec une courbure croissante pendant la déformation. La courbure qui en résulte durant la déformation est une mesure de l'extension parallèle à la direction axiale. Ainsi ₂ décroit progressivement. Avec une sinuosité prononcée, la contrainte parallèle à la direction axiale peut se réduire, à une profordeur donné au dessous du poids de la surcharge, originellement ₃. Les contraintes principales maximum et minimum échangent leur position, et la composante horizontale du rejet de la faille est alors favorisée là où la réaction à la déformation devient négligeable par suite de fracture de cisaillement. Le pourcentage de l'extension axiale peut s'exprimer par une équation qui compare la longueur de la courbe finale à la longueur originelle. Avec la connaissance des propriétés physiques des roches, on peut prévoir, le moment où les contraintes s'échangent durant l'évolution de la structure, ainsi que la réaction structurale qui en résulte.

---

, , ₂ . _{1 3}, . , , , , . . ₃ . , , , , , . . , . , , , . .. ₂ , , , , . . ₃. , , , . , . , , , , .

     

Compression of α-MnS (alabandite) and a new high-pressure phase

The compressibility of -Mns (alabandite) was determined by x-ray analysis using a Mao-Bell type diamond anvil cell. The zero pressure bulk modulus (K₀) is 74±2 GPa with the pressure derivative of the bulk modulus (K_o) fixed at four. Allowing (K_o) to vary yielded a statistically better fit with K₀ = 88±6 GPa and k₀ = 2.2±0.6. Our data combined with the data of McCammon (1991) gave K_o = 73±1 GPa with k_o fixed at four. A fit with k_o allowed to vary yielded k_o = 75±2 GPa and k_o = 3.7±0.4. Alabandite transformed from the B1 structure (NaCl-type) to an unknown high-pressure phase at 26 GPa. The high-pressure phase has lower than hexagonal symmetry and it is stable to at least 46±4 GPa.Also affiliated with the James Franck Institute, University of Chicago     

A materials failure relation of accelerating creep as empirical description of damage accumulation

A general materials failure relation, , describes accelerating creep of materials with rate coefficients andA, by relating rates of deformation, , to changes in deformation rate, (Voight, 1988). Time of failure can be extrapolated from inverse rate versus time data, and andA may be derived to permit one to calculate the failure time. The method is of value for quantitative hazard assessments.Mechanisms leading to damage accumulation during accelerating creep include creep fracture by stress corrosion and power law lattice deformation. These mechanisms are examined here as phenomenologically related to the materials failure relation. Apparently, both mechanisms favour , where is the parameter of the materials failure relation controlling the sensitivity to accelerating activity. For pure shear governed by power law creep of powerp, under constant load, =2.0 andA=p. Stress corrosion is widely described by Charles' equation, relating crack velocity to stress intensity during subcritical crack growth by the stress corrosion indexn. The relationship betweenn and is given by =(2n–2)/n.     

Recent advances in organic geochemistry

The study of the nature, distribution and interchange of organic matter in the geosphere and biosphere, and — in the narrower view — in sediments and fossils, is developing rapidly. One approach lies in the chromatographic isolation of individual components, followed either by their conclusive identification through direct comparison with known substances or by their structural elucidation using physical and chemical procedures.In favourable circumstances, single substances may be identified at the microgram level (10^–6gm.). Compounds of obvious biological origin are frequently present in geological materials and may be correlated with the original biological source. Thus, the hydrocarbons phytane (C₂₀H₄₂) and pristane (C₁₉H₄₀) found in crude oils and in sediments as old as 2 · 7 × 10⁹ years almost certainely derive from the phytyl (C₂₀H₃₉O-) side chain of chlorophyll.

---

Zusammenfassung Das Studium der Natur, der Verteilung und des Austausches des organischen Stoffes in der Geosphere und Biosphere, und — im engeren Sinne — in Sedimenten und Fossilien, macht rasche Fortschritte. Eine Methode besteht in der chromatographischen Isolierung einzelner Komponente. Darauf folgt entweder die entscheidende Identifizierung durch direkten Vergleich mit bekannten Substanzen oder die Aufklärung der Struktur durch physikalische und chemische Verfahren.In günstigen Fällen können einzelne Substanzen in der Größenordnung von Mikrogramm (10^–6g) identifiziert werden. Komponenten klarer biologischer Abstammung finden sich oft im geologischen Ausgangsmaterial und können mit der ursprünglichen biologischen Quelle in Beziehung gebracht werden. So stammen die in 2,7.10⁹ Jahre alten Rohölen und Sedimenten gefundenen Kohlenwasserstoffe Phytan (C₂₀H₄₂) und Pristan (C₁₉H₄₀) fast sicher aus der Phytylseitenkette (C₂₀H₃₉O-) des Chlorophylls.

---

Résumé L'étude de nature, de la distribution et de l'échange de la matière organique dans la géosphère et la biosphère et, plus précisément, dans les sédiments et les fossiles, progresse rapidement. Une méthode d'approche est la séparation chromatographique de composés, suivie ou bien par l'identification décisive par comparaison avec des substances connues ou bien par l'élucidation de leur structure à l'aide de méthodes physiques et chimiques.Dans des circonstances favorables, des substances pures peuvent être identifiées à l'échelle du microgramme (10^–6g). Des composés d'un évident origine biologique sont fréquemment présents dans les matières géologiques et peuvent être rapprochés de la source biologique. C'est ainsi que les hydrocarbures phytane (C₂₀H₄₂) et pristane (C₁₉H₄₀) trouvés dans des huiles brutes et des sédiments vieux de 2,7 · 10⁹ ans proviennent presque certainement de la chaîne latérale (C₂₀H₃₅O-) de la chlorophylle.

---

. , , . . . Phytan (_{20 42}) Pristan (_{19 40}) .

     

S1-cleavage fans in the Moselmulde of the Rheinische Schiefergebirge (Federal Republic of Germany) may be due to a D3-tectonic event: open folding and reverse faulting

A major problem in the Moselmulde is to explain the change in orientation of both S₁ and S₂ cleavages in distinct regional zones. Existing models used to explain this are not consistent with established field data. Penetrative S₁-fabrics gradually change attitude in sections perpendicular to their regional SW-NE strike. Involute constructions on S₁ reveal an open fold structure that could explain the gradually changing attitude. Misfits in the fold profile coincide with sites where narrow S₁-cleavage fans have been traces for over 50 km along strike in the field. S₃-fabrics, discovered at two sites along one of the three regional S₁-cleavage fans, suggest that the fans are partly due to penetrative microfolding, microshearing and pressure solution. It is suggested here that the large scale structure of the Moselmulde is essentially due to a deformational event, D₃, which caused apparent narrow cleavage fans along moderately NW-dipping reverse faults. The gradual change in attitude of the bedding, S₁- and S₂-cleavage in the zones separated by the regional fans is due to open folding associated with the reverse fault motion. The faults may be generated from a sole thrust which is reconstructed in my profiles at 10 to 15 km depth.

---

Zusammenfassung Ein Hauptproblem bei der Moselmulde besteht darin, die Veränderung des Fallwinkels der beiden S₁- und S₂-Schieferungen in verschiedenen regionalen Zonen zu erklären. Erklärungen bestehender Modelle stimmen nicht mit den gesicherten Feldergebnissen überein. S₁-Gefüge verändern allmählich ihren Fallwinkel senkrecht zu ihrer regionalen SW-NO-Richtung. »Involute« Profil-Konstruktionen von S₁ zeigen eine offene Faltenstruktur, die die allmähliche Veränderung des Fallwinkels erklären könnten. Diskontinuitäten in dem Falten-Profilschnitt fallen zusammen mit Gebieten, wo man die Achseln von engen Fächern und Meilern der S₁-Schieferflächen über 50 km entlang ihrem Azimut im Feld verfolgen konnte. S₃-Gefüge, entdeckt an zwei Stellen entlang einem der drei regionalen S₁-Spaltungsmeiler, weisen darauf hin, daß die Fächer und Meiler teilweise penetrativen Kleinfalten, Microscherzonen und »pressure solution« zuzuschreiben sind. Es wird hier die Vermutung geäußert, daß die regionale Struktur der Moselmulde im wesentlichen das Ergebnis einer Deformation, D₃, ist, die Fächer und Meilerstellung der Schieferflächen entlang mäßig NW-abtauchenden Aufschiebungen bildete. Die allmähliche Veränderung des Fallwinkels der Gesteinslagen und die S₁- und S₂-Schieferungen in den Zonen, die durch die regionalen Fächer und Meiler getrennt werden, wird durch eine offene Faltung verursacht, verbunden mit den Aufschiebungen. Die Aufschiebungen könnten durch einen »sole thrust« bewirkt worden sein, der in meinen Profilschnitten auf einer Tiefe von 10 bis 15 km konstruiert wird.

---

Résumé Un problème majeur dans le bassin de la Moselle est posé par les variations de pente des schistosités S₁ et S₂ dans diverses aires régionales. Les modèles invoqués d'ordinaire pour expliquer de telles dispositions ne sont pas en accord avec les données de terrain. Le long des sections perpendiculaires à la direction générale SW-NE, la schistosité pénétrative S₁ change progressivement d'attitude. Les constructions »involute« opérées sur S₁, montrent une structure en pli ouvert qui peut expliquer ces variations. Le profil de ce pli comporte des discontinuités qui correspondent à des zônes où S₁ est disposée en éventail étroit, zônes que l'on suit en direction sur plus de 50 Km.En deux points le long d'une de ces trois zônes on observe la présence d'une fabrique S₃ qui permet d'attribuer en partie les éventails à un processus pénétratif de microplissement, de microcisaillement et de dissolution (»pressure-so-lution«). L'auteur propose de considérer la structure à grande échelle du bassin de la Moselle comme l'expression d'une déformation D₃ qui aurait engendré les dispositions en éventail le long de failles inverses à pente NW modérée. Le changement progressif d'attitude de la stratification et des schistosités S₁ et S₂ dans les aires séparées par les zônes en question est le résultat d'un plissement ouvert, associé aux chevauchements. Les chevauchements pourraient être en relation avec un charriage de socle qui peut être construit dans les coupes géologiques à une profondeur de 10 à 15 Km.

---

S₁ S₂ . . S₁ SWNO. , . , , 50 . S₃, , , , pressure solution. , , D₃, , NW. S₁ S₂ , , , , . sole thrust, 10 15 .

     

Kleintektonische Untersuchungen in der Umgebung der Station „General Bernardo O'Higgins“ Nordwestspitze der antarktischen Halbinsel

Zusammenfassung In der unmittelbaren Umgebung der chilenischen Antarktisstation General Bernardo O'Higgins sind die klastischen Sedimente kretazischen Alters innerhalb der alpidischen Orogenese nach drei verschiedenen Verformungsplänen gefaltet worden. Die B₁Achsen sind die ältesten und entsprechen mit ihrem ENE-Streichen dem Großbau der Nordspitze Grahamlands. B₂ streicht etwa 168°, B₃ rund 30^o.

---

In the immediate surroundings of the Chilean Station General Bernardo O'Higgins, the clastic sediments of Cretaceous age have been folded according to three different systems of deformation, during the alpine (Cretaceous-Tertiary) orogeny. The B₁ axes are the oldest ones, and their strike ENE corresponds to the general structures of northern Grahamland. B₂ strikes N 12° W approximately and B₃ roughly N 30° E.

---

Resumen En los alrededores inmediatos de la Base antártica chilena General Bernardo O'Higgins los sedimentos clásticos del Cretácico han sido plegados según tres diferentes sistemas de deformación durante la orogénesis alpídica. Los ejes B₁ son los más antiguos y su rumbo ENE corresponde a la estructura general de la parte Norte de la Tierra de Graham. B₂ tiene rumbo N 12° W aproximadamente, B₃ alrededor de N 30° E.

---

Résumé Aux alentours de la Station chilienne Général Bernardo O'Higgins, les sédiments clastiques du Crétacé ont été plissés selon trois différents systèmes de déformation pendant l'orogénèse alpine (Cretacique-Tertiaire). Les axes B₁ sont les plus anciens, et leur direction ENE correspond à la structure générale de la partie Nord de la Terre de Graham. B₂ a une direction approximative de N 12° W, et B₃ de N 30° E environ.

---

General Bernardo O'Higgins , .