219 research outputs found
CRYSTAL-CHEMISTRY AND INTRACRYSTALLINE RELATIONSHIPS OF ORTHO-PYROXENE IN A SUITE OF HIGH-PRESSURE ULTRAMAFIC NODULES FROM THE NEWER VOLCANICS OF VICTORIA, AUSTRALIA
No full textA suite of orthopyroxenes from spinel lherzolite xenoliths associated with basanites occurring in the Victorian (Australia) post-Pliocene 'Newer Volcanics' province was investigated by means of a crystal chemical methodology which provides accurate site occupancy and site configuration parameters.
The M1 configuration is essentially constrained by Al(IV) rather than Fe2+. In addition, Fe3+, Cr3+ and Ti4+ are confined to M1 (Molin, 1989) and Al(IV) to TB. M2 is controlled by Fe(M2)2+ half arrow right over half arrow left constrained by (Fe2+ + Ca)M2 > 0.14 atoms per formula unit (p.fu.). Cation substitution in TB and M2 constrains the sum of the volumes of the respective polyhedra V(TB) + V(M2) to remain essentially constant. Therefore, M2 favours the retention of the large Fe2+ up to melting-point, causing nonideality of this iron-depleted orthopyroxene. As a consequence, the investigated orthopyroxene can be considered an ultimate Fe2+ carrier during partial mantle melting
Kinetics of Fe2+-Mg order-disorder in orthopyroxene: experimental studies and applications to cooling rates of rochs
No full textWe determined the forward rate constant (K+)
for the Fe2+–Mg order–disorder between the M2 and
M1 sites of orthopyroxene (OPx), which is described by
the homogeneous reaction Fe2+ (M2) + Mg(M1) M
Mg(M2) + Fe2+ (M1), by both ordering and disordering
experiments at isothermal condition and also by
continuous cooling experiments. The rate constant was
determined as a function of temperature in the range of
550–750C, oxygen fugacity between quartz–fayalite–
iron and Ni–NiO buffers, and at compositions of 16 and
50 mol% ferrosilite component. The K+ value derived
from disordering experiment was found to be larger than
that derived from ordering experiment at 550C, while at
T>580C, these two values are essentially the same. The
fO2 dependence of the rate constant can be described by
the relation K+ a (fO2)n with n=5.5–6.5, which is
compatible with the theoretically expected relation. The
Arrhenius relation at the WI buffer condition is given by
lnðCoKþÞ 1⁄4
41511 12600XFe
T ðKÞ
þ 28:26
þ 5:27 XFe; min1
where Co represents the total number of M2 + M1 sites
occupied by Fe2+ and Mg per unit volume of the crystal.
The above relation can be used to calculate the cooling
rates of natural OPx crystals around the closure temperature
(Tc) of Fe–Mg ordering, which are usually below
300C for slowly cooled rocks. We determined the
Fe–Mg ordering states of several OPx crystals ( Fs50)
from the Central Gneissic Complex (Khtada Lake),
British Columbia, which yields Tc 290C. Numerical
simulation of the change of Fe2+-Mg ordering in OPx as
a function of temperature using the above expression of
rate constant and a non-linear cooling model yields
quenched values of ordering states that are in agreement
with the observed values for cooling rates of 11–17C/
Myr below 300C. The inferred cooling rate is in
agreement with the available geochronological constraints
Fe2+-Mg order-disorder in orthopyroxene: equilibrium fractionation between the octahedral sites and thermodynamic analysis
No full textThe equilibrium intracrystalline distribution coefficient, k*(D), of Fe* (i.e. Fe2+ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe2+-Mg orthopyroxene crystals (FS14 FS15 and Fs(49)) were determined by annealing experiments at several temperatures between 550 and 1000 degrees C and single crystal X-ray structure refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs(23)) which were collected earlier by Molin et al. (1991) between 700 and 1000 degrees C, were re-refined. The data were processed through two different refinement programs (SHELXL-93 and RFINE90) using both unit and individual weights and also both ionic and atomic scattering factors. The calculated site occupancies were found to agree within their estimated standard errors. However, the use of ionic scattering factors led to significantly better goodness of fit and agreement index, and smaller standard deviations of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted refinements yielded significantly smaller standard deviations of the site occupancies than the unweighted refinements even when the same set of reflections was used in the two procedures. The site occupancy data from this study were combined with selected published data to develop expressions of k*(D) as a function of temperature and composition. Calculation of the excess configurational entropy, Delta S-XS, suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a "simple mixture" The calculated Delta S-XS values and the excess Gibbs free energy of mixing suggested by available cation exchange data lead to a slightly negative enthalpy of mixing in the orthopyroxene solid solution
Orthopyroxene from the Serra de Magé meteorite: structure refinement and estimation of C2/c pyroxene contributions to apparent Pbca diffraction violations
No full textANTARCTIC FRO90011 LODRANITE - COOLING HISTORY FROM PYROXENE CRYSTAL-CHEMISTRY AND MICROSTRUCTURE
No full textThe Antarctic FR090011 lodranite shows an achondritic coarse granoblastic texture with an approximately chondritic composition. The crystal chemistry of Pbca and C2/c pyroxene single crystals and the TEM texture of Pbca pyroxene were investigated to reconstruct the evolutionary history of the meteorite parent body. The structural behaviour of orthopyroxene (En(85)Fs(12.4)Wo(2.6)) is well constrained in the En-Fs trend. Orthopyroxene Fe2+-Mg ordering in the M1 and M2 sites yields a closure temperature (T-c) of 570 +/- 20 degrees C and a cooling rate 10 degrees C/day near the T-c. Clinopyroxene is a low-iron augite (En(51)Fs(5)Wo(44)). Clinopyroxene Fe2+-Mg ordering in the M1 and M2 sites yields a T-c of 513 +/- 50 degrees C, which is compatible with the coaling rate derived from the orthopyroxene crystals. TEM investigation on orthopyroxene showed a few thin lamellae not larger than 20 Angstrom parallel to (100) and rare one unit cell Guinier-Preston zones. TEM-EDS analysis did not reveal significant chemical differences between lamella-rich and lamella-free zones. The above data are interpreted as being due to fast cooling resulting from radiative heat loss of small fragments ejected from a parent body, which were subsequently reassembled into a larger body
Ortho-pyroxene From the Serra De Mage Meteorite - Structure Refinement and Estimation of C2/c Pyroxene Contributions To Apparent Pbca Diffraction Violations
No full textTransmission electron microscopy (TEM) of microstructures in the orthopyroxene of the Serra de Mage cumulate eucrite shows augite lamellae thicker than 500 Angstrom, irregularly spaced within the orthopyroxene matrix and parallel to (100), with a(Opx)* = a(Aug)*, [101](Opx)*, = c(Aug)*, and b(Opx)* = b(Aug)*. Streaking along a* owing to stacking disorder was observed in selected-area electron diffraction (SAD) patterns. Slabs of clinopyroxene were observed, but no Guinier-Preston zones were detected. Single-crystal X-ray diffraction (XRD) data were collected and structure refinements completed in space group Pbca, converging to R(obs) = 2.81 and 5.17% for the two samples. Because of the presence of some Okl forbidden reflections with k = 2n + 1 and l = 4n + 2, structure refinement was attempted in space group P2(1)ca, as suggested by previous studies, and in space groups P12(1)/c1 and P112(1)/a, as suggested by the Laue symmetry of the reflections, which is 2/m rather than mmm. However, the high residual indices R and the anomalous values of many displacement parameters and bond distances did not support the lower-symmetry space groups. The violations of the extinction conditions of the Pbca space group and the monoclinic symmetry of the reflections are due to overlap of reflections from the C2/c augite lamellae. From the intensity of the reflections forbidden in Pbca space group, the calculated augite content ranges from approximately 4 to 12%
TEM TEXTURE AND X-RAY SINGLE-CRYSTAL STUDY OF SERRA-DE-MAGE ORTHO-PYROXENE
No full textA combined Transmission Electron Microscopy (TEM), X-ray single crystal diffraction (XRD) and microprobe study was undertaken on the orthopyroxene of Serra de Mage feldspar cumulate eucrite. Two single crystals, natural and equilibrated at several temperatures, were examined in order to identify the true symmetry of the orthopyroxene and to estimate the cooling history of the meteorite. TEM texture analysis showed augite lamellae larger than 500 A irregularly spaced within the orthopyroxene matrix, with orientation parallel to (100). XRD data were collected using a four-circle diffractometer. Structural refinement carried out in space group Pbca converged to a discrepancy factor of R = 2.81%. Because of the presence of some Okl 'forbidden' reflections that may be explained either by P21ca symmetry of the orthopyroxene or by overlap of reflections from the exsolved C2/c clinopyroxene, structural refinement in space group P21ca was also attempted. The resulting discrepancy factor was very close to that obtained in Pbca; however, the anomalous values of many thermal parameters and cation-oxygen bond distances do not support the choice of the P21ca space group. Calibration or KD (KD = (Fe(2+) + Mn)M1(MgM2/(Fe(2+) + Mn)MZMgM1) vs. 1/T gave a closure temperature Tc for the intracrystalline Fe(2+)-Mg exchange reaction of about 470 C, consistent with a relatively slow cooling rate
Mechatronic Handling Solution for a Robotized Pizza-Chef Assistant
No full textIn the Industry 4.0 context, advanced mechatronic solutions are more and more adopted also outside the manufacturing field, e.g. food. Being pizza one of the most spread foods worldwide and since not all the tasks for preparing it need the dexterity of a pizza-chef, e.g. topping, an autonomous collaborative robot helping the pizza-chef would represent a possible and effective support solution. However, a crucial aspect in handling the ingredients for topping is the selection of a proper end-effector. Indeed, foodstuff is usually characterized by non-uniform shapes and dimensions as well as it is soft and/or fragile. As first step for conceiving and testing an intelligent (collaborative) robotic pizza topping system, this paper firstly investigates the commercially available low-cost grippers able to deal with foodstuff. Starting from this analysis, a versatile ad-hoc designed gripper able to handle most of the ingredient typically used for topping pizzas has been designed and tested
Energetic Preference In 5,6 and 6,6 Ring Junction Adducts of C-60 - Fulleroids and Methanofullerenes
No full textAugust Stimpfl Aquarelle, Zeichnungen ; Galerie Rolf Ohse, Bremen, 3.9.-9.10.1982 ; Galerie Döbele, Ravensburg, 16.10.-20.11.1982
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