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21.
S. K. Saxena M. C. Domeneghetti G. M. Molin V. Tazzoli 《Physics and Chemistry of Minerals》1989,16(5):421-427
Kinetic rates of Fe2+-Mg disordering in three orthopyroxenes (mean value of XFe = Fe2+/(Fe2++Mg) = 0.175,0.482,0.770 respectively) have been determined employing heating experiments and single crystal X-ray structural refinements. Disordering rate constants \((\vec K)\) (550800° C) for two pyroxenes are given by: ln \((\vec K)\) = 27.107(±5.177)?32062(±783)T?1(XFe = 0.175) ln \((\vec K)\) = 16.142(±0.057)?18227(±423)T?1(XFe = 0.770) The distribution coefficients KD (representing a steady state of disordering FeM2 + MgM1 ? FeM1 + MgM2) are given by: ln KD = 5.016(±0.223)-7033(±1473) T?1(XFe = 0.175) ln KD = 1.988(±0.122)-3809(±913)T?1(XFe = 0.770) These distribution coefficients provide the constraint of the disordering reaction on the value of the equilibrium constant for Fe2+-Mg order-disorder. Until the low temperature dependence of KD is well constrained, the calculation of cooling rates of pyroxenes and host rocks cannot be done reliably. 相似文献
22.
Summary The possibility of applying X-ray single crystal techniques to minerals pertaining to rocks experimentally crystallized in laboratory has been investigated as a new approach to precisely relate crystal-chemistry to pressure, temperature, fO2, and composition of the magma from which the crystal formed.A clinopyroxene from the experimental lunar basalt (74275 composition) ofGreen et al. (1975) crystallized at 12 Kb and 1320 °C, was successfully refined to an R factor of 0.041. Its structure configuration turned out to be exceptional, expecially in relation to the record low Ca(M2) occupancy (0.57 atoms per formula unit henceforth a.f.u.), but comparable with the other lunar C2/c clinopyroxene previously studied with X-ray single crystal methodology: augite 12052 (Takeda 1972 a). Comparative analysis of the two lunar clinopyroxene structures revealed their common high temperature origin and the high-pressure imprint of the experimental specimen relative to the natural one.Both lunar samples differ radically from the studied terrestrial clinopyroxenes of C2/c diopside-like structure: the main difference of lunar clinopyroxenes with respect to terrestrial analogues is the high Ally (T) (AlIV > 0.20 a.f.u.) in spite of low Ca(M2). Relative to the ideal diopside structure, Si4+ AlIV and Ca Mg + Fe2+ substitutions in T and M2 sites, respectively, turned out to be not compatible with terrestrial clinopyroxenes.
With 4 Figures 相似文献
Kristallographische methoden zur untersuchung experimenteller gesteine: struktur verfeinerung von C2/c klinopyroxen aus dem experimentellen hochdruck mond-basalt 74275
Zusammenfassung Die Anwendung von Einkristallverfahren auf Minerale aus experimentell hergestellten Gesteinen wurde untersucht; sie wird als ein neuer Weg zur quantitativen Korrelation der Kristallchemie mit Druck, Temperatur, fO2 und der Zusammensetzung des Magmas, aus dem der Kristall gebildet wurde, gesehen.Die Struktur eines Klinopyroxens aus dem experimentellen Mond-Basalt (Zusammensetzung 74275) vonGreen et al. (1975), der bei 1320 °C kristallisierte, wurde mit Erfolg bis auf einen R-Faktor von 0.041 verfeinert. Es zeigte sich, daß seine Struktur-Konfiguration außergewöhnlich war, besonders im Hinblick auf die besonders niedrige Ca(M2) Besetzungsdichte (0.57 Atome pro Formeleinheit). Sie ist jedoch vergleichbar mit dem anderen lunaren C2/c Klinopyroxen der bisher mit Röntgen-Einkristallverfahren untersucht wurde, nämlich Augit 12052) (Takeda, 1972a). Vergleichsanalysen der beiden lunaren Klinopyroxen-Strukturen lassen ihre gemeinsame Hochtemperatur-Entstehung erkennen, sowie den — im Gegensatz zu der natürlichen Probe — deutlichen Einfluß hoher Drucke auf die experimentell hergestellte Probe.Beide lunare Proben unterscheiden sich deutlich von den untersuchten terrestrischen Klinopyroxenen mit Diopsid-artiger C2/c Struktur: der Hauptunterschied zwischen lunaren Klinopyroxenen und ihren terrestrischen Analogen ist der hohe AlIV (T) (AlIV > 0.20 a.f.u.) trotz niedrigem Ca(M2). Im Vergleich mit der idealen Diopsidstruktur zeigte sich daß, Si4+ AlIV und Ca Mg + Fe2+ Substitutionen an T resp. M2 Plätzen nicht kompatibel mit terrestrischen Klinopyroxenen sind.
With 4 Figures 相似文献
23.
M. Chiara Domeneghetti Anna M. Fioretti Fernando Cmara Gianmario Molin Vittorio Tazzoli 《Meteoritics & planetary science》2007,42(10):1703-1710
Abstract— A single orthopyroxene crystal from the Martian meteorite Allan Hills (ALH) 84001 was studied by X‐ray diffraction (XRD) and electron microprobe analysis (EMPA) to retrieve information about its thermal history. Both sets of data were used to measure the Fe2+‐Mg order degree between the M1 and M2 sites expressed by the distribution coefficient kD. The 529 ± 30°C closure temperature (Tc) of the Fe2+‐Mg ordering process of ALH 84001 orthopyroxene (Fs28) was calculated using Stimpfl (2005a, 2005b) ln kD versus 1/T equation obtained for intermediate iron sample. At this Tc, the orthopyroxene cooling rate, calculated by Ganguly's (1982) numerical method, was 0.1 °C/day. This study puts new constraints on the last high‐temperature thermal episode recorded by orthopyroxene. With reference to the geological history (Treiman 1998), we ascribe this episode to the I3 event, and we interpret the Tc of 529 °C as a lower limit for this impact heating. Our data confirm that experimentally defined physical conditions for the formation of magnetite from decomposition of carbonates took place on the Martian surface during event I3. 相似文献
24.
A. Dal Negro S. Carbonin C. Domeneghetti G. M. Molin A. Cundari E. M. Piccirillo 《Contributions to Mineralogy and Petrology》1984,86(3):221-229
A clinopyroxene suite from lherzolite inclusions associated with the Victorian (Australia) “Newer Volcanics” has been investigated
with the aim of understanding the clinopyroxene crystal-chemical response to increasing temperature (e.g. a melting model
and/or crystallization processes prevailing at high pressure).
The M1 clinopyroxene polyhedron dominates the intracrystalline physical-chemical variations, essentially given by the triple
substitution AlVIFe
M1
2
Ti4+⇌Cr3+ Fe3+Mg
M1
2+
corresponding to an increase in the volume of M1 with increasing Mg/Mg+Fe2+ (mg) for the clinopyroxene. A relative Ca2+ increase in M2 ensures the necessary charge balance. However, Na+ occupancy of M2 persists to the highest mg values, i.e. maximum thermal stability, where the volume of M2 is the largest due to Fe
M2
2+
depletion. The variations of M1 and M2 volumes are greater than, and opposite to, the variations in the volume of T (tetrahedron)
by factors of ca. 3 and 1.5, respectively. Inclusions with relatively low clinopyroxene content (Mt. Porndon specimens) show
distinct intracrystalline variations, essentially reflecting lower AlVI, i.e. higher volume of M1, and implying a lower pressure regime compared to clinopyroxene-rich analogues (Mt. Leura specimens).
The intracrystalline relationships of the Mt. Porndon clinopyroxene suggest that the host peridotite inclusions survived larger
degree of mantle melting at shallower depths relative to the Leura analogues. 相似文献
25.
Summer Moisture Variability in East Central Sweden Since the Mid-Eighteenth Century Recorded in Tree Rings 总被引：2，自引：0，他引：2
Hans W. Linderholm Mats Niklasson Tina Molin 《Geografiska Annaler: Series A, Physical Geography》2004,86(3):277-287
To make predictions of future climate it is necessary to understand the past climate—temperature as well as precipitation. While a wealth of temperature proxies exist from northern latitudes, there is still a lack of information about past precipitation variability. Here we present a 300‐year‐long tree‐ring width chronology from xeric‐site Scots pines (Pinus sylvestris L.) in Tyresta National Park, east central Sweden. Tree‐ring widths were compared to the long observed temperature and precipitation records from Stockholm during 1786–2000. Analyses of the climate/growth relationship showed that, in general, May–June precipitation had a dominating influence on pine growth. However, during dry periods, negative responses to June–July temperature were stronger, especially evident in the late nineteenth century. Periods of below‐average growth were associated with dry conditions in May–June, but occasionally periods of wet and cool summers also produced narrow rings. Periods of above‐average growth were linked to wet, but sporadically also cool and dry, early summers. The years between 1815 and 1833 appear to be particularly dry in the 300‐year context. Since growth anomalies are found in other Swedish drought‐sensitive tree‐ring chronologies during this period, it is likely that this dry period had a regional extent. This is the first tree‐ring chronology from southern Sweden that provides multi‐century information of past summer drought and moisture variability with high resolution and the study will add important information regarding past climate variability in southern Sweden. 相似文献
26.
Second-order diffraction loads upon three-dimensional bodies 总被引：2，自引：0，他引：2
Bernard Molin 《Applied Ocean Research》1979,1(4):197-202
A non-linear theory is presented to derive second-order waveloads, in terms of the wave-steepness. All components of the second-order potential are included, which makes the solution valid for all range of wave frequencies. Numerical results are shown for a vertical cylinder. 相似文献
27.
G. Diego Gatta Romano Rinaldi K. S. Knight G. Molin G. Artioli 《Physics and Chemistry of Minerals》2007,34(3):185-200
The temperature induced structural evolution and thermoelastic behaviour of a natural (Pbca) orthopyroxene (Opx), with chemical formula M2(Mg0.856Ca0.025Fe2+
0.119) M1(Mg0.957Fe2+
0.011Fe3+
0.016Cr0.011Al0.005)Al0.032Si1.968O6, from a suite of high pressure ultramafic nodules of mantle origin, have been investigated by in-situ neutron powder diffraction
at several temperatures starting from 1,200°C down to 150°C. Unit-cell parameter variations as a function of T show no phase transition within this temperature range. The volume thermal expansion coefficient, α = V
−1(∂V/∂T) P0, varies linearly with T. The axial thermal expansion coefficients, αj = l
j−1(∂l
j/∂T)P0, increase non-linearly with T. The principal Lagrangian unit-strain coefficients (ɛ//a, ɛ//b, ɛ//c), increase continuously with T. However, the orientation of the unit-strain ellipsoid appears to change with T. With decreasing T, the values of the unit-strain coefficients along the b and c axes tend to converge. The orientation at ΔT = 1,080°C is maintained down to the lowest temperature (150°C). The two non-equivalent tetrahedral chains, TA
n
OA3n
and TB
n
OB3n
, are kinked differently. At room-T, the TB
n
OB3n
chain is more strongly kinked by about 23° than the TA
n
OA3n
chain. With increasing T, the difference decreases by 3° for the TB
n
OB3n
chain. The intersite cation exchange reaction between M1 and M2 (Mg2+ and Fe2+) shows a slight residual order at 1,200°C followed by reordering with decreasing temperature although seemingly not with
a definite progressive trend. At the lowest temperature reached (150°C), reordering has occurred with the same value of partitioning
coefficient K
D as that before heating. The absence of the expected phase transition is most likely due to the presence of minor amounts
of Fe3+, Al, Ca and Cr which must play a crucial role on the thermoelastic behaviour and phase stability fields in natural Opx, with
consequent important petrologic and geological implications. 相似文献
28.
M. c. Domeneghetti G. m. Molin M. Triscari M. Zema 《Meteoritics & planetary science》2000,35(2):347-354
Abstract— Single crystals of orthopyroxene from small fragments of the Kapoeta, Old Homestead 001, and Hughes 002 howardites were studied by x‐ray diffraction and microprobe analyses. The Fe‐Mg equilibrium distribution coefficients kD of the crystals were used to calculate the closure temperatures (Tc) using the calibration by Stimpfl et al. (1999). The compositions, the presence of exsolved augite lamellae, and the Tc values (from 365 to 385 °C) obtained for Kapoeta orthopyroxene s suggest that our fragment comes from a diogenitic cumulate clast. The more Fe‐rich composition, the absence of exsolved lamellae, and the higher Tc values (from 583 to 605 °C) measured in the Old Homestead 001 orthopyroxenes suggest that this fragment comes from a cumulitic clast affected by fast cooling at high temperature. For the Hughes 002 orthopyroxenes, close in composition to Old Homestead 001, the different Tc values (339, 358, and 607 °C) recorded by the various crystals and the presence of augite lamellae in the crystals with the lowest Tc support the hypothesis that this howardite sample is an unheated breccia containing a mixture of cumulitic orthopyroxenes with different thermal histories. 相似文献
29.
The 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 Fs49) were determined by annealing experiments at several temperatures between 550 and 1000 °C and single crystal X-ray structure
refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs23), which were collected earlier by Molin et al. (1991) between 700 and 1000 °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, ΔS
XS, suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a “simple mixture”. The calculated ΔS
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.
Received: 25 August 1998 / Accepted: 10 March 1999 相似文献
30.
