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1.
C. Carbonin A. Dal Negro S. Ganeo E. M. Piccirillo 《Contributions to Mineralogy and Petrology》1991,108(1-2):34-42
The crystallochemical variations of clinopyroxene in response to changes in fO
2 and melt composition have been determined for a basalt-pantellerite suite (Boseti Complex, Main Ethiopian Rift) by crystal
structure refinement and microprobe analysis. The pyroxene evolutionary trend has both a “Ca-minimum” and late iron enrichment.
During crystallization from basalts to trachytes, clinopyroxene geometry depends mainly on the relationships between T and
M2 sites; for example, high SiO2 activity in the magma causes high Si occupancy in T site, which in turn requires low Ca occupancy in M2 site in order to
fulfill the local charge balance requirements. In contrast, clinopyroxene crystallized from acid melts is characterized by
high Fe2+ (M1) content and therefore by a very large M1 site. Longer 〈M1-O1〉 and M1-O2 bond lengths require shorter T-O1 and T-O2 bond
lengths and high Si occupancy in T site. It is concluded that the “Ca-minimum” in the clinopyroxene structure is regarded
as the lowest value at which the charge balance requirements are satisfied in a C2/c clinopyroxene structure. 相似文献
2.
P. Nimis 《Mineralogy and Petrology》1995,53(1-3):49-61
Summary A detailed crystal-chemical study of clinopyroxenes from the peridotite-pyroxenite association from Zabargad Island (Red Sea) has been carried out to decline the intercrystalline relationships in mantle-derived clinopyroxenes equilibrated at low pressure conditions (plagioclase facies:Pl-Cpx).Pl-Cpx typically show larger cell volume (>437 °A3) compared with those from spinel and garnet-spinel peridotite nodules (Sp-Cpx). The larger cell volume is mainly achieved through higher Mgm, and louver AlVI occupancies, which strongly increase the M l volume. Concurrently, overcharging on the O3 oxygens due te, the high CaM2 ( 0.828 atoms per formula unit, a.f.u.) and low NaM2 (< 0.037 a.f.u.) occupancies requires lengthening of T-0 distances and increase of the T volume. Consequently: i) for the saine M 1 volume,Pl-Cpx have larger cell volume compared withSp-Cpx; ii) for a given trivalent cations (R3+) content in M1, AlIV is higher inPl-Cpx than inSp-Cpx. Plots of cell volume vs Ml volume and of M1-O2 vs T-Onbr bond lengths are a simple way to illustrate the complex intracrystalline relationships which control (Ca Na)M2, (Si AlIV)T and (Mg R3+)M1 substitutions, and thus permit sensitive qualitative discrimination of the pressure regimes of equilibration of mantle clinopyroxenes.
With 6 Figures 相似文献
Klinopyroxene aus Plagioklas-Peridotiten der Insel Zabargad (Rotes Meer) und Verqleich zwischen Hoch- und Tiefdruck-Klinopyroxenen des Mantels
Zusammenfassung Klinopyroxene aus der Peridotit-Pyroxenit-Assoziation der Insel Zabargad (Rotes Meer) wurden eingehend kristallchemisch untersucht, um die inter-kristallinen Bezie-hungen in aus dem Mantel stammenden Klinopyroxenen zu definieren, die unter niedrigem Druck (Plagioklas-Fazies,PI-Cpx) equilibriert wurden.PI-Cpx zeigen typisch ein größeres Zellvolumen (>437 Å3) im Vergleich zu jenen aus Knollen von Spinell- und Granat-Spinell-Peridotiten (Sp-Cpx). Das größere Zellvolumen wird hauptsächlich durch größere MgM1- und kleinere AlVI-Besetzungen erreicht, welche das M1-Volumen stark erhöhen. Gleichlaufend verlangt ein Ladungsüberschuß an den O3-Sauerstoffen durch hohe CaM2-Besetzungen (> 0,828 Atome pro Formeleinheit) und niedrige NaM2-Besetzungen ( 0,037 Atome pro Formeleinheit) eine Verlängerung der T-O-Abstände und ein Anwachsen des T-Volumens. Folglich haben für das gleiche M1-Volumen diePl-Cpx ein größeres Zellvolumen gegenüber denSp-Cpx, und ferner ist für einen gegebenen Gehalt an dreiwertigen Kationen (R3+) auf M1 das AlIV inPI-Cpx höher als inSp-Cpx. Diagramme von Zellvolumen gegen Volumen von M1, sowie von M1-O2 gegen T-Onbr sind ein einfaches Mittel, um die komplexen interkristallinen Beziehungen aufzuzeigen, welche die Substitutionen (Ca Na)M2, (Si AIIV)T und (Mg R3+)M1 beherrschen, sie erlauben eine empfindliche qualitative Unterscheidung der Druckverhältnisse bei der Equilibrierung von Klinopyroxenen aus dem Mantel.
With 6 Figures 相似文献
3.
L. Secco 《Mineralogy and Petrology》1988,39(3-4):175-185
Summary The clinopyroxene suites from lherzolite nodules from Mts. Leura and Noorat (Victoria, Australia) have been investigated by X-ray diffraction and electron probe microanalyses (Dal Negro et al., 1984; Cundari et al., 1986).The evolution of the host nodule is shown by general depletion of AlIv, AVI, Ti4+, Fe2+ and enrichment of Si, Mgm,, Fe3+, Cr3+, Ca, while Na depletion occurs only in the clinopyroxenes from Mt. Noorat. Different mechanisms of cation substitution are thus involved in the two clinopyroxene suites, suggesting different total pressures of equilibration in the stability field of spinel.Modifications involving the M1 octahedron are mainly ascribed to variable amounts of trivalent cations, the volume of the M 1 site increasing with decreasing content of trivalent cations in each suite. The differences in M 1 site configuration between the Mt. Leura and Mt. Noorat clinopyroxene suites are ascribed to the different amounts of A1v1 and FeM,.The volume of the T tetrahedral site is generally related to A1Iv in each suite. An increase in T site volume from Mt. Leura to Mt. Noorat clinopyroxenes was found however, due to lengthening of the T-Obrg bond lengths, for similar AIIv contents. The volume of the M2 site, generally related to Ca content, was generally higher in the Noorat clinopyroxenes for similar Ca (and Ca + Na) contents, due to the longer M2-O3C1 bond length, strongly related to Na content. Cell volume is linearly correlated to M 1 volume in each suite, but is definitely higher in the Noorat clinopyroxenes for similar M 1 volume. All the structural data show that the total pressure of equilibration in the Noorat clinopyroxenes was lower than that in the Leura clinopyroxenes, as suggested by chemical data.
With 2 Figures 相似文献
Die Kristallchemie von hochdruck-klinopyroxenen aus spinell-lherzolithknollen von Mt. Leura und Mt. Noorat, Victoria, Australien
Zusammenfassung Die Klinopyroxenparagenesen aus Lherzolitheinschlüssen von Mt. Leura und Mt. Noorat (Victoria, Australien) wurden mittels Röntgendiffraktometrie und Elektronenmikrosonde untersucht (Dal Negro et al., 1984; Cundari et al., 1986). Die Lherzolitheinschlüsse zeigen generell eine Abnahme von AlIV, AlVI, Ti4+, Fe MI 2+ und eine Zunahme von Si, Mgm,, Fe3+, Cr3+ und Ca. Eine Abnahme von Na tritt nur in Klinopyroxenen von Mt. Noorat auf. Verschiedene Substitutions-Mechanismen der Kationen weisen auf verschiedene Equilibrationsdrucke im Stabilitätsbereich der Spinelle hin und sind daher in den beiden Klinopyroxenserien zu berücksichtigen. Modifikationen in der oktaedrischen M1-Position wurden vor allem dem wechselnden Anteil an dreiwertigen Kationen zugeschrieben, wobei in jeder der beiden Serien das Volumen der Ml-Position mit abnehmenden Gehalten dreiwertiger Kationen zunimmt. Die unterschiedliche Konfiguration der M1-Position der Klinopyroxene von Mt. Leura und Mt. Noorat wird mit unterschiedlichen Gehalten an AlVI und Fe MI 2+ , in Zusammenhang gebracht.Das Volumen der tetraedrischen T-Position ist normalerweise mit den Gehalten an AllIV in jeder Serie verknüpft. Ein Vergleich der Klinopyroxene von Mt. Leura und Mt. Noorat zeigte jedoch, daß bei gleichen Gehalten an AlVI das Volumen der tetraedrischen Position infolge einer Aufwertung der T-Obrg.-Bindungen zunimmt. Das Volumen der M2-Position, üblicherweise mit dem Ca-Gehalt korreliert, ist in den Klinopyroxenen von Mt. Noorat bei ähnlichen Ca (und Ca f Na) Gehalten infolge größerer M2--O3C1-Abstände größer. Sie stehen also mit den Na-Gehalten in Verbindung.Das Volumen der Elementarzelle korreliert mit dem der M1-Position in jeder Serie; es ist aber in den Klinopyroxenen von Mt. Noorat deutlich höher. Die Ergebnisse der Strukturuntersuchungen zeigen-wie auch die chemischen Daten-, daß die genannten Equilibrationsdrucke für die Klinopyroxene von Mt. Noorat niedriger waren als für die von Mt. Leura.
With 2 Figures 相似文献
4.
G. Rossi R. Oberti A. Dal Negro G. M. Molin M. Mellini 《Physics and Chemistry of Minerals》1987,14(6):514-520
The systematic study of both natural and synthetic clinopyroxenes often indicates the presence in the difference Fourier map of a maximum of residual density of up to 0.8 electrons, here labelledM2′, close to theM2 site along the diad axis, defining a square pyramid co-ordination polyhedron. To investigate the nature and the crystalchemical implications of this feature, a limited but representative set of clinopyroxenes of volcanic, metamorphic and synthetic origin has been investigated by X-ray structure refinement (at 0.7 and 0.4 Å resolution), by microprobe analysis and by transmission electron microscopy. The most important results are: a) at increasing resolution, the height of theM2′ peak increases while its co-ordinates move towardM2; b) as (Ca + Na) content approaches 1.0 atom per formula unit,M2′ vanishes; c)M2′ has been found in clinopyroxenes which show differing incipient exsolution microstructures, from spinodal decomposition to non-periodic fluctuations, as well as in homogeneous specimens. The presence ofM2′ is interpreted in terms of the simultaneous coexistence in the crystals of two different structural models, approximately diopside and clinoenstatite. An accurate evaluation of the totalM2 + M2′ site occupancy is strongly suggested in XREF work, particularly when thermodynamic and kinetic considerations have to be obtained with accurate determinations of site occupancy factors as a starting point. 相似文献
5.
Tj. Peters 《Contributions to Mineralogy and Petrology》1968,18(1):65-75
The primary minerals of a partly serpentinized Alpine type ultrabasic mass, lying in a zone of low Alpine metamorphism, were analysed with an electron microprobe. The distribution coefficients of Mg and Fe in coexisting orthopyroxenes, clinopyroxenes and olivines indicates an equilibrium temperature of around 1400°C. The Ca/Ca+Mg ratios in the clinopyroxenes indicate temperatures between 800° and 1000°C. This discrepancy in temperature estimations can partly be explained by the high Al2O3-contents of the clinopyroxenes, since a good correlation between Al2O3-contents and Ca/Ca+Mg ratio was found. The Na2O-content of the clinopyroxenes decreases in the successive pyroxenites that differentiated from the main lherzolite. From the mineralogical composition of the different rock types and the chemical compositions of the minerals it is concluded that the Totalp peridotite originated in the Upper Mantle.In the Malenco serpentinite, the clinopyroxenes formed during the rather strong Alpine metamorphism are much poorer in Al2O3 and Na2O than the primary clinopyroxenes. On the other hand, the olivines, grown postdeformationally during the Alpine metamorphism, are not much different in composition to the primary olivines. 相似文献
6.
7.
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
8.
A model is proposed for the thermodynamic properties of multicomponent pyroxenes in the composition space defined by the end-member component CaMgSi2O6 and the exchange components Fe(Mg)-1, TiAl2(MgSi2)-1, Fe3+(Al)-1, Fe3+Al(MgSi)-1, and Mg(Ca)-1. It is formulated for the simplifying assumptions that: (1) a molecular mixing type approximation describes changes in the molar configurational entropy associated with the coupled exchange substitutions TiAl2MgSi2, Fe3+AlMgSi, and Al2MgSi (and their ferroan equivalents), and (2) Fe2+ and Mg2+, and Al3+ and Fe3+ display long-range non-convergent ordering between M2 and octahedral M1 sites, and octahedral M1 and tetrahedral sites, respectively. The molar vibrational Gibbs energy is described by a Taylor expansion of second degree in seven linearly independent composition and ordering variables, which is extended to third degree to account for asymmetry in the mixing of Ca and Mg, and Ca and Fe on the M2 site, and is further modified for the assumption that the standard state properties of Ca end-member components of clinopyroxenes are linearly dependent on the coordination number of Ca2+ on the M2 site. The model is shown to be consistent with miscibility gap feaures of pyroxenes in the system CaMgSi2O6–CaTiAl2O6–CaAl2SiO6. In subsequent papers, the model is calibrated for the simplifying assumptions that: (1) all regular-solution-type parameters are constants independent of temperature, (2) Pbca and C2/c end-members have identical heat capacities and coefficients of thermal expansion and compressibility, and (3) the heat capacities and coefficients of thermal expansion and compressibility are zero for all reciprocal reactions relating Pbca and pigeonite or high-calcium pyroxene C2/c endmember components. 相似文献
9.
Crystal-chemistry of clinopyroxenes from potassic and ultrapotassic rocks in central Italy: implications on their genesis 总被引：2，自引：0，他引：2
Daniela Cellai Sandro Conticelli Silvio Menchetti 《Contributions to Mineralogy and Petrology》1994,116(3):301-315
The clinopyroxenes mentioned have been investigated by single crystal X-ray diffraction combined with electron microprobe analysis. The aim of this study was to characterize the crystal-chemical variations of clinopyroxenes in order to delineate the intracrystalline constraints which are characteristic of specific magmatic environments. Clinopyroxenes (cpx) crystallized from peralkaline ultrapotassic melt with kamafugitic and lamproitic affinities are characterized by high Si contents, which are insensitive to variations in silica abundance and silica saturation of the melt. The high Si occupancy in clinopyroxenes from kamafugitic magma is coupled to large M1 (i.e. Mg and Fe2+) and M2 (high Ca occupancy) sites, whereas in clinopyroxenes from magmas with lamproitic affinity, high Si content is combined with large M1 but small M2 sites. Clinopyroxenes from Romantype alkaline potassic and ultrapostassic rocks are characterized by an expanded tetrahedron (high IVA1 content) and small M1 site which is combined with small M2 polyhedron in clinopyroxenes from the potassic rocks and large M2 site in those from the ultrapotassic rocks. 相似文献
10.
Summary The crystal-chemistry of clinopyroxene from a suite of Sunda arc volcanic rocks was investigated by X-ray structure refinement and microprobe analysis. Relationships with clinopyroxene from volcanic rocks of the intraplate environment were highlighted.Sunda clinopyroxenes were distinguished into two groups. The first consists of clinopyroxene from SiO2-saturated rock types and crystal rim from SiO2-undersaturated rock types, the second of crystal core from SiO2-undersaturated rock types. The crystal chemical behaviour of the two groups differs significantly, mainly in the geometrical relationships between M2 and T sites. While in the first group M2-O3 and T-O3 bond lengths shorten and lengthen respectively due to Ca and AlIV increase, in the second group in which M2 site is almost fully coordinated by Ca, -O3 bond lengths are forced to lengthen in spite of low AlIV content, in order to mantain the local charge balance around the O3 oxygens.It is noteworthy that Sunda clinopyroxene shows strong analogies with that from basaltic rocks from the Ethiopian plateau and K-rich lavas from the Roman Province respectively, reflecting similarities between the composition and mineralogical assemblages of the host rocks, although they all came from different tectonic environments. A relationship between intracrystalline Mg-Fe 2+ disorder and the explosive character of the volcanism is demonstrated.
Kristallchemie von Klinopyroxenen aus Gesteinen des Vulkanbogens der Sunda-Inseln
Zusammenfassung Die Kristallchemie von Klinopyroxenen aus einer Serie von vulkanischen Gesteinen des Sunda-Bogens wurde mit Röntgen-Strukturverfeinerungen und Mikrosonden-analysen untersucht. Beziehungen zu Klinopyroxenen vulkanischer Gesteine aus intra-Plattenpositionen werden beleuchtet. Die Sunda-Klinopyroxene wurden in zwei Gruppen eingeteilt. Die erste umfaßt Klinopyroxene SiO2-gesättigter Gesteinstypen und Kristallsäume SiO2-untersättigter Gesteinstypen, die zweite Kristallkerne SiO2-untersättigter Gesteinstypen. Das kristallchemische Verhalten der beiden Gruppen unterscheidet sich beträchtlich, hauptsächlich in den geometrischen Beziehungen zwischen den M2 und T Positionen. Während in der ersten Gruppe die M2-O3 und T-O3 Bindungsldngen durch Zunahme von Ca und AlIV kleiner bzw. größer werden, sind in der zweiten Gruppe die T-O3 Bindungslängen trotz niedrigen AlIV Gehaltes zu einer Vergrößerung gezwungen, damit um die O3 Atome der lokale Ladungsausgleich erhalten bleibt.Es ist bemerkenswert, daß die Sunda-Klinopyroxene große Analogien zu denen aus basaltischen Gesteinen des Abessinischen Plateaus und aus K-reichen Laven sowohl der Römischen Provinz als auch des Westafrikanischen Grabens zeigen. Dies spiegelt Ähnlichkeiten zwischen Zusammensetzung und Mineralbestand der Muttergesteine wider, obwohl sie all aus tektonisch unterschiedlichen Positionen kommen.Eine Beziehung zwischen intrakristalliner Mg-Fe2+ Unordnung und dem explosiven Charakter des Vulkanismus wird aufgezeigt.相似文献
11.
In order to clarify the structural configurations observed in Diss in the Ca-rich region of the Di-En join (in which TEM observations show neither exsolution microstructures nor evidence of spinodal decomposition) single crystals large enough for X-ray diffraction analyses, with composition (Ca0.66Mg0.34)MgSi2O6, have been equilibrated close to the solvus atT=1350° C for 317 h, and quenched at room temperature. The refinement in C2/c space group shows that in the M2 site Ca and Mg are fully ordered in two split positions (M2occ: 0.66 Ca; M2occ: 0.34 Mg). Since the average structure shows a relevant elongation of anisotropic thermal ellipsoids of the O2 and O3 oxygen atoms, the refinement has been carried out according to a split model for O2 and O3 atoms: Ca appears 8-coordinated (as in diopside) and Mg shows a sixfold coordination similar to that of high-pigeonite. This coordination for Mg is significantly different from the fourfold coordination (Zn-like in Zn-cpx) proposed previously and it is a more probable coordination for Mg from a crystalchemical point of view. The same results were obtained refining a Di80En20 cpx, equilibrated atT=1230° C, according to the same O-split model. The data support the coexistence of a Di-like configuration for Ca and of a highPig-like configuration for Mg away from the solvus also. AtT very near toT
solidus the different configurations, observed at room temperature in the quenched samples, should converge and Ca and Mg should retain a single disordered configuration in the M2 site. 相似文献
12.
Coordination of boron in nominally boron-free rock forming silicates: Evidence for incorporation of BO3 groups in clinopyroxene 总被引：1，自引：0，他引：1
Ulf Hålenius Henrik Skogby Sabrina Nazzareni Jeppa Resmark 《Geochimica et cosmochimica acta》2010,74(19):5672-1594
To explore mechanisms of B-incorporation in common chain silicates we have investigated synthetic diopside samples produced under boron-saturated conditions by 11B and 29Si magic-angle spinning (MAS) NMR and single-crystal NRA, FTIR, EMP and XRD/SREF techniques. Our samples contain 0.14-0.65 wt.% B2O3. NMR reveals that B is predominantly present in trigonal coordination in the clinopyroxene structure. This observation is supported by vibrational bands characteristic for B-O stretching in BO3 groups in the range 1250-1400 cm−1 in polarised single crystal FTIR-spectra. Single crystal structure refinements suggest that boron replaces Si at the T site. Combined, these results suggest that boron replacement for Si at the T-site leads to disruption of one of the T-O bonds of the nominal clinopyroxene structure resulting in replacement of SiO4 tetrahedra by BO3 groups. Our results show that high concentrations of boron can be incorporated in the nominally boron-free diopside. Elevated B-concentrations in the present calcic clinopyroxenes are accompanied by modifications of the diopside crystal structure involving the breaking of one T-O bond and simultaneous formation of vacancies at the octahedral M2 site. These structural modifications destabilize the structure and constitute thereby limiting factors for incorporating higher boron concentrations in diopside. 相似文献
13.
A clinopyroxene geobarometer for basaltic systems based on crystal-structure modeling 总被引：9，自引：0，他引：9
Paolo Nimis 《Contributions to Mineralogy and Petrology》1995,121(2):115-125
The crystal chemical response of basalt clinopyroxene to increasing pressure was investigated by means of crystal-structure
simulation (a procedure that enables modeling of the structural parameters of a clinopyroxene of known chemistry without requiring
direct X-ray diffraction analysis) using available experimental chemical data. Pressure proved the main physical variable
governing clinopyroxene behavior in a magmatic environment. The general internal consistency of the simulation data permitted
construction of an empirical geobarometer based on the relationship of cell volume (Vc) vs M1-site volume (VM1). The straightforward
geobarometric formulation in the absence of direct X-ray analysis is:
P(kbar)=698.443+4.985⋅AlT−26.826⋅Fe2+
M1−3.764⋅Fe3+
+53.989⋅AlM1+3.948⋅Ti+14.651⋅Cr
−700.431⋅Ca−666.629⋅Na−682.848⋅MgM2−691.138⋅Fe2+
M2−688.384⋅Mn−6.267⋅(MgM2)2−4.144⋅(Fe2+
M2
where:
(Fe2+
M1⋅MgM2)/(Fe2+
M2⋅MgM1)=e**(0.238⋅R3++0.289⋅CNM−2.315), CNM=Ca+Na+Mn, and R3+=AlM1+Fe3++Ti+Cr,
with cations in atoms per formula unit. The geobarometer reproduces experimental pressures within ±2 kbar (=1σ; max. dev.
≤5 kbar; N=29) in the range 0–24 kbar and is applicable to near-liquidus C2/c clinopyroxenes crystallized from basaltic melts in the absence of garnet (excepting high-Al2O3 basalts). It is therefore suitable for many natural clinopyroxenes occurring as mega- or phenocrysts or forming well-preserved
cumulate pyroxenites. If the above restrictions are not wholly satisfied, the Vc vs VM1 plot can also be used qualitatively
to deduce the relative pressure conditions of clinopyroxenes forming from similar batches of magma. The structural simulation of experimental data
also provided insight into the influence of minor chemical changes of the parental magma on the crystal chemistry of clinopyroxene
at high pressure. Within the considered compositional space at given P-T, a
CaO and a
SiO2 in the melt have opposite effects on M2- and T-site cation populations. As a result, under similar physical conditions, clinopyroxenes
from higher-CaO or more undersaturated basalts have higher VM2, VT and Vc and lower VM1. For basalts with normal contents of Al2O3 (<18 wt %), variations of major elements in the melt do not reduce the accuracy of the geobarometer.
Received: 3 April 1994 / Accepted: 23 December 1995 相似文献
14.
The model for the thermodynamic properties of multicomponent pyroxenes (Part I) is calibrated for ortho- and clinopyroxenes in the quadrilateral subsystem defined by the end-member components Mg2Si2O6, CaMgSi2O6, CaFeSi2O6, and Fe2Si2O6. This calibration accounts for: (1) Fe-Mg partitioning relations between orthopyroxenes and augites, and between pigeonites and augites, (2) miscibility gap features along the constituent binary joins CaMgSi2O6-Mg2Si2O6 and CaFeSi2O6-Fe2Si2O6, (3) calorimetric data for CaMgSi2O6-Mg2Si2O6 pyroxenes, and (4) the P-T-X systematics of both the reaction pigeonite=orthopyroxene+augite, and miscibility gap featurs, over the temperature and pressure ranges 800–1500°C and 0–30 kbar. The calibration is achieved with the simplifying assumption that all regular-solution-type parameters are constants independent of temperature. It is predicated on the assumptions that: (1) the Ca-Mg substitution is more nonideal in Pbca pyroxenes than in C2/c pyroxenes, and (2) entropies of about 3 and 6.5 J/K-mol are associated with the change of Ca from 6- to 8-fold coordination in the M2 site in magnesian and iron C2/c pyroxenes, respectively. The model predicts that Fe2+-Mg2+ M1-M2 site preferences in C2/c pyroxenes are highly dependent on Ca and Mg contents, with Fe2+ more strongly preferring M2 sites both in Ca-rich C2/c pyroxenes with a given Fe/(Fe+Mg) ratio, and in magnesian C2/c pyroxenes with intermediate Ca/(Ca+Fe+Mg) ratios.The proposed model is internally consistent with our previous analyses of the solution properties of spinels, rhombohedral oxides, and Fe-Mg olivines and orthpyroxenes. Results of our calibration extend an existing database to include estimates for the thermodynamic properties of the C2/c and Pbca pyroxene end-members clinoenstatite, clinoferrosilite, hedenbergite, orthodiopside, and orthohedenbergite. Phase relations within the quadrilateral and its constitutent subsystems are calculated for temperatures and pressures over the range 800–1700°C and 0–50 kbar and compare favorably with experimental constraints. 相似文献
15.
Summary The structural changes occurring in the clinopyroxenes with composition Di100, Di90En10 and Di80En20, due to the Ca-Mg substitution in the M2 site, have been studied. Evidence is given that with increasing Mg content a small percentage of the atoms converts from the M2 position to a new M2 position which is solely occupied by Mg. The maximum conversion of M2 to M2 found in this study is 7%. The closest parallel to the M2 geometry is found in the ZnSiO3 pyroxene (C2/c). The presence of this new site causes significant changes in the tetrahedral configuration, because the M2 atoms are not bonded to 03. The intermediate compositions, Di90En10 and Di80En20, may be thought of as the coexistence of two structural models: diopside and ZnSiO3 pyroxene (C2/c).
Abbreviations En Enstatite - Di Diopside - Hd Hedenbergite - Fs Ferrosilite - ClEn Clinoenstatite - Di100 pure diopside - Di90 Di90En10 (mol.-%) - Di80 Di80En20 - brg bridging With 6 Figures 相似文献
Kristallstrukturen Ca-reicher Klinopyroxene der CaMgSi2O6-Mg2Si2O6-Reihe
Zusammenfassung Es wurden die strukturellen Änderungen von Klinopyroxenen der Zusammensetzungen Di100, Di90En10 und Di80En20, die durch den Mg-Ersatz für Ca verursacht werden, untersucht. Es zeigt sich, daß mit steigendem Mg-Gehalt ein kleiner Teil der Atome der M2-Position zu einer neuen M2-Position wechselt; diese wird ausschließlich durch Mg besetzt. Der größte in dieser Arbeit gefundene Übergang von M2 nach M2 beträgt ca. 7%. Die stärksten Parallelen zur Geometrie um M2 werden im Pyroxen ZnSiO3 (C2/c) gefunden. Die Besetzung dieser neuen Position verursacht bedeutende Änderungen im Tetraederverband, da die M2-Atome nicht an O3 gebunden sind. Die Pyroxenstrukturen mit den intermediären Zusammensetzungen Di90En10 und Di80En20 können als Überlagerung zweier Modelle betrachtet werden: Diopsid und ZnSiO3-Pyroxen (C2/c).
Abbreviations En Enstatite - Di Diopside - Hd Hedenbergite - Fs Ferrosilite - ClEn Clinoenstatite - Di100 pure diopside - Di90 Di90En10 (mol.-%) - Di80 Di80En20 - brg bridging With 6 Figures 相似文献
16.
X-ray absorption spectroscopy, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques, have been used to determine the structure and speciation of complexes for Fe2+ and Fe3+ chloride solutions at a variety of pH's, ionic strengths, and chloride/iron ratios.Low intensity K-edge transition features and analysis of modified pair correlation functions, derived from Fourier transformation of EXAFS spectra, show a regular octahedral coordination of Fe(II) by water molecules with a first-shell Fe2+-O bond distance, closely matching octahedral Fe2+-O bonds obtained from solid oxide model compounds. Solution Fe2+-O bond distances decrease with chloride/iron ratio, pH, and total FeCl2 concentration. A slight intensification of the transition with increasing FeCl2 concentration suggests that chloride may begin to mix with water as a nearest-neighbor octahedral ligand. Fe3+ solutions show a pronounced increase in the transition intensities between 1.0 M FeCl3/7.8 M Cl? to 1.0 M FeCl3/ 15 M Cl?, indicating a coordination change from octahedral to tetrahedral complexes. EXAFS analyses of these solutions show an increase in first-shell Fe3+-ligand distances despite this apparent reduction in coordination number. This can be best explained by a change from regular octahedral complexes of ferric iron (either Fe(H2O)63+ or trans-Fe(H2O)4Cl2 or both; Fe3+-O bond distances of 2.10 Å) to tetra-chloro complexes [Fe3+-Cl bond distances of 2.25 Å]. 相似文献
17.
Babingtonite, Ca2Fe2+Fe3+[Si5O14(OH)] (Z?=?2, space group $ P\overline{1} $ ) from Yakuki mine (Japan), Grönsjöberget (Sweden), Kandivali Quarry (India), Baveno Quarry (Italy), Bråstad Mine (Norway), and Kouragahana (Japan), and manganbabingtonite, Ca2(Mn2+, Fe2+)Fe3+[Si5O14(OH)], from Iron Cap mine (USA) were studied using electron-microprobe analysis (EMPA), 57Fe Mössbauer analysis and single-crystal X-ray diffraction methods to determine the cation distribution at M1 and M2 and to analyze its effect on the crystal structure of babingtonite. Although all studied babingtonite crystals are relatively homogeneous, chemical zonation due to mainly Fe ? Mn substitution is observed in manganbabingtonite. Mössbauer spectra consist of two doublets with isomer shift (I.S.)?=?1.16–1.22 mm/s and quadrupole splitting (Q.S.)?=?2.33–2.50 mm/s and with I.S.?=?0.38–0.42 mm/s and Q.S.?=?0.82–0.90 mm/s, assigned to Fe2+ and Fe3+ at the M1 and M2 octahedral sites, respectively. The determined ratio of Fe2+/total Fe in manganbabingtonite (0.26) was smaller than that in the others (0.35–0.44) because of high Mn2+ content instead of Fe2+. The unit-cell parameters of babingtonite are a?=?7.466–7.478, b?=?11.624–11.642, c?=?6.681–6.690 Å, α?=?91.53–91.59, β?=?93.86–93.94, γ?=?104.20–104.34º, and V?=?560.2–562.3 Å3, and those of manganbabingtonite are a?=?7.4967(3), b?=?11.6632(4), c?=?6.7014(2) Å, α?=?91.602(2), β?=?93.989(2), γ?=?104.574(3)º, and V =565.09(5) Å3. Structural refinements converged to R 1 values of 1.64–3.16 %. The <M1-O> distance was lengthened due to the substitution of large octahedral cations such as Mn2+ for Fe2+. The increase of the M1-O8, M1-O8’ and M1-O13 lengths with mean ionic radii is slightly more pronounced than of the other M1-Oi lengths. The lengthened M1-O13 distance leads the positive correlation between Si5-O15-Si1 angle and M1-O13 distance. The increase of Si2-O3-Si1 and Si5-O12-Si4 angles due to the increase of mean ionic radius of M2 is also observed. 相似文献
18.
Subrata Ghose Fujio P. Okamura Haruo Ohashi 《Contributions to Mineralogy and Petrology》1986,92(4):530-535
The crystal structure of a synthetic CaFe3+Al-SiO6 pyroxene (20 kb, 1,375° C) with unit cell dimensions a=9.7797(16), b=8.7819(14), c=5.3685(5) Å, =105.78(1), space group C2/c has been refined by the method of least squares to an R-factor of 0.025 based on 812 reflections measured on an automatic single crystal diffractometer. The octahedral M1 site is occupied by 0.82 Fe3+ and 0.18 Al3+. Within the tetrahedral T site, Si4+ (0.50), Al3+ (0.41) and Fe3+ (0.09) ions are completely disordered, although submicroscopic domains with short-range order are very likely. The octahedral site preference energy of the Fe3+ ions with respect to Al3+ ions in CaFe3+AlSiO6 is about 10 kcal/mole, which is much higher than that found in Y3Al
x
Fe5–2O12 garnets. Topologically the structure of CaFe3+AlSiO6 is intermediate between that of diopside and calcium Tschermak's pyroxene, CaAlAlSiO6. For CaM3+ AlSiO6 clinopyroxenes an increase in the size of the M1 octahedron is accompanied by an increase in the average M2-0, bridging T-0 and 03-03 distances and kinking of the tetrahedral chain. 相似文献
19.
Renate Schumacher 《Contributions to Mineralogy and Petrology》1991,108(1-2):196-211
Calcic amphiboles coexisting with epidotegroup minerals (zoisite, clinozoisite, epidote) and/or clinopyroxene±plagioclase±quartz±garnet occur in amphibolites and calc-silicate rocks that underwent amphibolite to lower granulite-facies metamorphism in the Acadian metamorphic high of central Massachusetts, USA. Across the region, peak metamorphic conditions range from about 580° C and 6.2 kbar to 730° C and 6.3 kbar. The coexistence of most Ca-amphiboles with Fe3+-rich epidote-group minerals suggests the presence of Fe3+ in most of these amphiboles. An empirical Fe3+ estimation for the microprobe analyses is based on two constraints: the Na?Ca content of the M4 sites of Ca-saturated, gravimetrically analyzed hornblendes gives the relation: Ca(M4) c =-1.479 Na(M4) c +2 (c=corrected). The second constraint is the stoichiometric equation Ca(M4)+Na(M4)+FM=15, where FM is the sum of all cations exclusive of Ca, Na, and K. Solving the two equations simultaneously gives: 20.185=0.479 Ca(M4)+1.479 ΣFM. Starting with the uncorrected values of Ca(M4) u and ΣFM(M4) u (u = uncorrected) of the all ferrous formula, the normalization factor NF for calculating the corrected cations of the ferric formulas is: 20.185/(0.478 Ca(M4) u +1.479 ΣFM u ). From the deficient oxygen the Fe3+ content which is equal to 2(23-ΣOX) can be calculated. Determinations of Fe3+ contents of four hornblende separates by Mössbauer spectroscopy are in agreement with the calculated values. The Ca-amphiboles show systematic changes in composition with increasing grade of metamorphism within the amphibolite and lower granulite-facies zones: increasing edenite and tschermakite substitution, increasing Ti content, and increasing Fe2+/(Fe2++Mg) ratio. In addition, the coexisting clinopyroxenes are also characterized by an increase in Fe2+/(Fe2++Mg) ratio. In quartz-free rocks with coexisting Ca-amphibole and plagioclase there is an increase in the ratio X Ab/X Ed, where X Ab=Na/(Na+Ca) in plagioclase and X Ed=Na in the amphibole A-site. These chemical changes in mineral composition together with the disappearance of epidote at the transition to granulite-facies metamorphic conditions are attributed to the continuous reaction: albite+epidote+Fe-Mg hornblende→Fe?Mg clinopyroxene+anorthite+(NaAlSi-1)Hbl+H2O. 相似文献
20.
The mixing properties and the local structure of the Ca-Sr fluorapatite isomorphic system were studied using the original atomistic computer modeling approach for solid solutions. Calculations were carried out for a 4 × 4 × 4 supercell (2688 atoms) in conjunction with the high-performance computer SKIF MSU Chebyshev. In order to find the most probable (energetically preferable) distribution of Sr over the M1 and M2 cation sites in the fluorapatite solid solution, eight different distributions were tested. Structures with the most preferable x(Sr1): x(Sr2) ratios were used for further investigation of the effect of mixing on the lattice parameters and elasticity of the Ca-Sr fluorapatite solid solution and for the detailed analysis of the local structure. The local structure analysis included investigation of the individual and average M1-O, M2-O, M2-F bond lengths distortions, changes in the M1 and M2 polyhedra volumes as function of Sr content as well as the estimation of the structural relaxation parameters for all compositions of the solid solutions considered in this work. 相似文献