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31.
Synthetic titanite, CaTiOSiO4, and the series of (Ca1−x
Na
x
)(Ti1−x
Ta
x
)OSiO4 and Ca(Ti1−2x
Ta
x
Al
x
)OSiO4 solid solutions have been prepared by ceramic methods, and their crystal structure determined by the Rietveld analysis. At ambient conditions, titanite can contain up to 20 mol% NaTaOSiO4 or 60 mol% Ca(Al0.5Ta0.5)OSiO4. These limits might differ in natural samples due to combination with substitutions involving fluorine and/or hydroxyl replacing oxygen together with vacancies at cationic sites. All cations located at the vii
X- and vi
Y-sites in the structures of tantalian titanite are disordered. Expansion of the <Si–O> bond from 1.618 to 1.621 Å in CaTi0.8Ta0.1Al0.1OSiO4 and CaTi0.6Ta0.2Al0.2OSiO4 to 1.644 Å in the CaTi0.4Ta0.3Al0.3OSiO4 titanite suggests the possible presence of some Al3+ in the tetrahedral site replacing Si4+ in the latter. All tantalian titanites crystallize in the space group A2/a. This implies that both single-site and complex double-site substitutional schemes induce P21/a → A2/a phase transition(s). The (Ca1−x
Na
x
)(Ti1−x
Ta
x
)OSiO4 substitution scheme incorporates larger cations at both the vii
X and vi
Y sites, whereas the Ca(Ti1−2x
Ta
x
Al
x
)OSiO4 scheme involves only vi
Y-site (Al3+,Ta5+) cations with a slightly smaller “average” radius. Unit cell dimensions change insignificantly or increase incrementally with increase of average cationic radii in the (Ca1−x
Na
x
)(Ti1−x
Ta
x
)OSiO4 series, and with an insignificant decrease in the viR
Y
average cationic radii in the Ca(Ti1−2x
Ta
x
Al
x
)OSiO4 series. Both Ta-doped titanite and CaTiOSiO4 consist of distorted polyhedra with the XO7, YO6 coordination polyhedra and the SiO4 tetrahedron in tantalian titanite being less distorted compared to those of the pure CaTiOSiO4. 相似文献
32.
Subsolidus deuteric/hydrothermal alteration of eudialyte in lujavrite from the Pilansberg alkaline complex, South Africa 总被引:1,自引:0,他引:1
The most evolved rocks of the Pilansberg alkaline complex are aegirine lujavrites in which three varieties of eudialyte are recognized on the basis of textural relationships and composition. Manganoan eudialyte-I is a relict orthomagmatic phase occurring as poikilitic plates or as relict grains in pseudomorphed euhedral phenocrysts. Late eudialyte-II ranges in composition from manganoan eudialyte through kentbrooksite to taseqite-like varieties and is considered to be formed by cation exchange with eudialyte-I and alkaline fluids. Eudialyte-III is a hydrothermal phase replacing eudialyte-II, and has either taseqite-like (5–7.3 wt.% SrO, < 2.0 wt.% REE2O3) or kentbrooksite (< 1.5 wt.% SrO, 8.5 wt.% REE2O3) compositions. Three styles of replacement of eudialyte-I and -II are recognizable. Type 1 involves replacement by complex aggregates of zircon, fergusonite-(Ce), allanite-(Ce), britholite-(Ce), titanite, pyrochlore, albite and potassium feldspar, i.e. a “miaskitic” paragenesis. Type 2 alteration consists of complex aggregates dominated by deuteric Na–Zr-silicates (?catapleiite), stronalsite, strontium-apatite and lamprophyllite replacing eudialyte-I and -II and relicts of the “miaskitic paragenesis”, i.e. a highly sodic “agpaitic-to-hyperagpaitic” paragenesis. Type 3 replacement involves mantling of any residual eudialyte-II and zircon, and replacement of deuteric Na–Zr-silicates by eudialyte-III together with barytolamprophyllite as late hydrothermal phases. Further alteration and replacement resulted in the superposition of natrolite, britholite, pyrochlore, allanite and diverse Ba- and Mn-based minerals onto the types 2 and 3 assemblages, and ultimately to the deposition of allanite-(La), La-dominant REE carbonates and rarely a silica phase. All of the alteration styles are considered to have occurred in situ under subsolidus conditions (< 450 °C) by interaction of pre-existing eudialyte and other minerals with deuteric, sodium- and chlorine-bearing aqueous fluids. The evolution of the replacement products is from a miaskitic through an agpaitic to a hyperagpaitic paragenesis and ultimately back to a low agpaitic-to-miaskitic assemblage, reflecting changes in the a(Na+)/a(Cl−) ratio and alkalinity of the deuteric/hydrothermal fluids. 相似文献
33.
Johanna Marin-Carbonne Kevin D. McKeegan Andrew M. Davis Glenn J. MacPherson Ruslan A. Mendybaev Frank M. Richter 《Meteoritics & planetary science》2023,58(4):546-571
Oxygen, magnesium, and silicon isotopic abundances in Vigarano 1623-5 were studied using secondary ion mass spectrometry to investigate correlations between isotopic and petrologic properties of this unique forsterite-bearing FUN inclusion. Vigarano 1623-5 displays large, correlated mass-dependent fractionation effects, tightly linked to mineralogy within distinct petrologic units of the inclusion. The pyroxene-rich and melilite-rich interior parts of the inclusion display highly mass-fractionated isotopic compositions of oxygen, magnesium, and silicon, consistent with Rayleigh distillation during evaporation of a melt with initial oxygen composition close to a solar composition. However, the chemical composition, enriched in magnesium and silicon, suggests a precursor already fractionated by prior melt evaporation. A discontinuous igneous rim was produced by a flash-melting event followed by isotopic exchange in the rim melilite with planetary-like oxygen, mechanical fragmentation, and reassembly with an accretionary rim of heterogeneous materials. Al-rich minerals in 1623-5 show evidence for having crystallized with live 26Al but at less than the “canonical” level of most CV calcium-aluminum-rich inclusions. However, well-defined 26Al-26Mg isochrons are not found and temporal implications are ambiguous. 相似文献
34.
Parastoo Ghaznavi Yogita Kadlag David Haberthür Ruslan Hlushchuk Ingo Leya 《Meteoritics & planetary science》2023,58(6):897-900
Micro-computed tomography (μCT) is a fast and powerful technology for studying textural, physical, and chemical properties of solid objects in three dimensions. While regularly used for sample documentation and curation, it is often assumed that μCT techniques are essentially nondestructive or at least very little destructive. However, there are very few studies proving or rejecting the assumption of nondestructiveness. Here we study whether X-ray tomographic imaging affects the noble gas budget of matrix samples from the CV3 carbonaceous chondrite Allende. We irradiated powdered and homogenized matrix samples in the Bruker SkyScan 1272 μCT instrument at three different X-ray tube acceleration voltages of 30, 70, and 100 keV. By comparing the noble gas concentrations and especially the elemental and isotopic ratios of the irradiated samples with data for two non-irradiated aliquots, we found no significant differences. Our study therefore demonstrates that X-ray tomographic imaging has no measurable effect on the noble gas budget and can therefore safely be used for sample characterization prior to noble gas studies. 相似文献