The spinel coexisting with diamonds is often used as indicator mineral to evaluate the diamond-bearing potentiality of the kimberlite. In order to figure out the characteristics of spinels in the No. 30 kimberlite pipe, this paper has carried out detailed morphological observation and electron probe analysis on the spinels. The results show that spinels are elliptical and sub-Angular grains, with large particle diameter( 5(H)-1000 μm). A small amount of spinels shows erosional embay men t shape. With high contents of Cr2O3( up to 66. 56% ) and MgO ( 8. 88% - 16. 68% ) , and low TiO2, contents (mostlySpinels of No. 30 kimberlite pipe are mantle xenoerysts of the original mantle rock of perido-Tite. No. 30 kimberlite pipe has a certain capability to carry the diamond. Comparing with the kimberlite of the No. 50 pipe in the area and of the Changmazhuang, pipe in Mengying, Shandong, the diamond-bearing capability of the kimberlite of the No. 30 pipe is obviously weak. 相似文献
The crystal structure of MgFe2O4 was investigated by in situ X-ray diffraction at high pressure, using YAG laser annealing in a diamond anvil cell. Magnesioferrite
undergoes a phase transformation at about 25 GPa, which leads to a CaMn2O4-type polymorph about 8% denser, as determined using Rietveld analysis. The consequences of the occurrence of this dense MgFe2O4 form on the high-pressure phase transformations in the (MgSi)0.75(FeIII)0.5O3 system were investigated. After laser annealing at about 20 GPa, we observe decomposition to two phases: stishovite and a
spinel-derived structure with orthorhombic symmetry and probably intermediate composition between MgFe2O4 and Mg2SiO4. At pressures above 35 GPa, we observe recombination of these products to a single phase with Pbnm perovskite structure.
We thus conclude for the formation of Mg3Fe2Si3O12 perovskite.
Received: 27 March 2000 / Accepted: 1 October 2000 相似文献
Two MgAl2O4 stoichiometric spinel crystals, one natural and one synthetic, were heated from 25 to 950 °C and studied in situ by single-crystal
X-ray diffraction. The natural crystal, quenched from 850 °C, was further heated and cooled. Thermal expansion was characterized,
and cation partitioning at the various temperatures was determined according to a model purposely constructed for high-temperature
bond lengths. It was found that the structural evolution of the samples with temperature depended on order–disorder at room
temperature. At the temperatures lower than the beginning of cation exchange, thermal expansion was completely reversible
and the oxygen coordinate remained stable in spite of varying temperatures. At the temperature at which cation exchange starts,
the disordered samples first tend to order and then to disorder at higher temperatures, at variance with the ordered sample,
which tends to disorder steadily. In general, the evolution of the spinel structural state on cooling and heating over the
same temperature range and the same time intervals does not follow the same path. In particular, in the 600–950 °C range,
only partially reversible order–disorder processes occurred in the time span used for the experiments.
Received: 16 July 2001 / Accepted: 8 January 2002 相似文献
We present results from low-temperature heat capacity measurements of spinels along the solid solution between MgAl2O4 and MgCr2O4. The data also include new low-temperature heat capacity measurements for MgAl2O4 spinel. Heat capacities were measured between 1.5 and 300 K, and thermochemical functions were derived from the results.
No heat capacity anomaly was observed for MgAl2O4 spinel; however, we observe a low-temperature heat capacity anomaly for Cr-bearing spinels at temperatures below 15 K. From
our data we calculate standard entropies (298.15 K) for Mg(Cr,Al)2O4 spinels. We suggest a standard entropy for MgAl2O4 of 80.9 ± 0.6 J mol−1 K−1. For the solid solution between MgAl2O4 and MgCr2O4, we observe a linear increase of the standard entropies from 80.9 J mol−1 K−1 for MgAl2O4 to 118.3 J mol−1 K−1 for MgCr2O4. 相似文献
High-calcium, nepheline-normative ankaramitic basalts (MgO > 10 wt.%, CaO/Al2O3 > 1) from Rinjani volcano, Lombok (Sunda arc, Indonesia) contain phenocrysts of clinopyroxene and olivine (Fo85–92) with inclusions of spinel (Cr# 58–77) and crystallised melt. Olivine crystals have variable but on average low NiO (0.10–0.23 wt.%) and high CaO (0.22–0.35 wt.%) contents for their forsterite number. The CaO content of Fo89–91 olivine is negatively correlated with the Al2O3 content of enclosed spinel (9–15 wt.%) and positively correlated with the CaO/Al2O3 ratios of melt inclusions (0.9–1.5). Major and trace element patterns of melt inclusions are similar to that of the host rock, indicating that the magma could have formed by accumulation of small batches of melt, with compositions similar to the melt inclusions. The liquidus temperature of the magma was 1275 °C, and its oxygen fugacity ≤ FMQ + 2.5. Correlations between K2O, Zr, Th and LREE in the melt inclusions are interpreted to reflect variable degrees of melting of the source; correlations between Al2O3, Na2O, Y and HREE are influenced by variations in the mineralogy of the source. The melts probably formed from a water-poor, clinopyroxene-rich mantle source. 相似文献
A suite of spinel lherzolite and wehrlite xenoliths from a Devonian kimberlite dyke near Kandalaksha, Kola Peninsula, Russia, has been studied to determine the nature of the lithospheric mantle beneath the northern Baltic Shield. Olivine modal estimates and Fo content in the spinel lherzolite xenoliths reveal that the lithosphere beneath the Archaean–Proterozoic crust has some similarities to Phanerozoic lithospheric mantle elsewhere. Modal metasomatism is indicated by the presence of Ti-rich and Ti-poor phlogopite, pargasite, apatite and picroilmenite in the xenoliths. Wehrlite xenoliths are considered to represent localised high-pressure cumulates from mafic–ultramafic melts trapped within the mantle as veins or lenses. Equilibration temperatures range from 775 to 969 °C for the spinel lherzolite xenoliths and from 817 to 904 °C for the wehrlites.
Laser ablation ICP-MS data for incompatible trace elements in primary clinopyroxenes and metasomatic amphiboles from the spinel lherzolites show moderate levels of LREE enrichment. Replacement clinopyroxenes in the wehrlites are less enriched in LREE but richer in TiO2. Fractional melt modelling for Y and Yb concentrations in clinopyroxenes from the spinel lherzolites indicates 7–8% partial melting of a primitive source. Such a volume of partial melt could be related to the 2.4–2.5 Ga intrusion of basaltic magmas (now metamorphosed to garnet granulites) in the lower crust of the northern Baltic Shield. The lithosphere beneath the Kola Peninsula has undergone several episodes of metasomatism. Both the spinel lherzolites and wehrlites were subjected to an incomplete carbonatitic metasomatic event, probably related to an early carbonatitic phase associated with the 360–380 Ma Devonian alkaline magmatism. This resulted in crystallisation of secondary clinopyroxene rims at the expense of primary orthopyroxenes, with development of secondary forsteritic olivine and apatite. Two separate metasomatic events resulted in the crystallisation of the Ti–Fe-rich amphibole, phlogopite and ilmenite in the wehrlites and the low Ti–Fe amphibole and phlogopite in the spinel lherzolites. Alternatively, a single metasomatic event with a chemically evolving melt may have produced the significant compositional differences seen in the amphibole and phlogopite between the spinel lherzolites and wehrlites. The calculated REE pattern of a melt in equilibrium with clinopyroxenes from a cpx-rich pocket is identical to that of the kimberlite host, indicating a close petrological relationship. 相似文献
The magnetic behaviour and Curie temperatures (TC) of spinelloids and spinels in the Fe3O4–Fe2SiO4 and Fe3O4–(Mg,Fe)2SiO4 systems have been determined from magnetic susceptibility (k) measurements in the temperature range –192 to 700 °C. Spinelloid II is ferrimagnetic at room temperature and the k measurements display a characteristic asymmetric hump before reaching a TC at 190 °C. Spinelloid V from the Mg-free system is paramagnetic at room temperature and hysteresis loops at various low temperatures indicate a ferri- to superparamagnetic transition before reaching the TC. The TC shows a non-linear variation with composition between –50 and –183 °C with decreasing magnetite component (XFe3O4). The substitution of Mg in spinelloid V further decreases TC. Spinelloid III is paramagnetic over nearly the total temperature range. Ferrimagnetic models for spinelloid II and spinelloid V are proposed. The TC of Fe3O4–Fe2SiO4 spinel solid solutions gradually decrease with increasing Si content. Spinel is ferrimagnetic at least to a composition of XFe3O4=0.20, constraining a ferrimagnetic to antiferromagnetic transition to occur at a composition of XFe3O4<0.20. A contribution of the studied ferrimagnetic phases for crustal anomalies on the Earth can be excluded because they lose their magnetization at relatively low temperatures. However, their relevance for magnetic anomalies on other planets (Mars?), where these high-pressure Fe-rich minerals could survive their exhumation or were formed by impacts, has to be considered. 相似文献
The broadly N70°–90°E-trending dykes swarm at Kekem cut across the Paleoproterozoic-to-Achean terranes of West Cameroon remobilized during the Pan-African orogeny. They are picrite basalts and basalts with tholeiitic/transitional affinity, as shown by mineralogical and geochemical data, with variable major and trace element contents, MgO ranges from 7.3 to 12.4 wt.%, Cr from 190 to 411 ppm, Ni from 15 to 234 ppm. All the dykes are light REE enriched with LaN/YbN values of 5.3–8.1, suggesting a co-magmatic origin. They originated from a 2.8% partial melting of a spinel-mantle source with no or little crustal input. The geochemical features of Kekem dykes are similar to those of Paleozoic and Mesozoic dykes recorded in North and Central Africa, suggesting multiple reactivations of pre-existing fractures that resulted in the fragmentation of western Gondwana and the opening of Central and South Atlantic Oceans. 相似文献
Flux-grown Fe3+-bearing spinel s.s.–hercynite solid-solution crystals, (Mg1-yFe2+y)Al2O4 (0 < y≤ 1), have been investigated by means of electron microprobe technique and Mössbauer and electronic spectroscopy. Obtained results show that different electronic processes cause intense optical absorption bands in the near-infrared spectral region. In addition to an electronic d–d transition in single-ion IVFe2+, observed at 5200 cm?1, intense and broad bands at 9500 and 14 500 cm?1 are assigned to exchange-coupled pair (ECP) and intervalence charge-transfer (IVCT) transitions in VIFe2+VIFe3+clusters, respectively. The net linear extinction coefficients of these bands (α) were calibrated against Fe2+ and Fe3+ concentrations and site distributions previously defined by combined microchemical, Mössbauer, and XRD structural refinement data. The following expressions were obtained: where α is measured in cm?1 and concentrations are expressed in mol?l?1. The present results show that optical absorption spectroscopy may be used as a probe to obtain high spatial resolution (?~ 10 μm) information on Fe2+ ordering as well as on Fe3+ concentrations in minerals belonging to the spinel group. 相似文献