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1.
Most of the Al3+ entering the pyroxenes does so by substituting for tetrahedral Si4+. This creates a charge imbalance that requires the simultaneous entry of Cr3+, Ti4+, Fe3+ or Al3+ into octahedral sites. Cr3+, because of its high crystal field stabilisation energy (CFSE), is the most important of these elements to enter the early-formed pyrosenes but it is replaced by Ti4+ later in fractionation when the Cr3+ content of the melt becomes depleted. The dependence of Cr3+ and Ti4+ on charge balance controls their partition between coexisting pyroxenes and olivines. Ca-rich pyroxene which contains more Al3+ than Ca-poor pyroxene also has more Ti4+ and Cr3+ whereas olivine, which contains negligible Al3+, has low Cr3+ and Ti4+. The Al3+ content of pyroxenes is influenced by changes in P, T, \(a_{{\text{SiO}}_{\text{2}} }\) and \(a_{{\text{Al}}_{\text{2}} {\text{O}}_{\text{3}} }\) of the magma and by the nature of the ion providing charge balance in the octahedral site. Of these \(a_{{\text{SiO}}_{\text{2}} }\) is dominant and variations in the Al3+ content of the Jimberlana pyroxenes correspond closely with the expected changes in the \(a_{{\text{SiO}}_{\text{2}} }\) of the melt. The substitution of divalent ions, such as Mn2+ and Ni2+, in the pyroxene lattice is by replacement of Fe2+ or Mg2+ in the octahedral M 3 and M 2 sites and is therefore independent of charge balance. If there are no size restrictions, the principal factor to be considered is the CFSE the ion receives in octahedral co-ordination. Ni2+, which receives a high CFSE, partitions strongly between the early-formed pyroxenes and olivines and therefore becomes depleted in the magma with fractionation. Conversely Mn2+, which receives zero CFSE, concentrates in the magma with fractionation and becomes a more important substitute in the later-formed pyroxenes. Its geochemical behaviour is controlled by its size. The narrow miscibility gap of the Jimberlana pyroxenes and the high En content of the Ca-poor pyroxenes at the bronzite pigeonite changeover suggest that these pyroxenes crystallised at a higher temperature than pyroxenes of comparable composition from other intrusions.  相似文献   

2.
The electron spin resonance (ESR) spectrum of Cr3+ in a synthetic single crystal of forsterite doped with Cr2O3 was studied at room temperature in the X-band frequency range. The dependence of the observed spectra on the crystal orientation with respect to the applied magnetic field was investigated. The ESR spectra are described by the spin Hamiltonian \(H = \beta HgS + D(S_Z^{\text{2}} - {\text{1/3}}S{\text{(}}S{\text{ + 1)) + }}E{\text{(}}S_x^{\text{2}} - S_y^{\text{2}} {\text{)}}\) with S=3/2. The spin resonance reveals that the chromium ions are located at both the M1 and M2 positions. Other possible substitutional or interstitial Cr3+ positions may be possible, but were not observed. The site occupancy numbers of Cr3+ at M1 and M2 are roughly 1.2×10?4 and 0.8×10?4, respectively, assuming that chromium is oxidized completely. The preference of the chromium ions for M1 was interpreted qualitatively in terms of crystal field criteria. The rhombic and axial spin Hamiltonian parameters, D and E, and the directions of the magnetic axes obtained for M1 and M2 are consistent with the respective oxygen coordination polyhedra.  相似文献   

3.
A representative collection (138 analyses) of chromites from kimberlites of the Botuobinskaya pipe in Yakutia was studied. With allowance for the Cr, Ti, and Al contents, the chromites are subdivided into the low-Cr aluminous group A and the high-Cr and high-Ti group B. The chromites of group A with their compositional variations controlled by the Al3+-Cr3+ isomorphism are not related to kimberlite in composition and reveal attributes of restites. The chromites of group B with heterovalent Ti4+ + Fe2+ + Fe3+-3Cr3+ isomorphism vary in their composition in line with the compositional variations of kimberlites, thus demonstrating their primary magmatic origin. The chromites of the second group crystallized simultaneously with olivines from kimberlites, and both minerals could have formed nodules of spinel dunite.  相似文献   

4.
The crystal structure and site preference of Co2+ in a synthetic Co1.10Mg0.90SiO4 olivine have been determined from single crystal X-ray diffraction data collected on an automatic diffractometer. The R factor is 0.044 for 612 reflections. The site occupancies are: Ml site: Co 0.730±0.006; Mg 0.270; M2 site: Co 0.370, Mg 0.630. The Gibbs free energy change, ΔG° for the ion-exchange reaction between M1 and M2 sites is ?4.06 kcals/mole, assuming ideal mixing at each set of sites. This energy may be called ‘site preference energy’ of Co2+ in olivine. The strong preference of Co2+ for the M1 site can be quantitatively explained by two competing forces: preference of ions larger than Mg2+ for the M2 site and stronger covalent bonding of transition metal ions at the M1 site. For Fe2+, Mg2+, these two effects nearly neutralize each other, explaining the lack of considerable cation-ordering in Fe-Mg olivines.  相似文献   

5.
Variously colored gem-quality topazes from Ouro Preto, Minas Gerais, Brazil, were studied by optical absorption spectroscopy and photoluminescence methods. In the near infrared range (750–2500 nm) the absorption spectra display an identical pattern of narrow intense absorption lines caused by overtones and combination vibrations of OH groups, which do not relate to the coloration of the topazes studied. Their colors were found to be caused by combination of three sets of absorption features, (1), (2), and (3) in the visible and near-UV range, which are due to different color center. (1) denotes a pair of broad split bands with maxima 18 000 and 25 000 cm–1 caused by electronic spin-allowed dd transitions of Cr3+ ions. They cause a light rose to deep violet color and characteristic pleochroism of Cr3+-containing topazes. Photoluminescence evidences of at least three different types of Cr3+ complexes which, most probably, differ by ligand surroundings, O4F2, O4F(OH) and O4(OH) (2) Corresponds to the intense weakly polarized UV absorption edge. Two different parts, the thermally stable one, caused by ligand-to-metal charge transfer, and the thermally unstable one, caused by some defect center(s), contribute to the edge. (3) denotes a system of two broad unstructured bands with maxima around 19 000 cm–1 (X>Y Z) and 24 000 cm–1 (Y Z X). They cause the unique orange color and characteristic pleochroism of Brazilian Imperial topazes. Combinations of (1) and (3) absorption features cause various yellow-rose colors of the samples. Investigations of natural irradiated and thermally treated topazes show that the color centers (1) and (3) transform to each other at annealing and X- or gamma irradiation. The color of natural orange-red Imperial topazes is assumed to be caused by Cr4+, stabilized by other impurity ions and/or defect irradiation EPR centers. At T=300 °C Cr4+ reduces to Cr3+, the color of Imperial topazes changes to pale rose, caused by spin-allowed bands of Cr3+. In artificially irradiated crystals the (3)-center, Cr4+, may be induced according to the reaction 2Cr3+ Cr4+ + Cr2+, which involves chromium pairs in adjacent Al sites of the structure. Such artificially induced color is unstable at room temperature and in daylight. The process of the decay of (3)-centers may be described as a recombination Cr4++Cr2+ 2Cr3+ that results in vanishing of the (3)-bands accompanied by the appearance or increase in Cr3+ dd bands, the original orange color turning to a pale rose.  相似文献   

6.
The behaviour of tetrahedrally coordinated and octahedrally coordinated Cr3+ ions in diopside is discussed from the study on the join CaMg-Si2O6-CaCrCrSiO6. The molecule CaCrCrSiO6 decomposes into uvarovite+eskolaite and its maximum solubility in diopside is 6.7 wt percent at 940 ° C. Crystalline phases are diopside ss (ss is abbreviation of solid solution), uvarovite ss, wollastonite ss, spinel and eskolaite. The diopside ss is blue in colour. Its optical spectra were measured in the wavelenght range of 325–2600 nm, and assigned after tetrahedral configuration Td and octahedral configuration Oh. It is estimated that octahedral Cr3+ ions are in high spin state, while tetrahedral Cr3+ ions may be probably in low spin state. The t and B are 10,300–10,370 cm–1 and 429–432 cm–1. The CFSE for tetrahedral low spin Cr3+ ions is nearly the same as that for octahedral high spin Cr3+ ions. The ionic radii of tetrahedral low spin Cr3+ ions calculated are 0.47–0.53 Å, shrinked from the radius of octahedral high spin Cr3+ ion (0.615 Å) as much as 14–24 percent. Petrologic implications of the result are also discussed.The first half of the D. Sc. dissertation of K. Ikeda presented to Hokkaido University in June, 1976  相似文献   

7.
Olivine phenocrysts in microporphyritic xenoliths in the St. Mesmin chondrite (LL-breccia) show parallel rimward variations of FeO (10 to > -30wt.%), CaO (0·1–0·4%), MnO (0·2–0·8%) and Cr2O3 (0·2–0·7%). Aluminum is near background levels and does not vary systematically with iron. Titanium, sodium and nickel are below the limit of detection. Covariation of Fe and Cr in this olivine distinguishes it from the olivines in lunar mare basalts, in which Cr varies inversely with Fe.Transmission electron microscopy of the St. Mesmin olivine suggests that it is free of submicroscopic inclusions and exsolution lamellae and that the chromium present occurs in solid solution in the olivine. Charge balance and ionic radius considerations suggest that it occurs as Cr2+, whose effective ionic radius is nearly identical to that of Mn2+.The different Fe-Cr relationships observed in the lunar basaltic and St. Mesmin olivines reflect different crystallization sequences. Chromian spinel coprecipitated with olivine in the lunar basalts, reducing the activity of chromium in the melt and leading to the observed anticorrelation of Fe and Cr in olivine (butler, 1972). By contrast, olivine precipitated first in the St. Mesmin microporphyry and was the only solid phase present until more than half of the rock had crystallized. Parallel variation of Fe and Cr reflects crystallization from a melt in which the activity of chromium was increasing.  相似文献   

8.
Synthetic diopsides in the join CaMgSi2O6 CaCrAlSiO6 have been studied by means of crystal-field theory. These diopsides are either blue or pale green in colour. The former forms at lower temperatures and the latter at higher temperatures. The optical spectra and colours can be unequivocally explained based on the assumption that Cr3+ions occupy both tetrahedral and octahedral sites in the diopsides. In the blue diopsides Cr3+ions are present in two kinds of spin state, i.e., tetrahedrally coordinated low spin and octahedrally coordinated high spin. On heating the blue diopsides, tetrahedral occupancy of chromium decreases sharply due to spin transformation from tetrahedral low spin to octahedral high spin. Above 1,160° C nearly all chromium ions are present in octahedral sites with high spin state and the diopsides become pale green in colour. Some petrogenetic applications of the resultes are discussed.  相似文献   

9.
Olivine melilitites from Namaqualand, South Africa are characterized by a broad range in olivine compositions on the scale of individual hand specimens. It is possible to distinguish four petrographically and chemically distinct olivine populations in both the northern and southern pipe clusters studied: (a) Scarce anhedral or subhedral olivines that display marked disequilibrium features with the surrounding matrix, and which are characterized by having high iron and extremely low nickel contents (referred to as HILN olivines) relative to the other olivines in the same rock, (b) A dominant population of euhedral and often skeletal (hopper) olivines that are richer in Mg and Ni than the HILN olivines in the same rock. There are in addition unusual hopper olivines that are petrographically similar to the skeletal olivines, but show aberrant zonation patterns. Hopper and HILN type olivines contain fluid and carbonate inclusions which apparently record the loss of a vapour phase and an immiscible carbonate liquid during magma ascent, (c) A third population consists of large rounded olivines (megacrysts), up to 40 mm in greatest diameter. Individuals are chemically homogeneous, but megacrysts from the same pipe collectively define a trend of decreasing Mg and Ni (Fo92, 0.36% Ni to Fo75, 0.17% Ni). The most fayalitic megacrysts are depleted in Mg and Ni relative to the hopper olivines in the same rock, (d) Scarce magnesium-rich (Fo91) anhedral olivines which show strained extinction are believed to be xenocrysts.It is suggested that the HILN-type olivines crystallized from primitive carbonate-rich magmas under conditions of low oxygen fugacity, intermediate between the Ni-NiO and Fe-FeO buffers. Mineral-melt partition coefficients for the transition elements determined in basaltic systems are considered to be inappropriate to such carbonate-rich melts. Loss of volatiles and an immiscible carbonate liquid during magma ascent resulted in an increase in oxygen activity, a decrease in the Fe-Mg distribution coefficient (K D ) for olivine and liquid and an increase in liquidus temperatures. These effects led to the rapid crystallization of Mg- and Ni-enriched skeletal hopper olivines. The unusual hoppers crystallized later than the HILN olivines but prior to the normal hoppers, under conditions chracterized by rapid and independent changes in oxygen activity and partition coefficients associated with the loss of volatiles and an immiscible carbonate liquid. The range in chemistry which characterizes the megacryst-olivine suite is believed to record physico-chemical changes to the magmas subsequent to separation from a mantle source area, but prior to crystallization of the HILN olivines. Most important of these changes was an increasing degree of polymerization of the liquid structure and a progressive decrease in oxygen activity as the molar ratio (CO 3 2- /(CO 3 2- + CO2)) in the magma increased with decreasing pressure. Increasing polymerization of the liquid resulted in an increase in olivine-liquid partition coefficients for transition elements.Olivines in kimberlites show compositional characteristics and zonation patterns similar to those recognized in the olivine melilitites which, coupled with ilmenite compositions, suggests that the two magma types initially evolved along similar physico-chemical paths.  相似文献   

10.
Based on the measurements of refractive index,specific gravity,unit cell parameter,and mineral chemistry and infrared absorption spectrum analyses of pyropes in kimberlites from China,systematic studies of the Physical properties and compositional variations of pyropes of different colors and diverse paragenetic types,within and between kimberlite provinces have been undertaken,The origin of pyropes in the Kimberlites and the depth of their formation have been discussed.Pyropes of the purple series are different from those of the orange series in physical and chemical properties,for exaple,pyropes of the puple series are higher in α0,RI,SG,Cr2O3,MgO,Cr/(Cr Al),Mg/(Mg Fe),and Mg/(Mg Ca),and lower in Al2O3,Fe2O3 FeO than those of the orange series.The classification of garnets in kimberlites from china by the Dawson and Stephens‘ method(1975) has been undertaken and clearly demonstrates that pyropes of diamond-rich kimberlites contain much more groups than those of diamond-poor,especially diamond-free kimberlites.The higher in α0,RI,SG,Cr2O(3.Cr/(Cr Al),knorringite and Cr-component the pyropes are ,the richer in diamond the kimberlites will be.The infrared absorption spectrum patterns of pyropes change with their chemical composition regularly,as reflected in the shape and position of infrared absorption peaks.Two absortpion bands at 862-901 cm^-1 will grade into degeneration from splitting and the absorption band positions of pyropes shift toward lower frequency with increasing Cr2O3 content and Cr/(Cr Al) ratio of pyropes,LREE contents of orange pyrope megacrysts are similar to those of porple pyrope macrocrysts,but the former is higher in HREE than the latter,showing their different chondrite-normalized patterns.The formation pressures of pyropes calculated by Cr-component,Ca-component,knorringite molecules of pyropes show that some pyropes of the purple series in diamondiferous kimberlites fall into the diamond stability field.but all pyropes of diamond-free kimberlites lie outside the diamond stability field.The megacrysts were formed through early crystallization of kimberlites magma at high pressure condition,the majority of the purple pyrope macrocrysts have been derived from disaggregated xenoliths but the minoirty of them appear to be fragments of the discrete megacryst pyropes,or phenocrysts.  相似文献   

11.
The exsolutious of diopside and magnetite occur as intergrowth and orient within olivine from the mantle dunite, Luobusa ophiolite, Tibet. The dunite is very fresh with a mineral assemblage of olivine (〉95%) + chromite (1%-4%) + diopside (〈1%). Two types of olivine are found in thin sections: one (Fo = 94) is coarse-grained, elongated with development of kink bands, wavy extinction and irregular margins; and the other (Fo = 96) is fine-grained and poly-angied. Some of the olivine grains contain minor Ca, Cr and Ni. Besides the exsolutions in olivine, three micron-size inclusions are also discovered. Analyzed through energy dispersive system (EDS) with unitary analytical method, the average compositions of the inclusions are: Na20, 3.12%-3.84%; MgO, 19.51%-23.79%; Al2O3, 9.33%-11.31%; SiO2, 44.89%-46.29%; CaO, 11.46%-12.90%; Cr2O3, 0.74%-2.29%; FeO, 4.26%- 5.27%, which is quite similar to those of amphibole. Diopside is anhedral f'dling between olivines, or as micro-inclusions oriented in olivines. Chromite appears euhedral distributed between olivines, sometimes with apparent compositional zone. From core to rim of the chromite, Fe content increases and Cr decreases; and A! and Mg drop greatly on the rim. There is always incomplete magnetite zone around the chromite. Compared with the nodular chromite in the same section, the euhedral chromite has higher Fe3O4 and lower MgCr2O4 and MgAI2O4 end member contents, which means it formed under higher oxygen fugacity environment. With a geothermometer estimation, the equilibrium crystalline temperature is 820℃-960℃ for olivine and nodular chromite, 630℃-770℃ for olivine and euhedral chromite, and 350℃-550℃ for olivine and exsoluted magnetite, showing that the exsolutions occurred late at low temperature. Thus we propose that previously depleted mantle harzburgite reacted with the melt containing Na, Al and Ca, and produced an olivine solid solution added with Na^+, Al^3+, Ca^2+, Fe^3+, Cr^3+. With temperature d  相似文献   

12.
For the first time ever, the luminescence spectra of Cr3+ centers in two chlorite crystals are presented. Chromium ions occupy the strong crystal-field site M4 in the brucite sheet and the intermediate crystal-field site in the inner octahedral sheet for purple and green chlorite, respectively. We discuss the influence of an effective positive charge on the Cr3+ ion and an effective negative charge of ligands on the differences in the values of the Dq and B parameters. It is concluded that the presence of Fe2+ ions and other point defects, as well as concentration quenching, causes the very short luminescence lifetimes of chromium ions.  相似文献   

13.
The electron paramagnetic resonance (EPR) spectrum of Cr3+ in synthetic crystals of forsterite consists primarily of lines of Cr3+ “isolated” at the M1 and M2 positions in a “perfect” crystal environment without local charge compensation. In addition it shows two nonequivalent superhyperfine-split sextets with different intensities which are due to strong interaction of the Cr3+ electron spin S (S=3/2) with an adjacent nuclear spin I(I=5/2). Electron nuclear double resonance (ENDOR) experiments revealed that the sextets result from Cr3+ (M1) - Al3+ and Cr3+ (M2) - Al3+ pairs, Al being located at adjacent tetrahedral Si sites. The g, D, A, and A′ tensor components of the Cr3+ - Al3+ pairs have been determined at room temperature. The values of the pairs are distinct although they are not very different from the corresponding data of “isolated” Cr3+. From the intensities of the EPR spectra the relative concentration of the Cr3+ - Al3+ pairs with respect to “isolated” Cr3+ has been estimated.  相似文献   

14.
Based on the results of more than 600 electron microprobe analyses of 25 minerals the distribution pattern of the Cr6+ impurity in vanadates, phosphates, and arsenates collected in oxidation zones of six ore deposits of the Urals was studied. Among them are Pb minerals of the brackebuschite, apatite, adelite, and tsumcorite groups and alunite supergroup, as well as carminite, cornwallite, and bayidonite. Vanadates and arsenates with brackebuschite-type structures show a high affinity to Cr6+. The maximum content of the Cr6+ impurity is characteristic of minerals with specified Fe3+ trivalent cations (ferribushmakinite, arsenbrackebuschite, and gartrellite) or Al3+ (plumbogummite and bushmakinite). The prevailing scheme of isomorphous substitution, according to which chromium enters into the compositions of these minerals, is heterovalent: Cr6+ + M 2+Т 5+ + M 3+ (where Т = V, As, P; M 3+ = Fe, Al; M 2+ = Сu, Zn), whereas the role of isovalent substitutions Cr6+ → S6+ and Cr6+ → Mo6+ in oxosalts that formed in mineral occurrences of the Urals is insignificant.  相似文献   

15.
Polarized electronic absorption spectra, Ea(∥X), Eb(∥Y) and Ec(∥Z), in the energy range 3000–5000?cm–1 were obtained for the orthorhombic thenardite-type phase Cr2SiO4, unique in its Cr2+-allocation suggesting some metal-metal bonding in Cr2+Cr2+ pairs with Cr-Cr distance 2.75?Å along [001]. The spectra were scanned at 273 and 120?K on single crystal platelets ∥(100), containing optical Y and Z, and ∥(010), containing optical X and Z, with thicknesses 12.3 and 15.6?μm, respectively. Microscope-spectrometric techniques with a spatial resolution of 20?μm and 1?nm spectral resolution were used. The orientations were obtained by means of X-ray precession photographs. The xenomorphic, strongly pleochroic crystal fragments (X deeply greenish-blue, Y faint blue almost colourless, Z deeply purple almost opaque) were extracted from polycrystalline Cr2SiO4, synthesized at 35?kbar, above 1440?°C from high purity Cr2O3, Cr (10% excess) and SiO2 in chromium capsules. The Cr2SiO4-phase was identified by X-ray diffraction (XRD). Four strongly polarized bands, at about 13500 (I), 15700 (II), 18700 (III) and 19700 (IV) cm–1, in the absorption spectra of Cr2SiO4 single crystals show properties (temperature behaviour of linear and integral absorption coefficients, polarization behaviour, molar absorptivities) which are compatible with an assignment to localized spin-allowed transitions of Cr2+ in a distorted square planar coordination of point symmetry C2. The crystal field parameter of Cr2+ is estimated to be 10?Dq?10700?cm–1. A relatively intense, sharp band at 18400?cm–1 and three other minor features can, from their small half widths, be assigned to spin-forbidden dd-transitions of Cr2+. The intensity of such bands strongly decreases on decreasing temperature. The large half widths, near 5000?cm–1 of band III are indicative of some Cr-Cr interactions, i.e. δ-δ* transitions of Cr2 4+, whereas the latter alone would be in conflict with the strong polarization of bands I and II parallel [100]. Therefore, it is concluded that the spectra obtained can best be interpreted assuming both dd-transitions of localized d-electrons at Cr2+ as well as δ-δ* transitions of Cr2 4+ pairs with metal-metal interaction. To explain this, a dynamic exchange process 2 Crloc 2+?Cr2, cpl 4+ is suggested wherein the half life times of the ground states of both exchanging species are significantly longer than those of the respective optically excited states, such that the spectra show both dd- and δ-δ*-transitions.  相似文献   

16.
The natural occurrence of hydroxide in olivine   总被引:1,自引:0,他引:1  
Polarized infrared (IR) spectra of olivine single crystals from 17 different localities show a tremendous variability in both mode and abundance of hydroxide (OH) incorporation. Kimberlitic olivines contain the most total OH at an estimated concentration level of 976 H/106Si, whereas olivines from basalts contain the least at 3 H/106Si. Olivines of metamorphic and hydrothermal origin have widely varying concentration levels intermediate between those of basalts and kimberlites. Over 30 distinct OH absorption bands have been identified. Most of these bands are not unique to individual localities but may be found in samples from several different localities. Pleochroism is consistent among localities, but relative band intensities vary. No evidence is found for molecular H2 in olivine. Hydrous minerals have been identified in olivine by their characteristic OH absorption bands. Serpentine is commonly found and is clearly distinguishable from intrinsic OH. Talc is present in one sample. Prominent OH bands at 3572 and 3525 cm?1 are attributed to humite group minerals. San Carlos, Arizona, olivines annealed in the presence of H2O develop absorption bands which are found in natural samples, however the OH absorption spectra of these annealed olivines are not identical to those of any single natural crystal. Sharp-band OH abundances in annealed samples are an order of magnitude lower than the maximum measured in natural specimens. The mechanical properties determined from these annealed olivines may not be directly applicable to mantle olivine because both the OH sites and concentrations are different.  相似文献   

17.
Carbonate-rich, SiO2-poor residua are developed in some kimberlites solidifying as ocelli, layers, or discrete dikes which satisfy petrographic definitions of carbonatite. Arguments that these rocks have mineralogies, antecedents, and comagmatic rocks differing from those of the carbonatites in alkaline rock complexes, including the specific observation that kimberlites and carbonatites contain ilmenites and spinels of different composition, have been used to refute the alleged kimberlite-carbonatite relationship. New microprobe analyses of ilmenites and spinels from carbonate-rich rocks associated with kimberlites in three South African localities correspond to spinels and ilmenites of carbonatites from alkalic complexes, or have characteristics intermediate between those of carbonatites and kimberlites. The ilmenites are distinguished from kimberlite ilmenites by higher MnO, FeTiO3, and Nb2O5, and by negligible Cr2O3. The spinels are distinguished from kimberlite spinels by their Al2O3 and Cr2O3 contents. There is clearly a genetic relationship between the kimberlites and the carbonate-rich rocks, despite the observation that their ilmenites and spinels are distinctly different, which indicates that the same observation is not a valid argument against a petrogenetic relationship between kimberlites and carbonatites. These rocks are among the diverse products from mantle processes influenced by CO2, and we believe that the petrogenetic links among them are forged in the upper mantle. We see insufficient justification to deny the name carbonatite to carbonate-rich rocks associated with kimberlites if they satisfy the petrographic definition in terms of major mineralogy.  相似文献   

18.
Orissa is an important area for gem variety of corundum deposits in India. Spectroscopic studies, such as ESR, OAS on samples from Sardapur, Orissa, were carried out to ascertain the colour cause of corundum. Electron spin resonance (ESR) spectroscopic study was carried out on the samples to detect the presence of paramagnetic ions i.e. Fe2+, Fe3+, Ti4+, Cr3+ and V3+ etc. The variable temperature experiment carried out to observe the effect of heating on peak valence state change in paramagnetic ions. Samples were cut and polished to obtain the optical absorption spectrum (OAS) to detect the colour causing transition ions/defect centres. The samples of gem variety were step heated up to 300°C for colour enhancement studies. EPMA analysis has revealed the low chromium concentration in the rubies. The varying hues of red in the corundum are due to the presence of bivalent and trivalent iron and charge transfer process along with Cr3+ absorption in the 550 nm region.  相似文献   

19.
Data on the structural and valence distribution of Cr and Fe in chrysoberyl and in alexandrite, its gem variety, are given. It is shown that the Cr3+ line in the natural Ural and Tanzania samples is the strongest in the M1 site and for the synthetic stones, in the M2 site. During the annealing of the alexandrite crystals, Cr3+ passes from the smaller M1 site into the larger M2 site. The M?ssbauer spectroscopy quantitatively determined the distribution of different valence Fe ions. The various proportions of both Fe2+ and Fe3+ ions isomorphically entering the octahedral sites in the BeAl2O4 crystal structure were established.  相似文献   

20.
The crystal structure and chemical composition of a crystal of (Mg14?x Cr x )(Si5?x Cr x )O24 (x ≈ 0.30) anhydrous Phase B (Anh-B) synthesized in the model system MgCr2O4–Mg2SiO4 at 12 GPa and 1600 °C have been investigated. The compound was found to be orthorhombic, space group Pmcb, with lattice parameters a = 5.900(1), b = 14.218(2), c = 10.029(2) Å, V = 841.3(2) Å3 and Z = 2. The structure was refined to R 1 = 0.065 using 1492 independent reflections. Chromium was found to substitute for both Mg at the M3 site (with a mean bond distance of 2.145 Å) and Si at the octahedral Si1 site (mean bond distance: 1.856 Å), according to the reaction Mg2+ + Si4+ = 2Cr3+. Such substitutions cause a reduction in the volume of the M3 site and an increase in the volume of the Si-dominant octahedron with respect to the values typically observed for pure Anh-B and Fe2+-bearing Anh-B. Taking into account that Cr3+ is not expected to be Jahn–Teller active, it appears that both the Cr3+–for–Mg and Cr3+–for–Si substitutions in the Anh-B structure decrease the distortion of the octahedra. Electron microprobe analysis gave the Mg13.66(8)Si4.70(6)Cr0.62(4)O24 stoichiometry for the studied phase. The successful synthesis of this phase provides new information for the possible mineral assemblages occurring in the Earth’s deep upper mantle and shed new light on the so-called X discontinuity that has been observed at 275–345 km depth in several subcontinental and subduction zone environments.  相似文献   

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