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Crystal chemcal study of R-3c natural oxides along the eskolaite - karelianite - hematite (Cr2O3 -V2O3-Fe2O3) join
No full textSix natural crystals from the Sludyanka crystalline complex belonging to the eskolaite (Cr2O3)–
karelianite (V2O3)hematite (Fe2O3) solid solution were studied by means of X-ray diffraction and
electron microprobe. The Fe3+-poor samples show a general increase in a and c cell parameters with
increasing mean cationic radius (MCR), consistent with that shown by the synthetic crystals along the
eskolaite–karelianite join. The Fe3+-richer sample deviates significantly from the behaviour shown by
the Fe3+-poor ones, similar to synthetic andnatural hematites; with increasing MCR, the a and c cell
parameters increase linearly along the eskolaite–karelianite join. However, for the samples rich in Fe3+,
from karelianite to hematite, a shows a slightly steeper slope whereas the c parameter decreases
strongly. The octahedral distortion increases slightly as a function of MCR along the eskolaite–
karelianite join, whereas it increases markedly for Fe3+-rich samples. The evolution of the octahedral
edges and of the octahedral distortions as a function of MCR are responsible for the behaviour of the
unit-cell parameters along the eskolaite–karelianite–hematite join
Distinctive Crystal-chemistry and Site Configuration of the Clinopyroxene From Alkali Basaltic Rocks - the Nyambeni Clinopyroxene Suite, Kenya
No full textA crystal chemical investigation of clinopyroxenes from a suite of nepheline-bearing lavas located in the Nyambeni Range of Kenya has delineated the polyhedral site configurations and related intracrystalline relationships. These are distinct from those determined for the clinopyroxene in an analogous suite of leucite-bearing lavas from the Sabatini volcanoes in the Roman Region of Italy (Dal Negro et al. 1985).
The Nyambeni clinopyroxene, varying from salite to hedenbergite, preferentially accepts Na in the M2 site to balance increasing Fe2+ and Si, respectively, whereas the Sabatini clinopyroxene is confined within the salite field and preferentially accepts Aliv to balance the effect of increasing (Fe3++Ti4++Alvi+Cr3+)M1.
The Fe2+/Fe3+ and K/Na ratios of the host rocks emerge as significant factors in determining the different polyhedral configurations and evolutions of the clinopyroxene from the two lava suites, respectively. The resulting Mg-Fe2+ order-disorder relationships in M1–M2 are also distinct in the two clinopyroxenes. A high degree of MgFe2+ order in M1–M2 corresponds to the largest configurational, hence energetic, difference between M1 and M2 in the Nyambeni clinopyroxene, whereas the converse applies to the Sabatini clinopyroxene.
In view of the significant crystal chemical differences and distinct evolution trends, it is proposed that salites from alkali volcanic rocks may be referred to as Nyambeni-type or Sabatini-type, respectively
THE EFFECT OF CATION ORDERING ON THE HIGH-PRESSURE CRYSTAL STRUCTURE BEHAVIOUR of natural spinel MgAl2O4
No full textAbstract 3.3.i
Residual Electron-density At the M2 Site In C2/c Clinopyroxenes - Relationships With Bulk Chemistry and Sub-solidus Evolution
No full textThe 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 labelledM2prime, 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 theM2prime peak increases while its co-ordinates move towardM2; b) as (Ca + Na) content approaches 1.0 atom per formula unit,M2prime vanishes; c)M2prime 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 ofM2prime 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 + M2prime 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
Crystal-chemical examination of natural diopsides: some geometrical indications of Si-Ti tetrahedral substitution
No full textThe crystal structures of 20 natural diopsides have been refined. For some samples, microprobe analyses show a Si deficiency in the tetrahedron with no A1 present; this suggests that some Ti4+ may enter the tetrahedral site. Geometrical variations in these samples with respect to the others are discussed in the light of this Si Ti4+ substitution. The volumes of their M1 sites are larger because of the absence of R3+ cations, the octahedron being very regular; in the same way, the volumes of M2 sites of these samples are greater not because of different site occupancies but because of significant charge modifications around the O3 and O1 oxygens, caused by the tetrahedral site. The low distortion of M1 site and the lengthening of (M2-O1) and M2-O3 bond lengths are a clue to the presence of small amounts of Ti at the tetrahedral site
Crystal chemistry of Ca-rich pyroxenes from undersaturated to oversaturated trachytic rocks, and their relationships with pyroxenes from basalts
No full textA study of 23 Ca-rich pyroxenes from silica-undersaturated and -oversaturated rocks, shows that all the pyroxenes from trachytes are characterized by a relatively large M1 site, related to its high Fe2+ occupancy. The volume of the M1 site is inversely related to that of the tetrahedral site. The high Si content of the pyroxene from trachytes causes significant increase in the longest M2-O3C2,D2 distances.
The most striking difference between the pyroxenes from trachytes and those from basalts is found in the volumes of the M1 and T sites, and in the degree of distortion of the M2 polyhedron (ΔM2).
The relationship between the β angle and ΔM2 provides an estimate of: (1) the degree of alkalinity in trachytic melts, and (2) indications on the nature of their possible parental “basaltic” magmas
Crystal chemistry and site configuration of the clinopyroxene from leucitite-bearing rocks and related significance: the Sabatini lavas, Roman region Italy
No full textA clinopyroxene suite from leucite–bearing lavas located in the Sabatini district of the Roman Volcanic Region has been investigated by single–crystal X–ray diffraction methods combined with electron probe microanalysis. The main aim of the study was to obtain crystallographic data necessary to evaluate intracrystaUine order–disorder relationships in natural clinopyroxenes and related equilibria–disequilibria phenomena in different host–rock types. Generally, the polyhedral site configurations of the clinopyroxenes investigated are interdependent and follow specific variation trends related to specific host–rock types. The major variations are found in the tetrahedron and the octahedron; while the polyhedron M2 is essentially unchanged and occupied by(Ca+Na). The degree of Mg–Fe2+; ordering in M1–M2, respectively, depends on the configuration of these sites, particularly M2, and is generally lower in late–crystallized (rim) clinopyroxenes than in the corresponding higher–temperature (core) from the same crystals. This is' attributed to configuration relationships unfavourable to the Mg–Fe2+ ordering and largely depending on the occupancy of M2. Polygenetic, early–crystallized clinopyroxenes, occasionally with resorbed margins, indicate crystal-liquid disequilibria related to crystal and–or magma mixing processes
Petrogenetic application of Mg-Fe2+ order-disorder in orthopyroxene to the cooling history of rocks
No full textA brief review of the theory and data on the Fe²⁺-Mg order-disorder in orthopyroxene and the relation among the kinetic rates of intracrystalline and intercrystalline ion-exchanges and cooling of the host rocks has been presented. The recognition of these competitive processes makes it possible to constrain the cooling history of the host rocks. The currently existing data on heated pyroxenes introduce great uncertainties in the calculations but it appears that many metamorphic granulites may have cooled rather rapidly. The calculated cooling rates lie in the ranges of 0.06 to 19°/year and 2°/minute to ll°/day for metamorphics and volcanics respectively. These rates are subject to errors of several orders of magnitude and may be meaningless if the nature of the rate process is different below about 400 °C.Il est présenté un court compte rendu de la théorie et des données concernant l'ordre-désordre de Fe²⁺-Mg dans les orthopyroxènes et le rapport entre la cinétique des échanges ioniques intracristallins et intercristallins et le refroidissement des roches. La connaissance de ces processus compétitifs permet de cerner l'histoire du refroidissement des roches hôtes. Les données que l'on possède sur les pyroxènes chauffés font apparaître de grosses incertitudes dans les calculs mais il paraît évident que beaucoup de granulites métamorphiques peuvent subir un processus de refroidissement assez rapide. Les vitesses de refroidissement calculées se situent entre 0,06 et 19°/ an et 2°/minute et ll°/jour respectivement pour les roches métamorphiques et volcaniques. Ces vitesses sont sujettes à des erreurs de plusieurs ordres de grandeur et peuvent ne pas être significatives si la vitesse du processus subit des variations en-dessous de 400 °C environ.Saxena Surendra Kumar, Dal Negro Alberto. Petrogenetic application of Mg-Fe2+ order-disorder in orthopyroxene to the cooling history of rocks. In: Bulletin de Minéralogie, volume 106, 4, 1983. pp. 443-449
Crystal chemistry of Ti3+-Ti4+-bearing synthetic diopsides
No full textA series of Ti-doped diopsides were synthesised by flux growth in a boron-rich melt under low fO2 conditions. Following chemical characterisation by EMPA and nuclear reaction microanalysis, their crystal structures were refined by single crystal X-ray diffraction. The diopside samples were doped with increasing amounts of Ti, resulting in a coupled increase in Na and B. The tetrahedral Si is substituted by Ti4+ and B, whereas Mg is substituted by Ti3+ and Ti4+ in the M1 octahedron with simultaneous charge compensation by coupled substitution of Na for Ca in the M2 octahedron. The occurrences of Ti4+ in the tetrahedral site and Ti3+ in the M1 octahedron are in agreement with optical absorption studies performed on the same samples. The diopside framework is strongly influenced by the incorporation of Ti4+ and Ti3+, with the M1 and M2 sites becoming more distorted as the substitutions increase, accompanied by a decreasing distortion of the T-site and the T-chain and an increase of the c lattice parameter
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