1,721,042 research outputs found
Crystal chemistry of a clinopyroxene series in ultrafemic xenoliths from North-Eastern Brazil
No full textA series of clinopyroxenes (Cpx) in peridotitic
spinel nodules from Rio Grande do Norte, North-Eastern
Brazil, was investigated by X-ray diffraction and electron
microprobe analyses and compared with an analogous series
from spinel peridotite nodules from Mt. Leura, Victoria,
Australia. The examined Cpx series is mainly characterized
by increasing Mg/(Mg+Fe 2+) ratio (i.e. increase of
refractory character of the host nodules), by the substitutions
of A1 vi vs Mg in the MI site and Navs (Ca+Mg)
in the M2 site, whereas A1 iv in the T site remains substantially
constant. The strong Na depletion in the Brazilian
Cpx causes a different charge balance from that in the Australian
Cpx, in which Na remains quite constant. This feature,
associated with the differences in polyhedral and cell
volumes which are significantly higher in the Brazilian Cpx
than in those of the Australian Cpx, suggests that the Brazilian
Cpx crystal chemistry may be related to a lower pressure
regime, compared with that of the Australian Cpx series
Temperature, composition and fO2 effects on intersite distribution on Mg and Fe2+ in olivines
No full textmodello cristallochimico sugli effetti della fugacità di ossigeno ai fini della solubilità del ferro in miscele ortosilicatiche Pbn
CRYSTAL CHEMISTRY OF CLINOPYROXENES IN SPINEL-PERIDOTITE MANTLE XENOLITHS FROM THE FERNANDO DE NORONHA OCEANIC ISLAND (NE-BRAZIL)
No full textSix c2/c clinopyroxenes (cpx) of spinel peridotite mantle xenoliths enclosed in the alkali basalts from the Fernando de Noronha (FN) oceanic island (NE-Brazil) were studied by means of X-ray diffraction (single crystal) and electron microprobe analyses. Fn-cpx crystal chemistry was compared with those of clinopyroxenes of spinel-peridotite mantle xenoliths enclosed in alkali basalts related to continental extensional tectonics from NE-Brazil and Mt. Leura (Australia). The equilibration pressure estimates by FN-cpx crystal chemistry through cell and site volumes relationships evidences as the mantle xenoliths from Fernando de Noronha oceanic island were sampled at higher depth with respect to the continental analogues from NE- Brazil
Characterization of the pressure induced Ringwoodite toMg-perovskite and Mg-wüstite phase transition by Catastrophe Theory
No full textClinopyroxene of spinel-peridotite mantle xenoliths from lake Nji (Cameroon volcanic line, W Africa): crystal chemistry and petrological implications
No full textThe crystal chemistry of six clinopyroxenes enclosed in protogranular spinel-peridotite mantle xenoliths from Lake Nji (Cameroon, W Africa) was studied by means of single-crystal X-ray diffraction and electron microprobe. These spinel-peridotite xenoliths are characterised by clinopyroxene contents distinctly higher than those reported by Lee et al. (1996) for spinel-peridotite xenoliths of the same region (19–11% and 15–8% respectively) and by high clinopyroxene/orthopyroxene ratios: 0.7–2.7 (present study) and about 0.4 (Lee et al. 1996). The clinopyroxene crystal chemistry indicates that the xenoliths show the compositional features of a mantle source contaminated by small-volume melts, responsible for the high clinopyroxene content. The protogranular texture of the Cameroon xenoliths (upper portion of lithospheric mantle) testifies that the spinel peridotite and the associated small-volume melts completely re-equilibrated at the spinel-peridotite facies. This is also supported by the petrological and geochemical data of Lee et al. (1996) on the Etinde–Biu Plateau spinel-peridotite xenoliths, which underwent Late Proterozoic–Early Paleozoic enrichment in incompatible trace elements
New insight on element bioaccumulation performance of two lichen biomonitors: When morpho-chemical details mark the difference
No full textIn biomonitoring of airborne elements with native lichens, two allopatric species could be used jointly only when bioaccumulation mechanisms are the same, and differences negligible. The present study aims at (i) investigating the bioaccumulation performance for 12 elements in one-year-old lobes of Flavoparmelia caperata and Xanthoria parietina (i.e. the two lichen native biomonitors most used in the peri-Mediterranean regions) collected side-by-side on the same trees at 17 sampling sites. Both species were characterized morphologically and by X-ray diffraction analysis to evaluate specific thallus area and oxalate content, respectively. Results show that Ca and chalcophile elements (Cd, Cu, Pb and Zn) were more concentrated in F. caperata, lithophile and siderophile elements (Cr, Fe, and Ti) in X. parietina, whereas Al, Li, Mn, and Ni were accumulated in similar amounts. The outstanding content of Ca-oxalate deposits in F. caperata underlies the higher content of Ca, Cd and Pb observed in this species. On the contrary, the micro-anatomy of the upper cortex of X. parietina, which is more porous than in F. caperata, allows a higher transfer of litophile and siderophile elements. The adoption of interspecific calibration factors for biomonitoring purposes is thoroughly discussed, and the choice of using a single native species made by the most recent guidelines for biomonitoring airborne elements with lichens is fully supported
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