A. Mottana

Symmetry dependence of x-ray absorption near-edge structure at the metal K edge of 3d transition metal compounds

The pre-edge features in a system with even symmetry, apart from quadrupolar transition contribution, are mainly dipolar in character, associated with the existence of unoccupied states made up of mixed cation np with higher-neighboring cation- (n-1) d orbitals, and reflect the density of states due to the medium-range order of the system, while in odd symmetry materials these pre-edge features are the result of a transition from the Is to a final density of states of p symmetry due to an unsymmetrical mixing of the ligand wave functions with the central cation d orbitals. In the latter case, they contain not only the p but also the d base of orbitals, similar to a tetrahedral configuration. These results are validated for Fe as a photoabsorber by comparing x-ray absorption near-edge spectra of Fe2SiO4 (fayalite) and Fe2O3 (hematite) to ab initio full multiple scattering calculations at the Fe K edge, but pertain to all systems containing sixfold-coordinated cations

Crystal-chemical evaluation of garnet and omphacite microprobe analyses: Its bearing on the classification of eclogites

Abstract The reliability of 920 clinopyroxene and 804 garnet microprobe analyses performed from 1964 through 1984 has been evaluated on the basis of crystal-chemical constraints resulting from the latest structural investigations. Accepted were 343 pyroxenes and 28 garnets which were increased to 444 and 287 respectively, after relaxation of some of the initial rigorous constraints, so as to meet the number suitable for statistical processing. The graphical portray of the accepted data permits the detection of the chemical elements significant in discriminating these eclogite minerals in three groups, related to the geological settings of the enclosing rocks and therefore to their P - T conditions of formation and to their small variations in bulk chemistry. The best discriminant plots were found to be the Ca + Mgue5f8Na + Al VI ue5f8Na + Fe 3+ triangle for clinopyroxenes (essentially the Di + Jd + Acm triangle), and the Caue5f8Mgue5f8Fe 2+ triangle for garnets (essentially the Gross-Pyr-Alm triangle). Their updated versions are reported. In addition, the data suggest that the P - T regime acting during subduction metamorphism was remarkably constant in space and in time; they hint for a future possible division of common regional-metamorphic eclogites into three isofacial groups, provided that the statistics will be increased and additional geological information becomes available; finally, they point out that the peculiar crystal chemistry of the minerals in kimberlite xenoliths may require reconsideration of the constraints inferred from crustal occurrences.

The eclogite-bearing series of Isla Margarita, Venezuela: Geochemistry of metabasic lithologies in the La Rinconada and Juan Griego Groups

Abstract XRF analysis for major and minor elements in 94 samples of the metabasic La Rinconada Group and in 28 samples of metabasic lithologies in the predominantly metasedimentary, overlying Juan Griego Group of Isla Margarita, Venezuela, has been carried out to compare the bulk compositions of the two eclogite-bearing metabasic suites. The two units differ in the age and geological setting of the protolith. The La Rinconada Group has a monotonous basaltic character, but shows irregular variations in bulk composition which are interpreted to reflect variations in the type of protolith, ranging from basaltic lava flow to gabbroic, cumulitic dyke. As a whole, the La Rinconada Group is related to a source of tholeiitic composition in a MORB-type volcanic-hypabyssal environment. The Juan Griego metabasic rocks are compositionally more homogeneous and uniformly basaltic in their major elements, again reflecting a tholeiitic source and a MORB-type environment. Despite their contrasting geological setting, the two suites of metabasic rocks can be inferred to have had a similar source and have probably developed in a passive, “Atlantic”-type continental margin setting. We infer that the La Rinconada Group represents early oceanic crust later covered by the siliceous, continent-derived clastics of the Juan Griego Group, in which the minor basaltic intrusions represent the last stages of waning magmatic activity.

Xanes analysis on pyroxenes with different ca concentration in M2 site

X-ray Absorption Near-Edge Structure (XANES) analysis of the calcium K-edge of a series of natural pyroxenes is reported. The samples belong to the solid solution series diopside (CaMgSi2O6) — jadeite (NaAlSi2O6). In diopside, the M2 site is occupied by Ca only, but along the join Na substitutes Ca in this position. From XANES analysis of different samples we found a distortion of the polyhedron around the M2 site varying as a function of Ca content. This is probably due to compression of the site in a selected direction with an unchanged average distance, so that the coordination around the Ca atom changes from the 4-2-2 configuration typical of the diopside structure to the 6-2 configuration typical of Na in the jadeite structure. Intermediate pyroxenes exhibit both configurations, and acquire therefrom the structural order as already detected by X-ray diffraction techniques.

Equilibration conditions of eclogite lenses from Isla Margarita, Venezuela: Implications for the tectonic evolution of the metasedimentary Juan Griego Group

Abstract The constituent primary and secondary minerals in a suite of fifteen samples of eclogite, amphibole-eclogite and garnet-amphibolite, scattered as boudins and pods in metapelitic schists and gneisses on Isla Margarita, Venezuela, have been analysed by electron microprobe to augment existing bulk-rock chemical data. The large sample population available allows spurious effects of stoichiometric Fe3+ calculation procedures and bulk-rock influence to be recognised and eliminated. All samples belong to one population with a relatively homogeneous character. The equilibration temperatures are derived from Fe 2+ Mg fractionation between garnet and clinopyroxene and range from 525–650 °C. Jadeite contents of clinopyroxene and the persistence of minor stable albite constrain pressures to between 13 and 19 kbar. Combined with new evidence for high-pressure metamorphism in the enclosing metapelites and with existing data on an eclogitic metabasic unit exposed in north-eastern Isla Margarita (La Rinconada Group), these data show convincingly that the Margarita crustal block is and has been a relatively coherent unit ever since the entire complex suffered high-pressure metamorphism in Late Mesozoic times.

Detection of order-disorder in pyroxenes of the jadeite-diopside series via XAS at the CaNa and MgAl K-Edges

Abstract Soft X-ray absorption spectroscopy studies have been carried out on a number of natural clinopyroxenes of different origin straddling the compositional join between diopside and jadeite with only minor substitutions of Fe 2+ for Mg, and of Fe 3+ for Al. Calculation of the spectra is successfully made by the program package developed by C.R. Natoli and his coworkers, which is based on the multichannel multiple scattering formalism.

New data on merlinoite and related zeolites

A new occurrence of the rare zeolite merlinoite is described from Fosso Attici, near Sacrofano, north of Rome, Italy. Merlinoite occurs in little cavities of a fassaitic ejectum, as elongated, well developed, yellowish prisms (up to5 mm long). Merlinoite from Fosso Attici is orthorhombic (pseudotetragonal), s.g.Immm, witha = 14.136(6);b = 14.232(2);c = 10.040(1) (in Å). The crystal-chemical formula based on an average of 20 electron microprobe point-analyses and thermo-gravimetric determination of H2O is: (K5.02Na0.66Ba0.32Sr0.07Fe0.05Ca1.60Mn0.08Mg0.03) (Al10.79Si21.21) O64x 20.10 H2O.Infrared spectra of merlinoites from Fosso Attici, from the type locality Cupaello (Latium, Italy) and of several natural phillipsites were measured in the range 4000-300 cm-1 and compared with sinthetic Linde W zeolite. The IR spectra show that both merlinoite and Linde W zeolite are structurally related to zeolites with double octagonal rings (D-8-R) of (Si, Al) O4 tetrahedra. The IR method allows a rapid distinction between merlinoite and phillipsite.RiassuntoNel presente lavoro viene descritto un nuovo ritrovamento della rara zeolite merlinoite a Fosso Attici, Sacrofano, nord di Roma, Italia. Il minerale si presenta in prismi gialiognoli allungati (fino a 5 mm) e ben sviluppati in piccole cavità di un proietto rassaitico. La merlinoite di Fosso Attici è ortorombica (pseudotetragonale, gruppo spazialeImm, con dimensioni di cella (in Å):a = 14.136(6);b = 14.232(2);c = 10.040(1). La formula cristallochimica (media di 20 analisi puntuali in microsonda e determinazione termogravimctrica dell’H2O) è: (K5.02Na0.66Ba0.32Sr0.07Fe0.05Ca1.60Mn0.08Mg0.03) (Al10.79Si21.21) O64x 20.10 H2O.Gli spettri infrarosso della merlinoite di Fosso Attici, dell’olotipo di Cupaello e di svariate phillipsiti naturali sono stati registrati nell’intervallo 4000-300 confrontati con lo spettro della zeolite Linde W. Gli spettri IR mostrano che la merlinoite e la Linde W possono essere strutturalmente assimilate alle zeoliti con anello doppio ottagonale (D-8-R) di tetraedri (Si, A1)O4. Inoltre 1’IR permette una rapida distinzione tra 1a merlinoite e 1a phillipsite.

Nuovi minerali di terre rare nei proietti sanidinitici delle piroclastiti della provincia magmatica romana

I proietti olocristallini composti in prevalenza da sanidino che si rinvengono nelle piroclastiti della Provincia Potassica Romana contengono minerali rari, tra cui alcuni quasi certamente nuovi. Vengono qui descritti un borato trigonale di Al e terre rare, tra cui Ce prevale su La, parte delle quali sono sostituite da Ca + Th; e un boroberilliosilicato di Ca, Fe e terre rare, in cui Y prevale su Ce e Nd, appartenente al gruppo della gadolinite e percio di simmetria monoclina.The holocrystalline ejecta mainly made up of sanidine occurring in the pyroclastic rocks of the Quaternary Roman Potassic Province of Latium, Italy, contain numerous rare minerals, among which two REE-bearing, probably new ones are preliminarly described. Mineral A is a trigonal borate of Al and REE, where Ce > La, and with a little substitution of Ca + Th for trivalent REE. Mineral B is a boroberilliosilicate of Ca, Fe and REE, where Y > Ce ≈ Nd, belonging to the gadolinite group, and of monoclinic symmetry.RiassuntoI proietti olocristallini composti in prevalenza da sanidino che si rinvengono nelle piroclastiti della Provincia Potassica Romana contengono minerali rari, tra cui alcuni quasi certamente nuovi. Vengono qui descritti un borato trigonale di Al e terre rare, tra cui Ce prevale su La, parte delle quali sono sostituite da Ca + Th; e un boroberilliosilicato di Ca, Fe e terre rare, in cui Y prevale su Ce e Nd, appartenente al gruppo della gadolinite e perciò di simmetria monoclina.

Petrogenesis of the eclogites from Soazza, Switzerland

Abstract The metabasite bodies interbedded in the kyanite—staurolite-subfacies gneisses of the Adula Nappe in the vicinity of Soazza (Mesolcina Valley) preserve the eclogite assemblage in their cores and grade into common amphibolite towards their margins. Garnets vary irregularly in composition from sample to sample. Omphacites included in garnets have Jd 32 , those occurring as large crystals Jd 45 , but intergranular laths again Jd 35 . They all display the ordered P2/n space group. Amphibole formed late in the eclogite assemblage is a pargasitic hornblende while those occurring in amphibolites are edenites or Mg-hornblendes. Garnet—clinopyroxene pairs equilibrated first at T = 675° C and P ≥ 15 kbar then at a constant T = 760° C under P conditions decreasing from 15 to 10 kbar; the amphibolite-facies assemblages equilibrated at ≈ 570° C , 6 kbar. Chemically, the eclogite relics plot within the field of “common” ( B -type) eclogites in high-grade metamorphic rocks. Magmatic trends are still indicated by their minor elements, and discrimination diagrams suggest that these eclogites were originally tholeiitic basalts of oceanic affinity. In spite of now being within a crustal sequence, the Soazza eclogites are believed to have been a part of an oceanic crust that underwent subduction and “Eo-alpine” metamorphism to be emplaced later, as disrupted solid wedges, during the Alpine tectonism and then recrystallized under Barrovian amphibolite-facies regional conditions during the “Lepontine” phase.

Cummingtonite in the amphibolites of the South-Alpine basement complex (upper Lake Como region, Italy): Its relationships with hornblende

SummaryCummingtonite and hornblende occur together in metamorphosed basic rocks interlayered with lower-amphibolite facies sillimanite-muscovite-bearing schists and gneisses in the upper Lake Como region, South-Alpine basement complex of the Alps. Textural, chemical and spectroscopic studies indicate that they do not represent equilibrium pairs, but that hornblende was overgrown by cummingtonite during a late stage of either temperature increase or, more likely, of isothermal pressure release, by a continuous reaction which also produced the reverse zoning of plagioclase.ZusammenfassungIm Gebiet des oberen Como-Sees, im südalpinen Basement-Komplex, kommen Cummingtonit und Hornblende zusammen in metamorphen, basischen Gesteinen mit Zwischenlagen von Sillimanit-Muskovit-führenden Schiefern und Gneisen der unteren Amphibolit-Fazies vor. Untersuchung der Texturen, der chemischen Zusammensetzung und Infrarot-Spektroskopie zeigen, daß diese nicht Gleichgewichts-Paare darstellen, sondern daß Hornblende während eines späten Stadiums, entweder bei zunehmender Temperatur, oder eher während isothermalen Druck-Nachlasses durch eine kontinuierliche Reaktion von Cummingtonit überwachsen wurde, die auch für die reverse Zonierung der Plagioklase verantwortlich ist.