1,721,074 research outputs found
New data on the crystal-chemistry of fluoborite by means of SREF, SIMS, and EMP analysis
No full textThe crystal structure of fluoborite [Mg3F3(BO3)] was refined by Dal Negro and Tadini (1974) who provided a complete structural model. Previously, Takeuchi (1950) had refined an OH-dominant fluoborite (OH ~70%), but the limited quantity of data (extracted from two Weissenberg-Buerger photographs) did not permit the location of H atoms. Dal Negro and Tadini (1974) also could not locate H atoms because they used a crystal with near end-member composition. We have located the H bond in an OH-dominant fluoborite from the Betic Cordilleras (SE Spain). Excellent quality X-ray data on two crystals of fluoborite allowed discovery and refinement of the H position in this mineral. Electron microprobe (EMP) and secondary-ion mass spectrometry (SIMS) analyses of the light elements H, B, and F have resulted in the formulation of special procedures to obtain accurate, highquality quantitative data, which are presented in this paper. EMP, SIMS, and crystal structure refinement (SREF) data are in a good agreement. Linear equations are also presented to calculate the F content directly from cell parameters
Crystal chemistry of three tourmalines by SREF, EMPA, and SIMS
No full textThe crystal structures of three tourmaline crystals: (Na-0.49 K-0.01 Ca-0.48) (Mg-1.35 Fe-0.94(2+) Fe-0.49(3+) Ti-0.20) (Al-4.58 Fe-0.62(3+) Mg-0.80) (Si-5.99 Al-0.01) O-18 (BO3)(3.03) (OH)(3.18) F-0.18 O-0.64, a = 16.017(2), c = 7.256(2) Angstrom, V = 1612.2(4) Angstrom(3), R3m, Z = 3; (Na-0.64 K-0.01 Ca-0.03) (Mn-0.18 Fe-1.71(2+) Al-0.88 Li-0.11 Zn-0.03 Ti-0.07) (Al-5.67 Fe-0.28(3+) Mg-0.05) Si-5.76 Al-0.24) O-18 (BO3)(2.99) (OH)(3.96) F-0.17, a = 15.983(2), c = 7.152(2) Angstrom, V = 1582.1(4) Angstrom(3); (Na-0.81 K-0.01 Ca-0.01) (Mn-0.02 Mg-0.61 Fe-0.90(2+) Al0.80Li0.70Zn0.01 Ti-0.06) Al-6.00 (Si5.97Al0.03) O-18 (BO3)(2.93) (OH)(3.42) F-0.55 O-0.03, a = 15.921(3), c = 7.137(2) Angstrom, V = 1566.7(6) Angstrom(3), have been refined to R-indices of 1.3-2.2% using X-ray intensity data collected with a four-circle diffractometer using MoKalpha X-radiation. The crystals were analyzed by electron- and ion-microprobe techniques for all major and minor elements in the crystals. Unit formulae were calculated on the basis of 31 anions (O, OH, F) and the Fe3+/(Fe2+ + Fe3+) ratio was calculated for electroneutrality. The refined site-scattering values and the observed and distances were used to assign site populations that are in accord with the unit formulae calculated from the electron- and ion-microprobe compositions. The B contents are equal to 3.0 apfu (atoms per formula unit) within experimental error. In two of the crystals, (OH + F) = 4.0 apfu. However, the third crystal has (OH + F) = 3.36 apfu and O2- is dominant at the W(O1) site, and is an "oxy" tourmaline as defined by Hawthorne and Henry (1999). Non-spherical electron-density was observed at the X site, suggesting that there is some positional disorder at this site associated with occupancy of X by Ca and Na, possibly coupled with variable anion occupancy of the O1 site
An investigation of matrix effects in the analysis of fluorine in humite-group minerals by EMPA, SIMS, and SREF
No full textAccurate determination of F in minerals is a difficult task even when high F concentrations are present. Fluorine usually is determined by means of electron micro-probe analysis (EMPA) standardized on non-silicate-matrix compounds (e.g., fluorite), and some previous work has revealed the difficulties in determining F at high concentrations such as found in the humite-group minerals. Moreover, when both single-crystal structure refinement (SREF) and EMPA are available for the same crystal, the two estimates do not always agree. On the other hand, the secondary ion mass spectrometry (SIMS) technique is not easily applied at high F concentrations due to the existence of matrix effects related to the chemical composition and structure of the sample as well as to the concentration of the element itself. We tested the agreement among these analytical techniques in the estimation of high F contents and propose an analytical procedure for the analysis of fluorine. Our results indicate that careful selection of working conditions for EMPA of F together with appropriate correction, can yield accurate fluorine concentrations in minerals. Fluorine data extracted from refined site occupancies are systematically overestimated. New accurate working curves have been worked out for SIMS analysis of F taking Si and Mg, in turn, as the reference element for the matrix. Humite-group minerals show SIMS matrix effects on the order of similar to 10%. In analyzing fluoborite in the most unfavorable cases, the difference in Ion Yield (F/Mg) between "disoriented" humite-group minerals and "oriented" fluoborite samples can reach similar to 27%. Finally, a lower than expected IY(F/Si) from the F/Si working curve (made with humite minerals) is shown by topaz, which can be ascribed to chemical matrix effects, as well as to the covalent-type bonding between F and the major element in the matrix (Al)
Biotiti e cloriti nelle diverse facies del granito di Baveno-Mottarone (Verbania)
No full textIl plutone di Baveno – Mottarone è costituito da diverse varietà di granito, caratterizzate dalla diversa colorazione dei feldspati. La porzione maggiore e altimetricamente più bassa del plutone è costituita da granito bianco, mentre la parte alta è costituita dal tipico granito a feldspato rosa, da granito rosso (sia K-feldspato che oligoclasio colorati), entrambi miarolitici, e da una facies “di transizione” in cui la colorazione del K-feldspato varia irregolarmente da rosa tenue a bianco. I graniti sono tutti metalluminosi o leggermente peralluminosi; quelli a feldspati colorati si distinguono per un tenore più elevato in alcali e minore in Al2O3 (Boriani et al., 1992). Questa differenza tra granito bianco e graniti colorati è evidenziata anche dai diversi caratteri tipologici degli zirconi (Caironi, 1985). Nella facies bianca la biotite è pleocroica sul bruno e altera in una clorite debolmente pleocroica sul verde pallido. Nelle facies colorate la biotite è fortemente pleocroica sul bruno-verde scuro e anche la clorite è nettamente pleocroica sul verde scuro; inoltre, vicino alle lamelle cloritizzate sono presenti fluorite e micro-miaroli a miche chiare.
Cristalli di biotiti e cloriti, opportunamente selezionati da campioni rappresentativi delle diverse facies, sono stati analizzati mediante microsonda elettronica (EMP) per gli elementi maggiori, e mediante microsonda ionica Cameca IMS 4f (SIMS) per gli elementi in traccia Li, Be, B e F, in accordo a Ottolini et al. (2002).
Le biotiti del granito bianco, rispetto alle altre biotiti, risultano chiaramente più ricche di Mg (MgO = 3.26 – 4.63 wt % contro 1.61 – 2.53 wt %) e più povere di Fe (FeOtot=25.87 - 29.29 wt % contro 26.05 – 31.32 wt %); inoltre sono leggermente più ricche di Al (valore medio di Al2O3 = 18.17 wt % contro 16.23 wt %) e in generale un po’ più povere di Ti (valore medio di TiO2 = 2.08 contro 2.50 wt %). Nei diagrammi classificativi proposti da Abdel Rahman (1994) i due gruppi di biotiti sono sempre ben separati; i campioni provenienti dal granito bianco cadono nei campi caratteristici delle biotiti dei graniti peralluminosi, mentre le biotiti dei graniti colorati sono spostate verso, e in parte entro, i campi delle biotiti dei graniti alcalini. Inoltre le biotiti dei graniti a feldspati colorati si caratterizzano per la presenza di un tenore più elevato di Li2O (valore medio 1.20 vs 0.34 wt %) e di F (valore medio 0.92 vs 0.36 wt %). Gli altri elementi in traccia analizzati (Be e B), sempre molto bassi, non sembrano essere discriminanti per le biotiti dei graniti delle varie facies in quanto variano irregolarmente. Per quanto riguarda le cloriti, si può osservare che i loro caratteri composizionali riflettono quelli delle coesistenti biotiti soprattutto per quanto riguarda i contenuti di MgO e FeOtot, che risultano anche in questo caso discriminanti tra le cloriti della facies bianca e quelle delle facies colorate. Inoltre, nonostante il limitato numero di analisi di cloriti effettuate mediante SIMS, è possibile evidenziare la presenza di Li2O e F in quantità maggiore nei campioni appartenenti alle facies a feldspati colorati.
La diversa composizione in termini di elementi maggiori (soprattutto Fe, Mg e Al) sembra essere un carattere acquisito durante la cristallizzazione magmatica e legato ad un diverso chimismo del magma. Il maggior contenuto in F e Li delle biotiti dei graniti a feldspati colorati è da attribuire a fluidi arricchiti in F e Li, presenti probabilmente già a partire dalla fase tardomagmatica, come indica la presenza di fluorite nella roccia e la cristallizzazione nelle geodi, insieme a fluorite e miche litinifere, degli stessi minerali costituenti il granito
New chemical data on the clinopyroxene-garnet pair in the Alpe Arami eclogite, Central Alps, Switzerland
No full textIn the area of Gorduno, near Bellinzona, Central Alps, Switzerland, well-preserved eclogites crop out only at the margin of the
garnet peridotite massif of Alpe Arami (AA). These bodies of eclogite were not derived from cognate mafic layers of mantle
origin, and their thermal evolution is different from that inferred for the peridotite. Some questions are still open about the P–T
evolution of AA eclogites, and the data given in literature cover a broad range of variation. We have investigated the bulk
composition and the clinopyroxene–garnet trace-element distribution in a sample of eclogite from a lens occurring at the rim of
the peridotite body. We report new data on trace elements in the bulk rock, obtained by ICP–MS, as well as the results of SIMS
analyses on REE, Ti, V, Cr, Y, Sr, Zr, Sc carried out on the two major rock-forming minerals, i.e., clinopyroxene and garnet. We
also provide AAS data for Co, Ni and Cu in purified clinopyroxene and garnet concentrates, and O isotope ratios both on the bulk
rock and on clinopyroxene and garnet. We found that: i) the trace-element abundance in garnet and clinopyroxene may be
strongly influenced by the major-element composition; ii) their distribution within both mineral phases is inhomogeneous. In
particular, their highly variable contents highlight the complex evolution of the AA eclogite and emphasize the existence, at least
for some elements and at a very local scale, of chemical disequilibrium, also confirmed, in an independent way, by a significant
difference in 18 O between clinopyroxene and garnet. Therefore, the low temperature of equilibration estimated in eclogite
sample 70–AM–10 is not an artifact of the computational strategy adopted, but is the record of the lack of complete equilibration
between clinopyroxene and garnet with changing P–T conditions during the various metamorphic events.
......................................
Quantification of H, B and F in Kornerupine : Accuracy of SIMS and SREF (X-Ray Single-Crystal Structure-Refinement) Data
No full textWe use a multiple-analytical approach based on secondary-ion mass-spectrometry (SIMS), X-ray single-crystal structure refinement (SREF) and electron-probe micro-analysis (EPMA) to derive the complete crystal-chemical formula of a B-rich kornerupine-group mineral, prismatine, from Hrarigahy Madagascar: (Ca0.01Li0.02Mg0.20Fe0.102+) (Mg3.57Fe0.062+Al5.37) (Si3.84B0.91Al0.26)O-21 (OH1.08F0.07). SIMS matrix effects related to crystal structure were investigated by analyzing two grains with a known crystallographic orientation relative to the ion beam.
Boron orders at the T3 site. The refined site-scattering for T3, 6.33 eps (electrons per site) agrees well with the mean bond-length for this site (1.512 Angstrom), which indicates nearly complete occupancy by B (85% rel.). B2O3 (similar to4wt%), derived by SREF, agrees with the SIMS data within analytical uncertainty using Si as the inner reference for the matrix. The occupancy of the X site obtained by combining the SIMS and EPMA data (5.30 eps; electrons per site) agrees with the refined site-scattering value (5.75 eps). Trace quantities of Li and Ca are ordered at this site. SIMS data for H2O is in accord with the stoichiometric value, indicating complete occupancy at 010 by OH. Fluorine (similar to0.17 wt%) orders at 010: it corresponds to similar to0.07 atoms per formula unit (apfu) vs. 0.15 apfu (atoms per formula unit) by SREF, indicating a slight overestimation of F with SREF, as previously observed in fluoborite.
Our data show that SIMS chemical matrix effects are well-calibrated, and emphasize the usefulness of independent micro-analytical techniques in testing the mutual accuracy and consistency of experimental data
Clinoholmquistite discredited : the new amphibole end-member fluoro-sodic-pedrizite
No full textRe-examination of holotype "clinoholmquistite", ideally A□ BLi2C(Mg 3Al2) TSi8 O 22X(OH)2 (Ginzburg 1965) from the Tastyg spodumene deposit, Tuva, Siberia, Russia by EMP and SIMS analysis and structure refinement shows that the sample consists of a mixture of two distinct amphibole compositions, tremolite and a new amphibole end-member, fluoro-sodic-pedrizite, ideally ANa BLi2C(Mg2Al 2Li) TSi8 O22XF2 (IMA-CNMMN 2004-002). Fluoro-sodic-pedrizite from Tastyg has the following crystal-chemical formula and unit-cell parameters : A(Na0.64K 0.01) B(Li1.93Ca 0.04Na0.03) MI(Mg 1.69Fe0.312+) M2(Al1.98Cr0.01Zn0.01) M3(Li0.64Fe0.212+Mg 0.13Mn0.02) TI(Si 3.96Al0.04) T2Si4 O 22X(F1.10OH0.90), a = 9.368(8), b = 17.616(10), and c = 5.271(4) Å, β = 102.38(4 )°, V = 849.6 Å3, Z = 2. The structure has been refined to Robs = 2.3% (I > 3σI) and R all = 3.8%. Refined site-scattering values and site-geometries were used, together with EMP and SIMS results, to obtain site populations. Fluoro-sodic-pedrizite is the first amphibole end-member with dominant CLi found in Fe-poor geologic environments. The coexisting tremolite contains only 0.002 wt% Li 2O and 0.06 wt% B2O3, probably ordered at the T1 site. Crystal-chemical arguments, as well as preliminary experimental work, suggest clinoholmquistite is unstable
Strategies for quantification of light elements in minerals by SIMS : H, B and F
No full textUsing a large set of silicate crystals, characterized by Structure REFinement (SREF), Electron Probe Micro-Analysis (EPMA) and Secondary Ion Mass Spectrometry (SIMS), and mounted with known crystallographic orientation [1], we propose a new SIMS quantification for H, B and F (from ppm level to several wt.%), using 27Al+ and 44Ca+, in turn, as the reference isotope for the matrix, and propose suitable calibration standards to obtain accurate results. The final SIMS data are then compared to those obtained using Si as the reference element, with those available from EMPA (B and F), and with the crystallographic constraints derived from SREF investigation. The results of this study can be extended to the measurement of light elements in complex silicate or non-silicate samples
New SIMS Procedures for the Characterization of a Complex Silicate Matrix, Na 3 (REE,Th,Ca,U)Si 6 O 15 . 2.5H 2 O (Sazhinite), and Comparison with EMPA and SREF Results
No full textAnalytical methods based on secondary ion mass spectrometry were developed for the characterisation of a complex layered silicate REE-mineral, named sazhinite, for which a number of issues are still open regarding its chemistry and structure. Such procedures involved the analysis and quantification of light, volatile, alkaline, medium-Z, rare earth and actinide elements. The accuracy of the SIMS data is within the assigned precision of the concentration values assumed as reference in the calibration standards employed. REE and actinide data yield a good agreement in terms of calculated site scattering at the M site: 58.42 electrons per formula unit (epfu) vs. 60.39 epfu obtained by Single Crystal Structure-Refinement (SREF). Accuracy is estimated on the order of 5% rel. for H, Li, Be and B, and 10% rel. for F. Na analysis was crucial to solve the open questions about the structure, and excellent agreement was obtained by comparing data from SIMS (REE, Y, actinides, Na) + EMPA (SiO2, CaO, SO3 and K2O) with information derived from SREF: site scattering of the M site + Na sites = 92.56 epfu calculated from chemical data, against 91.95 epfu from SREF. Such procedures can be easily extended to the analysis of variously complex, silicate REE-minerals
- …
