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Crystal chemistry and petrologic significance of Fe3+-rich phlogopite from the Tapira carbonatite complex, Brazil
No full textThis contribution deals with the crystal chemistry of phlogopite and Fe3+-rich phlogopite from the Tapira alkaline-carbonatite complex (Brazil) to assess the petrological significance and genetic conditions of these rocks. The Tapira complex consists of a layered intrusion composed mainly of ultramafic rocks (dunite, wehrlite, clinopyroxenite, bebedourite, garnet-magnetitite, perovskite-magnetitite, and glimmerite) with subordinate carbonatite. The wide range of textural, optical, and crystal-chemical characteristics of phlogopite is related to the variation of f(O2), a(H2O), and a(CO2) as well as magma bulk-chemical composition during fractional crystallization. Phlogopite from alkaline-silicate rocks (ranging from dunite to bebedourite) is characterized by fairly constant Al content, moderate Fe-[4](3+) substitution, and variable amounts of Ti. The Fe-[4](3+) substitution, accompanied by crystals showing reverse pleochroism, increases during fractional crystallization. These features correspond to crystallization at low pressure and high f(O2) and a(H2O) in the presence of moderate saturation in Ti-bearing phases, Al2O3 in the magma, or both. Phlogopite from silicate-carbonatite rocks, classified as ferriphlogopite on the basis of strong reverse pleochroism related to Fe-[4](3+) tetrahedral substitution, also presents low to very low Al, Fe2+, and Ti contents. These features suggest very high f(O2), H2O, and CO2 conditions in the presence of strong saturation in Ti-bearing phases as well as very low Al2O3 content in the liquid.The crystal-structure refinements of Tapira phlogopite show that Fe3+ substitutes for Si in tetrahedral sites; Fe distribution is completely disordered, so the resulting space group is C2/m. The octahedral-site composition is similar to that of phlogopite, the octahedral sites being preferentially occupied by Mg. The presence of Fe3+ in the tetrahedral sheet enlarges the whole structure. This enlargement is reflected by an increase in cell-edge lengths and a decrease in beta-angle values. The increase in distortion of the tetrahedral ring (alpha angle up to similar to 11 degrees) is necessary for the tetrahedral and octahedral sheets to fit together
Structure and mineralogy of layer silicates: recent perspectives and new trends
No full textBecause of their many novel and advanced applications, there is increasing interest in layer silicates from the scientific and technical communities. Appropriate application of these minerals requires deep understanding of their properties and of the processes where they are involved. This chapter, by providing fundamental definitions and crystal structural and chemical data pertaining to layer silicates, aims to introduce this field to new researchers and technicians, by describing the fundamental features leading to different behaviours of layer silicates in different natural or technical processes. The subject addressed is vast and so the reader is referred in some cases to work already published. The focus here is on layer silicates for which detailed crystal structures are given in the literature and which are likely to be used in an applied way in the future. Layer-silicate minerals fulfilling these requirements are: (1) kaolin-serpentine group (e. g. kaolinite, dickite, nacrite, halloysite, hisingerite, odinite, lizardite, berthierine, amesite, cronstedtite, nepouite, kellyite, fraipontite, brindleyite, guidottiite, bementite, greenalite, caryopilite; minerals of the pyrosmalite series); (2) talc and pyrophyllite groups (e. g. pyrophyllite, ferripyrophyllite, willemseite); (3) mica group (i.e. some recent advances in crystal chemistry and structure of dioctahedral and trioctahedral micas); (4) smectite group (e. g. montmorillonite, saponite, hectorite, sauconite, stevensite, swinefordite); (5) vermiculite group; (6) chlorite group (e. g. trioctahedral chlorite such as clinochlore, di,trioctahedral and dioctahedral chlorites such as cookeite and sudoite); (7) some 2: 1 layer silicates involving a discontinuous octahedral sheet and a modulated tetrahedral sheet such as kalifersite, palygorskite and sepiolite; and (8) imogolite and allophane
Single-crystal, polarized-light, FTIR spectroscopy of masutomilite: short range order of cations and orientation of the O-H dipoles
No full textMasutomilite is a rare Mn-analogue of the Li-mica zinnwaldite; it was described by Harada et al. (1976) from the Shiga Prefecture, Japan, and later reported from few other localities (Němec, 1983, Čerńy and Burt, 1984). We relate here an infrared study of two masutomilite specimens from Central Urals, Russia (Mokrusha Pegmatite, Murzinka Region) and Idaho, USA (Sawtooth Mountains, Boise County), respectively. Both samples belong to the 1M polytype. The chemical composition, derived by combining electron microprobe analysis, inductively coupled plasma atomic emission spectrometry (ICP-AES) for Li, and results from the structure refinement are, on the basis of O10(F,OH)2 (Brigatti et al., 2007): (Si3.30Al0.70) (Al1.00Fe0.36Mg0.01Mn0.31Li1.32) (Ca0.01Na0.04K0.94) O10F1.91(OH)0.09 (Mokrusha mine, Russia) and (Si3.11Al0.89) (Al0.91Ti0.02Fe0.46Mg0.03Mn0.52Li1.06) (Na0.05K0.92Rb0.02) O10F1.89(OH)0.11 (Sawtooth Mountains, Boise County, Idaho, USA). Both samples have all Li > 1.0 apfu and variable Fe/Mn. The anionic site is dominated by F with low amounts of OH (0.09-0.12 apfu). Single-crystal, polarized-light FTIR spectra were collected using a NicPlan microscope equipped with a nitrogen-cooled MCT detector, a KBr beamsplitter and a gold wire grid polarizer, at 4 cm-1 resolution. The spot size was ~ 100 μm and 128 scans were averaged for each spectrum. The collected patterns are different as expected from the different chemical composition of the examined samples, however they show broad similarities and a similar behaviour under polarized light. They consist of a higher-frequency component which is centred at 3685 and 3659 cm-1 for masutomilite from Russia and Idaho, respectively; both bands are strongly polarised for E // α. A broader, multicomponent absorption is observed at around 3600 cm-1. In masutomilite from Idaho this band is split in two components of equal intensity, at 3685 and 3659 cm-1 respectively, plus a lower-frequency shoulder at ~ 3552 cm-1. This band shows maximum intensity for E // γ and a medium intensity for E // α, while showing reduced intensity for E // β. A lower frequency, broad band is finally present at ~3500 cm-1 in both samples; this feature is virtually unaffected by rotation of the polarizer. Following the model of Robert et al. (1989) for Li-bearing micas, the higher frequency bands at 3685 - 3659 cm-1 are assigned to OH-dipoles bonded to FeFeFe local environments (TRI-6 environments), while the multicomponent band around 3600 cm-1 is assigned to AlM2+M2+ (with M2+ = Fe or Mn) local environments (TRI-7 environments). The spectra show that all Li is associated to F at the anionic site. From measurement of the dichroic ratio (absorbance along a / absorbance along b, e.g. Serratosa and Bradley, 1962, Vedder and McDonald, 1963) the orientation of the O-H vectors with respect the (001) plane can be evaluated.
References
Brigatti, M.F., Mottana, A., Malferrari, D. and Cibin, G. (2007) Am. Mineral., 92, 1395-1400.
Harada, K., Honda, M., Nagashima, K., Kanisawa, S. (1976) Min. J., 8, 95-109.
Němec, J. (1983) Neues J. Miner., Mh., 537-540.
Robert, J.-J., Beny, J.-M., Beny, C., Volfinger, M. (1989) Can. Mineral., 27, 225-235.
Serratosa, J.M. and Bradley, W.F. (1962) Determination of the orientation of OH bond axis in layer silicates by infrared absorption. J. Phys. Chem., 62, 1164-1167.
Vedder, W. and McDonald, R.S. (1963) Vibrations of the OH ions in muscovite. J. Chem. Phys., 38, 1583-1590
Crystal chemistry of trioctahedral micas in the top sequences of the Colli Albani volcano, Roman Region, central Italy
No full textTrioctahedral mica crystals are frequently found in the volcanoclastic products emplaced during the final stages of the activity of the Colli Albani volcano (Roman Region, central Italy). In the youngest phreatomagmatic deposits, mica is found either as a minor mineral phase in holocrystalline ejecta, scoriae, and coherent pyroclastics, or as loose phenocryst in incoherent pyroclastics. Based on optical and electron microscope investigations, as well as crystal chemical and structural data, the micas selected for this study were divided into two groups, the first one encompassing brownish, Fe-rich crystals (type-A phlogopites, hereafter referred as t-A) with 0.68 < Mg/(Mg + Fetotal) < 0.85 and 0.062 < Tiapfu < 0.199, and the other one encompassing colorless, Mg-rich crystals (type-B phlogopites, hereafter referred as t-B) with Mg/(Mg + Fetotal) in excess of 0.85 and 0.007 < Tiapfu < 0.052. t-A phlogopites also show a tetrahedral ring cavity, overlapped tetrahedral hexagon area, and basal tetrahedron area greater than in the t-B phlogopites.
From a petrological point of view, the textural and chemical variations of t-A phlogopites are compatible with fractional crystallization processes taking place within the magma chamber. t-B phlogopites show evidence of a Ti-oxy substitution mechanism, thus suggesting high fO2 conditions. Textural and paragenetic features observed in t-B phlogopite-bearing rock samples, indicate a genesis by thermal metamorphism of a siliceous dolomitic limestone with the input of a variable amount of a potassic magma, possibly the same from which the t-A phlogopites formed, with the exception of one sample, for which a different parental magma is suggested. Indeed, there is an almost continuous spectrum of crystal-chemical and structural parameters starting from t-B and evolving towards t-A phlogopites. The t-A phlogopites displays complex, apparently meaningless relationships in their crystal-chemical parameters, thus indicating interplay of several substitution mechanisms. They possibly formed in polybaric conditions, since their cell volumes differ, but the total size of the cations hosted in the octahedral sheet is the same
Crystal structure and chemistry of ferroan phlogopites from the Albano maar lake (Colli Albani volcano, Central Italy)
No full textThis work is aimed at the crystal chemical and structural characterization of micas
from Colli Albani volcano. In particular all the samples considered in this study, with the
exception of sample CC, are from the ending volcanic sequences, usually referred as Albano
maar lake. Micas from the first three phreatomagmatic units are homogeneous in composition
or show well defined chemical trends. From the forth phreatomagmatic unit upward, chemical
composition is more variable, especially in the ejecta. The Fe/(Fe+Mg) ratio varies from 0.06
to 0.55, thus the mica chemical composition ranges from phlogopite to magnesian annite
through ferroan phlogopite. Heterovalent octahedral substitutions are related to Ti, Al and
very limited amount of Cr. In the interlayer Ba for K substitutions occur up to 0.06 apfu.
Crystal structure refinement were carried out in the space group C2/m and C2/c for 1M and
2M1 polytype, respectively (agreement factor, Robs: 0.021 ≤ Robs ≤ 0.035).
The crystal structure of micas studied appears to be mostly affected by annitic
substitution. An increase in the annitic component of the mica is reflected by a decrease in the
distortion affecting both individual tetrahedra (tetrahedral angle variance, TAV: 0.49 ≤ TAV
≤ 1.13°2) and the tetrahedral ring (6.59 ≤ α ≤ 10.34°). Interlayer topology was found to be
affected as well. A very well established correlation can be defined between Fe and Ti content
and thus an effect of Ti content over the interlayer topology may be introduced also.
The deprotonation of the anionic site was found in literature to be a powerful
petrogenetic indicator. A well established correlation was found between heterovalent
octahedral substitution and anionic site deprotonation. On the contrary heterovalent interlayer
substitutions are accompanied by F- for (OH)- substitution
What drives the distribution in nature of 3T vs. 2M(1) polytype in muscovites and phengites? A general assessment based on new data from metamorphic and igneous granitoid rocks
No full textPetrologic, chemical, and polytype data are presented for dioctahedral potassic micas from K-feldspar-bearing metamorphic and igneous rocks of acidic composition unaffected by high-pressure (HP) conditions. The paper aims to demonstrate that: (1) under non-HP conditions, in both metamorphic and igneous plutonic environments, a given bulk-rock compositional constraint imposes a more or less marked phengitic composition to dioctahedral potassic mica; and (2) this muscovite crystallizes as 2M1, notwithstanding its phengitic composition. The samples (157 in number) are from widespread provenances. We conclude that the growth of 3T polytype of muscovite is not a function of mica composition. This is consistent with the recent crystallographic knowledge on polytypism, cation ordering, elastic properties, and structural deformational mechanisms of muscovite, which address the stabilization of 3T with pressure.
Keywords: Muscovite, phengite, celadonitic substitution, polytypism, 2M1, 3T, pressure-polytype relationship, mica chemistry, polytype relationship, petrologic mineralog
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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