RICERCA ITALIANA     

Effect of petrological variables on mica crystal chemistry.

Università degli Studi di Modena e Reggio Emilia

Abstract

The main objectives of this research project are:
a) To increase the crystal chemical and crystal physical knowledge on natural micas to understand and predict their behaviour and role in well defined petrogenetic environments;
b) To work out or improve criteria, based on micas, describing the petrogenetic or geological processes either generating or re-equilibrating the phases.

Natural processes involving micas can be better described if thermodynamic, crystal chemical and crystal physical aspects are considered together as also evidenced by the significant added value that such information as the crystal chemical determination of each phase, as order-disorder pattern and as polytypic zoning can provide over the definition of reactions involving micas. The following research lines will be endeavoured:
i)determination of the crystal chemistry of individual phases;
ii)determination of order-disorder reaction especially for iso-electronic atom species;
iii)study of zoning in polytypes;
iv)modelling of crystal-chemical, geometrical and thermodynamic relations;
v)experiments on micas under non-ambient consitions;
vi)analysis of the short-range order mechanisms;
vii)determination of the redox state of the transition atoms.

Different experimental techniques will thus be used, such as ionic and electron microprobe, X-ray and neutron diffraction (on powder and crystals), electron (e.g. TEM, ESEM) and surface microscopy (e.g. AFM), spectroscopic analyses (e.g. XAS, XPS, Mössbauer).
Moreover the cross-disciplinary approach will allow the study to be performed on samples with a petrological significance, and to overcome drawbacks and uncertainties characterizing partial mono-disciplinary approaches. A deeper insight into petrogenetic correlations in micas, as well as into the points mentioned above, will be gained from a combined, cross disciplinary effort, involving all the five units cooperating in the project, where year-long experience on micas (advanced structural crystallography, spectroscopy, electron microscopy, geothermobarometry, petrology) will be merged together to provide optimised results. <<<

Principal Investigator

Maria Franca BRIGATTI Università degli Studi di MODENA e REGGIO EMILIA

Research Objectives

SYNTETIC LIST OF THE OBJECTIVES. i) To increase the crystal chemical and crystal physical knowledge on natural micas, understand and predict their behaviour and role in well defined petrogenetic environments; ii) To work out or improve criteria, based on micas, describing the petrogenetic or geological processes either generating or re-equilibrating the phases.

THE OBJECTIVES. During the last two decades, studies on micas drew the attention of several researchers and provided significant results towards a better structural definition, a deeper crystal chemical insight and an increased knowledge on the complex relationships between chemical, physical and topological properties of micas and their petrological environment. The Research Units (RU) joining this program led many of these studies.
Despite these significant progresses, much work on micas is still required both to validate the characterization of the phases, according also to the new IMA nomenclature (Rieder et al., 1998), and to better clarify the relationships between mica structure and crystal chemical composition, and their relationships with the petrological environments or processes.
The objectives of this project are thus both actual and completely matching the various competences of the RU here integrated. The high scientific profile of the RU, as well as their potential to implement cross-disciplinary researches and their ability to cooperate to reach common scientific goals, are believed to provide an extremely promising background to the project. They are outlined below.

COMPOSITION. Even if chemical composition of micas was defined several years ago, some points deserve further investigation, such as: i) a better definition of the anionic site chemical composition; ii) the effect of crystallization environment on the oxidation of transition metals; iii) a systematic and accurate determination of trace and light elements. These aspects also involve mica classification, which presents significant compositional gaps, even more evident when linking chemical composition to layer topology.

CRYSTAL CHEMISTRY. Although the crystal chemical definition of some series and of many end-members is well defined, much work still needs to be performed, to describe: i) the crystal chemical details of each 2:1 layer in different polytypic sequences; ii) the effect of each single site on the crystallization of different polytypes; iii) the cation ordering processes as a function of intensive and extensive variables in natural environments; iv) the effect of heterovalent substitutions on the whole layer; v) the behaviour of transition metals; vi) the structural location of light elements, with a particular focus for dioctahedral micas; vii) the effect of octahedral substitutions on the anionic site; viii) the relationships between micas and coexisting minerals; ix) the exchange vectors affecting Ti in mica structure; x) structural changes as a function of temperature and pressure. All the mentioned points are strictly related to the petrogenetic investigations developed inside the program, since micas are universally recognized as good indicators for thermal, baric and compositional variables prevailing during crystallization process and as "transporters" of chemical species and functional groups inside the Earth (e.g., Melzer & Wunder 2001; Schingaro et al. 2001 Guidotti & Sassi 2002; Zanazzi & Pavese 2002).

STRUCTURE AND MICROSTRUCTURE. Data and models introduced during these last years, (Nespolo & Durovic 2002), can provide a valuable tool and an excellent starting point for the interpretation of the complex reactions driving mica crystals to grow in natural environments. Polytypic sequences in micas will be thus accurately investigated, not only to provide structural data, when the refinement is not feasible due to excessive crystal deformation or polytypic complexity, but also to verify: i) the coexistence of different polytypes; ii) the presence of inhomogeneous sequences; iii) the alteration processes and mechanisms driving polytypic arrangement; iv) planar and structural defects. Moreover it should be considered that polytypic and compositional zoning have been related to the thermo-baric conditions and to the activity of fluids in the host rock (Ferraris et al. 2001; Ivaldi et al. 2001).

CHARACTERIZATION OF METAMORPHIC MICAS EVEN FOR GEOTHERMOBAROMETRIC PURPOSES. Dioctahedral and trioctahedral micas from metamorphic environments are well characterized from petro-mineralogical and chemical viewpoints (Guidotti & Sassi 2002; Cesare et al. 2003), even if a detailed structural, crystal chemical and crystal physical characterization is of paramount importance for better understanding the behaviour of micas from these environments. Moreover order-disorder relations, both long- and short-range, can provide geothermobarometric insights, as well as the different stacking, which was correlated to the kinetics of the reactions during crystal growth (Baronnet 1980).
The comparison of structural, crystal chemical and crystal physical properties of micas from metamorphic complexes, with experimental evidences (high pressure or temperature crystallographic studies) and/or theoretical calculations (Connolly 1990) can thus provide a deep insight onto temperature, pressure and composition of the host rock. Moreover established trends, such as the variation of d(060),(33-1) parameter as a function of pressure and temperature (Sassi & Scolari 1974; Guidotti & Sassi 1986, 1998), extensively used in literature, needs recalibration to take into account the great amount of recent analytical data.

CRYSTALLIZATION OF MICAS FROM MAGMATIC ENVIRONMENTS. Some promising, but still not conclusive, results were obtained on this topic (Brigatti et al. 1996; Brigatti et al. 2000). Major points need further investigation to identify chemical and physical constraints during crystallization: i) both long- and short-range cation ordering; ii) transition element oxidation conditions and their structural location; iii) identification of light and trace elements; iv) anionic site stereochemistry.

GENERAL STRATEGY. A deeper insight into petrogenetic implications of micas, as well as into the above mentioned points, will be gained from a combined, cross disciplinary effort, involving all the five units cooperating to the project, where year-long experience on micas (advanced structural crystallography, spectroscopy, electron microscopy, geothermobarometry, petrology) will be merged together to provide optimised petrological results.
Natural processes involving micas can be better understood and model when thermodynamic, crystal chemical and crystal physical aspects are considered all together in the ambit of rigorously constrained petrologic frames.
Different experimental techniques will thus be used, such as ionic and electron microprobe, X-ray and neutron diffraction (on powder and crystals), electron (e.g. TEM, ESEM) and surface microscopy (e.g., AFM), spectroscopic analyses (e.g. XAS, XPS, Mössbauer).
Moreover the cross-disciplinary approach will allow the study to be performed on samples with a petrological significance, and to overcome weak points and uncertainties characterizing partial mono-disciplinary approaches. <<<

First Results

The scientific objectives of the project are described in A form, and in greater detail in the B form by local RU. Also considering a coherent scientific approach and the overall project layout, results will be obtained step by step.

At the end of PHASE I: samples to be studied will be available; an early analytical characterization will be performed to identify samples promising for further analyses; the program for jointly executed researches will be detailed, agreed and implemented.At the end of PHASE II: the greatest part of experimental data will be available; a mid-term project meeting will be organized to discuss results and provide early interpretations; weak points deserving future attention will be identified; sampling, experimental, strategic and conceptual approaches will be discussed and optimised.At the end of PHASE III: all the experimental data will be available; data will be formatted and arranged into data-bases; statistical analyses will be performed; the interpretation of result will start.At the end of PHASE IV: the interpretation of results will be concluded; the documentation of results via scientific papers and communications at meetings will be on progress; a significant number of scientific papers will be rendered available to the scientific community before the end of the project time. <<<

Timescale

24 months

National and international background

A recent review on micas is reported in the volume "Micas: Crystal Chemistry and Metamorphic Petrology" (Mottana et al., Eds. 2002). This volume, also reporting an interesting "History" of micas by Cipriani (2002), was one of the results of the previous project on micas, financed by MURST. This review underlines that, in these last 20 years (i.e. after the printing of the previous review volume edited by Bailey in 1984), a great progress has been carried out on micas, by using many different experimental techniques, such as: 1) Powder X-ray diffraction, both considering or not the entire spectrum and single crystal X-ray powder diffraction both at room and high temperature and pressure (Brigatti & Guggenheim 2002, Ferraris & Ivaldi 2002, Zanazzi & Pavese 2002); 2) Neutron diffraction (Zanazzi & Pavese 2002); 3) Different spectroscopies (Mössbauer: Dyar 2002; Infrared: Beran 2002; XAS: Mottana et al. 2002); 4) Transmission electron microscopy (Kogure 2002). Moreover investigations on polytypes (Nespolo & Durovic 2002), on compositional variation of dioctahedral micas from different metamorphic environments (Guidotti & Sassi 2002) and on illites in low metamorphic environments (Arkai 2002) attracted significant interest too.
Literature also numbers a lot of extremely interesting approaches on Experimental Mineralogy (Massonne & Schreyer 1986, Robert et al. 1993, Icenhover & London 1995; Fechtelkord et al. 2003) which, being affected by metastability problems, are confined into extreme systems only.
The overall picture of the scientific background on micas should also consider the new classification introduced by IMA (Rieder et al. 1998) and some recent works, not reviewed in the above review volume, such as Boukili et al. (2001), Rancourt et al. (2001), Henry & Guidotti (2002), Brigatti et al. (2003b), Cesare et al. (2003), Redhammer & Roth (2003), Sanz et al. (2003), Tracy & Beard (2003), Comodi et al. (2004), Ishida et al. (2004), or not considered there for being outside the scope of the volume (e.g., Jeong & Kim 2003, Sainz-Diaz et al. 2003, Palin & Dove 2004, Palin et al. 2004).
Despite all these efforts, much experimental and theoretical work still remains to be done on phyllosilicates, and on micas in particular. Just to quote the most evident problems: i) the new classification introduced for micas (Rieder et al. 1998) pointed out the presence of compositional gaps, which need discussion and chemical, structural and spectroscopic evidence; ii) micas from low metamorphic grade environments often show a limited occupancy for the interlayer site, thus pointing out either the presence of vacancies or the occupancy by light cation groups, such as H3O+ and NH4 (Harlow et al. 2001, Nieto 2002, Wunder & Melzer 2002), not detected in electron microprobe analysis (Arkai 2002, Hovis et al. 2004); it is worth note that vacancies in the interlayer site significantly affect the thermo-elastic properties in micas and thus their equilibrium equation (Comodi et al. 1999, 2002; Zanazzi & Pavese 2002); iii) Polytypic sequences were often related to crystal growth, even if, despite the good theoretical characterization (Takeda & Ross 1995, Nespolo et al. 1997, Nespolo & Durovic 2002) and the potential petrological application (Sassi et al. 1994), limited use was so far observed; iv) Interesting results were provided by crystallographic and crystal chemical studies both at room and high T and P (Comodi et al. 2002, 2004; Pavese 2002; Pavese et al. 2003a), even if their extensive application to petrologically significant series is still missing; v) XAS spectroscopy is a valuable tool to describe the short-range ordering and transition elements oxidation state (e.g., Cruciani et al. 1995; Malitesta et al. 1995; Mottana et al. 2002, Tombolini et al. 2002 a,b, 2003; Cardelli et al. 2003) and it can be applied to small and deformed mica crystals, coming from low grade metamorphic environments, where single crystal X-ray diffraction fails (Manceau et al. 1990; Vantelon et al. 2003).
Another relevant aspect, in the previously introduced frame set, is the potential petrological application of micas. The understanding of the thermodynamic parameters, affecting the mineral stability, can be obtained by combining crystallographic and chemical data (Vieillard, 1995), thus also accounting for their petrological significance.
Micas are extensively used to provide quantitative information on P-T-t trends in host rocks, even if it is well known that, when applied to real systems, even the best geothermobarometer can fail significantly (Ferry & Spear 1978; Hodges & McKenna 1987, Holdaway et al. 1997, Holdaway 2000). The geothermometric equations so far available essentially take into consideration the Mg, Fe, and Al cations only. Other cations are either not considered or used through empirical corrections to deal with particular compositional fields (Indares & Martignole 1985; Holdaway et al. 1997). Another limitation is the uncertain location of elements inside the mica layer.
A good example of such difficulties is provided by the "biotite-garnet" geothermometer introduced by Thompson (1976) and experimentally calibrated by Ferry & Spear (1978), which started from a simplifying approach that considered the exchange between two different solid solutions: phlogopite-annite (micas) and pyrope-almandine (garnet), and thus, the Mg-Fe2+ exchange. This model was suggested for changes several times, first of all after new experimental data (Perchuk & Lavrenteva 1983, Gessmann et al. 1997), then to take into account other significant chemical elements (Indares & Martignole 1985, Aranovich et al. 1988) and finally to introduce the oxidation state of Fe, which is an extremely significant element (Güttler et al. 1989), since Fe can enter the structure in two different oxidation states and/or coordinations. Most of these evidences cannot be obtained from a simple microprobe analysis, but require more advanced structural, chemical and spectroscopic investigations. Other aspects to be considered are related to exchange mechanisms and vacancy location, to the chemical composition of the octahedral anionic site and to its relation with local and mean layer composition, to the order-disorder processes both from a crystal chemical and polytypic perspective. Finally, if additional data are not provided, the "biotite-garnet" geothermometer will not provide unique results, but rather offers different interpretation models (Holdaway 2000), due to the complexity and to the fast kinetics of the reactions involving micas, also changing as a function of microstructure (Ferraris et al. 2000, 2001; Usuki 2002). Despite the experimental and interpretation complexity, the "biotite-garnet" geothermometer is still one of the most reliable (Spear 1993; Holdaway 2000) and a prerequisite to use GASP geothermometer (Holdaway 2001).
As a general conclusion it can be stated that geothermobarometric models, when based only on mean chemical compositions of coexisting rock minerals, are of difficult petrological application and can be affected by significant errors. Much more reliable results can thus be obtained by combining chemical, crystallographic, spectroscopic, micro-structural and high temperature and pressure structural approaches (Hazen & Downs 2000, Pavese 2002), to provide a clear assessment of the stability of the mica crystals and a reliable petrological interpretation. It should anyway be considered that the experimental and interpretation demands, connected to the mentioned approach, are significant.
Micas can provide a significant insight also in assessing the rock forming conditions (thermodynamic parameters and fluid and gas activities) in igneous rocks. In alkaline-carbonatitic rocks, to quote an example, the chemical substitutions in the join phlogopite - tetra-ferriphlogopite were related to O2 fugacity and to H2O and CO2 activity (Gibson et al. 1995), whereas, in phlogopites from explosive volcanic sequences, Ti presence into the structure is related to the octahedral anionic site deprotonation, associated to the explosive activity of the magma (Feeley & Sharp 1996). Moreover Fe-rich phlogopites are considered H2O and K carriers into the upper mantle of the Earth (Kushiro et al. 1967) in ultramafic compositions (Wendtland & Eggler 1980; Sudo & Tatsumi 1990) and are believed to affect the large ion lithophile element ratios in the subduction zones (Melzer & Wunder 2001).
Extensive crystal chemical analyses on well-defined petrological series are almost completely missing in literature. In some areas of the Roman Magmatic Province micas are common and well crystallized, thus offering a good potential for petrological insight. In particular in Alban Hills, where the explosive magmatic activity dominated, the study of micas from freatomagmatic sequences, developed at the end of the process, could clarify their crystallization conditions (Funiciello et al. 2003). Another promising area is Mt. Vulture, especially considering the various petrogenetic/geological process experienced by this location (La Volpe & Principe 1989).
Other examples for the petrological significance of micas are provided by Fe-phlogopites in anatectic metapelites from El Joyazo (Cesare et al. 2003), where high Ti and low H content account for Ti-oxy substitution. Even higher TiO2 content were identified in xenoliths from Euganean Hills, which underwent a complex metamorphic history, partly as high-grade effects related to their incorporation within the trachytic magma, partly, presumably older, due to high-grade regional metamorphism (Sassi R. et al. 2004). The above mentioned mica from El Joyazo could offer a deeper knowledge of the Ti-involving substitution mechanisms.

A brief description of the Italian scientific background on micas follows. A particular reference will be given to the description of the researches carried out by UR joining the program. It is worth noting once again that, as also evidenced in the international overview above, the Italian groups played a significant role on micas throughout these last years.

MODENA (RU 1). Crystallographic studies on phyllosilicates, also relating crystal chemical features to crystallization conditions, have been characterizing Modena research for many years. In particular the compositional field of micas was defined by exchange vectors and numerous trioctahedral micas structures were refined (for a review: Brigatti & Guggenheim 2002 and Brigatti et al. 2003 a, b). Recently, crystal chemical investigations were introduced in a broader petrological context (Brigatti et al. 1996, 2000).

ROMA TRE (RU 2). This RU approached the study of di- and trioctahedral phyllosilicates, and more generally of rock forming silicates, by using synchrotron radiation adsorption spectroscopy techniques (Mottana et al. 1997; Mottana et al. 2002 ; Tombolini et al 2002 a, b, 2003). Theoretical models were derived on mica structure to validate the experimental results, when investigating extremely diluted atoms (Cardelli et al. 2003).

MILANO (RU 3). Milano approached the structural determination of 2M and 3T phengites, both from X-ray and neutron diffraction techniques, also including high temperature and pressure studies (Catti et al. 1989, 1994; Ivaldi et al. 2001; Pavese et al. 2002, 2003 a, b).

PADOVA (RU 4). Micas from metamorphic rocks were extensively studied in Padova, thus producing significant petrological results. Other studies involved unit cell dimensions of natural micas (Guidotti et al 1992), geothermometric information derived from muscovite-paragonite system (Blencoe et al. 1994; Guidotti et al. 1994a,b; Sassi et al. 1994) and on the geobarometric character of the muscovite-phengite join (Sassi & Scolari 1974; Guidotti & Sassi 1976, 1986, 1998, 2002). Other studies are related to polytypism of metamorphic muscovite (Sassi et al. 1994) and on high biotites crystallized at high T (Cesare et al 2003).

BARI (RU 5). Bari RU addressed the crystal chemical investigation of trioctahedral micas starting from a multi-analytical crystallographic and spectroscopic (in particular XPS and Mössbauer) approach (Schingaro et al. 2001).

In conclusion the program follows a cross-disciplinary approach, and the opportunities for synergies between joining RU are promising, as also evidenced by the already established cooperations. The scientific goals of the network are ambitious and request a multi-disciplinary cooperation. All the groups show a significant, internationally recognized competence, and a strong commitment to support the program activities by doing what they proved to do best in the past.
The overall picture on micas researches in Italy needs to mention the excellent results obtained in Perugia, both for what concerns high temperature and pressure studies (Comodi & Zanazzi 1995, 1997; Comodi et al. 2002, 2004) and phlogopites (Cruciani & Zanazzi 1994; Cruciani et al. 1995). <<<