Carbon-Bearing Cordierite from Allumiere (Tolfa Volcanic Center, Latium, Italy): Occurrence, Crystal-Structure and FTIR Microspectroscopy
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Per. Mineral. (2006), 75, 2-3, 113-126 http://go.to/permin An International Journal of O PERIODICO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage Carbon-bearing cordierite from Allumiere (Tolfa volcanic center, Latium, Italy): occurrence, crystal-structure and FTIR microspectroscopy Giancarlo Della Ventura1,2*, Fabio Bellatreccia1, Fernando Cámara2, Roberta Oberti2, Jean-Pierre Lorand3, Gian Carlo Parodi3, Gabriel Carlier3 and Dario Di Domenico4 1 Dip. Scienze Geologiche, Università Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy 2 CNR-Istituto di Geoscienze e Georisorse, Unità di Pavia, via Ferrata 1, I-27100 Pavia, Italy 3 Unité Scientifique de Minéralogie CNRS 7160, Dept. Histoire de la Terre, Museum National d’Histoire Naturelle, 61, rue Buffon, F-75005 Paris 4 Via VII Settembre 9, I-00051 Allumiere, Roma, Italy Abstract. — This paper reports the crystal- absorption coefficients available in the literature, H2O chemical study of cordierite from a volcanic ejectum = 0.02 wt% and CO2 = 0.22 wt% are calculated from from the Tolfa volcanic center (Latium, Italy). The the measured band intensities. The observed mineral studied rock was sampled within the pyroclastic assemblage and the volatile composition of cordierite deposit (“tufo caotico”) outcropping in a quarry at are coherent with a thermometamorphic overprint Colle dello Spizzicatore, Allumiere Community. on an early metapelite, which occurred at low-P (≤ The sample was studied by electron microprobe 2 Kbar), high-T (> 700°C) and extremely reducing analysis (EMP), single-crystal structure refinement conditions. and Fourier-transform infrared microspectroscopy (FTIR). Combination of these techniques allowed Key Words: cordierite, Allumiere, X-ray structure detailed characterisation of the amount, location and refinement, EMP analysis, polarised FTIR coordination of cations and volatile molecules in the spectroscopy, volatile composition. channels. Cordierite from Allumiere is orthorhombic, space Riassunto. — In questo lavoro viene descritto group Cccm; refined cell-parameters are (in Å): a = lo studio cristallochimico della cordierite di un 17.1580(6), b = 9.7573(3) and c = 9.3228(3). EMP proietto vulcanico dell’apparato della Tolfa (Lazio, analyses show an intermediate XMg [Mg/(Mg+Fe) = Italia). La roccia esaminata è stata campionata 0.58], and a very low alkali (Na + K = 0.042 apfu) all’interno del deposito piroclastico (“tufo caotico”) content. Single-crystal FTIR spectra are strongly affiorante in una cava a Colle dello Spizzicatore, polarised; the absorptions observed in the H2O- Comune di Allumiere. Il campione è stato esaminato stretching region can be assigned to H2O molecules attraverso analisi in microsonda elettronica (EMPA), oriented with the H…H vector // a and the molecular raffinamento strutturale su cristallo singolo e plane // (001). The absorptions observed in the C-O microspettroscopia infrarossa in trasformata di Fourier stretching region can be assigned to both CO2 and (FTIR). Combinando queste tecniche è stato possibile CO molecules oriented // a; the spectra also show the caratterizzare la quantità, posizione e coordinazione presence of 13C and 18O in the sample. Using the molar dei cationi e degli elementi volatili presenti nei canali strutturali del minerale. La cordierite di Allumiere è ortorombica, * Corresponding author, E-mail: [email protected] gruppo spaziale Cccm; i parametri di cella sono (in 114 G. Della Ventura, F. Bellatreccia, F. Cámara, R. Oberti, J.-P. Lorand, G.C. Parodi, G. Carlier and D. Di Domenico Å): a = 17,1580(6), b = 9,7573(3) e c = 9,3228(3). Le of studies, using a large variety of techniques, analisi microchimiche mostrano un rapporto Mg/Fe including optics (Armbruster and Bloss, 1982), [XMg = Mg/(Mg+Fe)] = 0,58, e un basso contenuto in X-ray and neutron diffraction (Cohen et al. 1977, alcali (Na + K = 0,042 apfu). Gli spettri FTIR ottenuti Armbruster and Bürgi 1982, Armbruster 1985), su cristallo singolo sono fortemente polarizzati; gli proton NMR spectroscopy (Carson et al. 1982), assorbimenti osservabili nella regione di stiramento thermogravimetry (Sugiura, 1959), secondary ion dell’H2O possono essere attribuiti a molecole di mass spectrometry, SIMS (Harley et al. 2002), acqua orientate con il vettore H…H // a ed il piano confocal micro-Raman spectroscopy (Kaindl et della molecola // (001). Gli assorbimenti presenti al. 2006) and ab-initio molecular calculations nella regione di stiramento dei legami C-O possono (Winkler et al. 1994). Most of these studies have essere attribuiti a molecole di CO2 e CO orientate been however done using infrared (IR) spectroscopy entrambe // a; gli spettri mostrano anche la presenza 13 18 (Farrell and Newnham 1967, Goldman et al. 1977, di C and O nel campione. Utilizzando i coefficienti Aines and Rossman 1984, Le Breton 1989, Kolesov di assorbimento molare disponibili in letteratura, è and Geiger 2000a, Khomenko and Langer 2005). possibile calcolare un contenuto di H O di 0,02 wt% 2 In fact, this technique, when coupled with Raman e di CO di 0,22 wt%. La composizione mineralogica 2 spectroscopy, is able to unambiguously locate della roccia, e la composizione dei volatili nei canali della cordierite sono concordi nel suggerire un evento the volatile molecules within the structural pores termometamorfico a bassa P (≤ 2 Kbar) ed alta T and to clarify their interaction with the silicate (> 700°C) avvenuto in condizioni estremamente framework; if properly calibrated it is a rapid and riducenti su una metapelite preesistente. relatively easy tool to characterise volatiles in cordierite. IR spectroscopy in fact requires only Parole chiave: cordierite, Allumiere, raffinamento small amounts of material, namely few milligrams strutturale, analisi microchimica, spettroscopia for powder analysis (Vry et al. 1990) and small FTIR in luce polarizzata, contenuto in volatili. (few tenths of μm across) grains for single-crystal analysis. Despite the numerous efforts, however, several problems still persist, especially regarding the location and behaviour of the H O molecules Introduction 2 within the channels. This point is crucial when attempting to use spectroscopy as an analytical tool Cordierite, (Mg,Fe)2Al4Si5O18•(n H2O, m CO2) is a powerful mineral probe to monitor the fluid for cordierite in petrological studies, because any conditions during high-grade metamorphism (Smith calibration for quantitative purposes must be based 1996, Harley et al. 2002). It is in fact a rare case on unambiguous assignments of the measured absorptions. of a microporous mineral stable under geological In this paper we describe a new finding conditions spanning from the amphibolite facies of cordierite in an unusual occurrence, and to the crustal anatexis and is able to incorporate provide its complete characterisation combining significant amounts of both H O and CO within 2 2 microchemistry, single-crystal structure its structural channels, as well as other molecular refinement, and FTIR analysis. species, such as hydrocarbons (Zimmermann 1981, Mottana et al. 1983, Khomenko and Langer 1999), Ar (Armbruster 1985, Schreyer 1985), Description of the sample N2 (Armbruster, 1985) and CO (Khomenko and Langer, 2005). Provided these channel constituents The examined sample was collected by one of us are preserved during the mineral equilibration and (D. Di Domenico) within the explosive pyroclastic its subsequent geological history, their content deposit known in literature as “tufi caotici” which can be used to constrain the volatile composition is characteristically found at the base of the Mt. (particularly with respect to H2O and CO2) of Sassetto ignimbritic formation (Negretti 1962). coexisting fluids or melts during crystallisation The sampled rock in particular outcrops in a quarry (e.g. Vry et al., 1990, Harley et al., 2002). The at Colle dello Spizzicatore, Allumiere Community, incorporation and orientation of H2O and CO2 in Tolfa volcanic center (Latium, Italy). The studied cordierite has been the object of a large number deposit of “tufi caotici” consists of accumulated Carbon-bearing cordierite from Allumiere (Tolfa volcanic center, Latium, Italy): occurrence, ... 115 ejected blocks, from centimetric to metric in Université Paris VI. Operating conditions were 15 dimension, cemented by a fine-grained glassy ash. kV accelerating voltage and 15 nA beam current, Several types of rocks occur in the deposit; there with a beam size of 5 μm; counting time was 10 are fragments of ignimbrites which show, under sec on both peak and background. Standards, the polarizing microscope, a glassy groundmass spectral lines and crystals used were: diopside with K-feldspar, labradoritic plagioclase, biotite, (SiKα, TAP; MgKα TAP; CaKα, PET), orthoclase ortho- and clino-pyroxene as main phenocrystals. (AlKα, TAP; KKα, PET), albite (NaKα, TAP), Other holocrystalline tephra have a syenitic Fe2O3 (FeKα LIF). Analytical errors are 1% rel. appearance and are constituted by dominant K- for major elements and 5% rel. for trace elements. feldspar and plagioclase, with minor pyroxene Data reduction was done using the PAP method and biotite; common accessories are rutile and (Pouchou and Pichoir, 1985). zircon (Negretti 1962). The host rock where the studied cordierite occurs, is found as blocks with dimensions up to 1 meter and appears as very fine-grained holocrystalline type. Studied