Beryl is an accessory mineral typical of pegmatitic rocks, with the ideal formula Al2Be3Si6O18 (space group P6/mcc). Its honeycomb structure is characterized by open channels parallel to the c axis. Chemical compositions of natural beryl substantially deviate from the ideal formula owing to complex cationic substitutions. Although beryl is a nominally anhydrous mineral, channels also typically contain H2O and CO2 molecules. H2O within the structure occurs in two distinct orientations, named type I or II, depending on the orientation of the twofold axis of the molecule relative to the c axis of beryl (Wood and Nassau 1968). Because of its size, the linear CO2 molecule is normal to the c axis and parallel to the larger dimension of the channel (Wood and Nassau, 1968; Charoy et al., 1996). Due to its bearing in petrology, the solubility of water and CO2 in the structurally related cordierite has been studied extensively while there are no data for beryl. We report here the preliminary results of an experimental study aimed at defining the solubility mechanisms of carbon dioxide into the beryl channels. Single-crystals of Cr-doped beryl (emerald) synthesized using the flux method by Hautefeuille and Perrey (1888) obtained from the Natural History Museum of Paris were used as starting materials. These crystals show a perfect hexagonal prismatic morphology and a size not exceeding 1 mm in length and 0.5 mm across; due to the synthesis procedures they are CO2-free and contain only few ppm water (Bellatreccia et al., 2008). Experiments were done using a non end-loaded piston cylinder at P in the range 200-500 MPa and T in the range 700-900°C. Samples were prepared as Pt tubes charged with emerald grains mixed with AgCO3 as a source for CO2. The run products were studied by optical methods using a spindle stage and by FTIR microspectroscopy. The results show that P has a strong effect on the solubility of carbon dioxide and that at least 24 hrs are needed to significantly diffuse CO2 into the beryl channels. Measurement of the refraction indexes show that after the treatment with CO2, the " index remains virtually constant while there is a strong increase of !. This is in agreement with the alignment of the linear CO2 molecule perpendicular to the c axis as also confirmed by polarized FTIR micro-spectroscopy. FTIR images obtained using an FPA detector show that carbon dioxide diffuses along the channel direction. References Bellatreccia, F. et al. (2008) N. Jb. Miner. Abh., 185, 11–16 Charoy, B. et al. (1996) Am. Mineral. 81, 395–403. Hautefeuille, P. and Perrey, A. (1888) Compte Rendues Acad. Sci. Paris, 106, 1800–1802. Wood, D.L. and Nassau, K. (1968) Am. Mineral. 53, 777–800.
DELLA VENTURA, G., Bellatreccia, F., Bellomo, L., Radica, F., Freda, C. (2012). Carbon dioxide into beryl: an experimental study. In European Mineralogical Conference.
Carbon dioxide into beryl: an experimental study
DELLA VENTURA, Giancarlo;BELLATRECCIA, FABIO;
2012-01-01
Abstract
Beryl is an accessory mineral typical of pegmatitic rocks, with the ideal formula Al2Be3Si6O18 (space group P6/mcc). Its honeycomb structure is characterized by open channels parallel to the c axis. Chemical compositions of natural beryl substantially deviate from the ideal formula owing to complex cationic substitutions. Although beryl is a nominally anhydrous mineral, channels also typically contain H2O and CO2 molecules. H2O within the structure occurs in two distinct orientations, named type I or II, depending on the orientation of the twofold axis of the molecule relative to the c axis of beryl (Wood and Nassau 1968). Because of its size, the linear CO2 molecule is normal to the c axis and parallel to the larger dimension of the channel (Wood and Nassau, 1968; Charoy et al., 1996). Due to its bearing in petrology, the solubility of water and CO2 in the structurally related cordierite has been studied extensively while there are no data for beryl. We report here the preliminary results of an experimental study aimed at defining the solubility mechanisms of carbon dioxide into the beryl channels. Single-crystals of Cr-doped beryl (emerald) synthesized using the flux method by Hautefeuille and Perrey (1888) obtained from the Natural History Museum of Paris were used as starting materials. These crystals show a perfect hexagonal prismatic morphology and a size not exceeding 1 mm in length and 0.5 mm across; due to the synthesis procedures they are CO2-free and contain only few ppm water (Bellatreccia et al., 2008). Experiments were done using a non end-loaded piston cylinder at P in the range 200-500 MPa and T in the range 700-900°C. Samples were prepared as Pt tubes charged with emerald grains mixed with AgCO3 as a source for CO2. The run products were studied by optical methods using a spindle stage and by FTIR microspectroscopy. The results show that P has a strong effect on the solubility of carbon dioxide and that at least 24 hrs are needed to significantly diffuse CO2 into the beryl channels. Measurement of the refraction indexes show that after the treatment with CO2, the " index remains virtually constant while there is a strong increase of !. This is in agreement with the alignment of the linear CO2 molecule perpendicular to the c axis as also confirmed by polarized FTIR micro-spectroscopy. FTIR images obtained using an FPA detector show that carbon dioxide diffuses along the channel direction. References Bellatreccia, F. et al. (2008) N. Jb. Miner. Abh., 185, 11–16 Charoy, B. et al. (1996) Am. Mineral. 81, 395–403. Hautefeuille, P. and Perrey, A. (1888) Compte Rendues Acad. Sci. Paris, 106, 1800–1802. Wood, D.L. and Nassau, K. (1968) Am. Mineral. 53, 777–800.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
