Cancrinite group minerals, [(Na,K)6(CO3,SO4)][(Na,Ca)2(H2O,Cl)2][Al6Si6O24], are modular microporous minerals, which are built up by a single module and may present increasing complexity structures depending on the number of layers and on the staking sequence [1]. Few studies on the thermal behavior of cancrinite and related phases are available. These have been undertaken up to date only on AB... stacking sequences: cancrinite [2,3,4], pitiglianoite [1,5], davyne and microsommite [6], and also very recently cancrisilite [4]. Non linear increase of cell parameters and eventual discontinuities have been described. Some have been related to dehydration [1,5], structure rearrangement [3] or even displacive phase transitions [6]. It is evident that these microporous materials have a non trivial thermal behavior and that additional studies may help to seed light. The next step in structural complexity of cancrinite group minerals is represented by bystrite [(Na,K)7Ca] [Al6Si6O24](S2-)1.5 • H2O (ABAC..., P31c) [7]. However, from a comparative point of view the presence of S2- anions characteristic of bystrite makes difficult comparison with AB... sequence minerals. Bystrite has two LOSOD cages, and two ϵ (cancrinite) cages per unit cell. Recently a new mineral of the cancrinite group minerals has been described: carbobystrite, ideally Na8[Al6Si6O24](CO3) • 4H2O [8], which is isostructural with bystrite. Its compositions is closely related to cancrinite - having (CO3) groups and H2O groups in the LOSOD cages and H2O groups in the ϵ cages - which makes it a perfect candidate for comparative work. Carbobystrite shows also Na-1K substitution in the LOSOD cages. A single crystal of carboystrite was studied by in situ high-T X-ray diffraction up to 525°C. Lattice parameters were collected each 25 °C steps and complete intensity data collections were performed at 25, 125, 225, 325, and 425 °C and then at room T after cooling down. Upon heating, the cell parameters increased in the range 25–300 °C, with the a lattice showing a very faint decrease at 75°C. After a couple of hours at 300°C the structure started to collapse of a 5 ‰ in volume. Upon heating the collapse progress down to ca. 1 %. Structure refinements show the starting of a dehydration reaction with loss of half a H2O group in the channels and migration of K from the LOSOD cages into the ϵ cages, with concomitant inverse migration of Na (as already observed in pitiglianoite by [5]). The process keeps going slowly and progressively involves also the total release of H2O from the ϵ cages. At 525 °C the crystal broke in smaller pieces compromising the collection of intensity data. Thus only lattice parameters were collected on cooling. Structure refinement of the larger fragment on a Bruker CCD at room-T reveals that the cracking of the crystal is due to the completion of dehydration (in agreement to what reported by [9] in pitiglianoite) by release of the remaining H2O groups from the LOSOD cages. No loss of (CO3) groups at the LOSOD cages is registered. After dehydration, Na and K in LOSOD cages occupy slightly different position at the top and lower windows formed by the 6-membered rings of tetrahedral and perpendicular to [001], showing an off-plane position. Same behavior is observed in the ϵ cages. References. [1] Bonaccorsi, E., Merlino, S. (2005): Rev Mineral & Geochem, 57, 241-290; [2] Ballirano, P., Maras, A., Caminiti, R., Sadun, C. (1995): Powder Diffr, 10, 173-177; [3] Hassan, I., Antao, S. M., Parise, J. B. (2006): Am Mineral, 91,1117–1124; [4] Ogorodova, L. P., Mel’chakova, L. V., Vigasina, M. F., Olysich, L. V., Pekov, I. V. (2009): Geochem Int, 47, 260–267; [5] Bonaccorsi, E., Della Ventura, G., Bellatreccia, F., Merlino, S. (2007): Microporous and Mesoporous Materials, 99, 225–235; [6] Bonaccorsi, E., Comodi, P., Merlino, S. (2005): Phys Chem Minerals, 22, 367-374; [7] Pobedimskaya, E.A., Terentieva, L.E., Sapozhnikov, A.N., Kashaev, A.A., Dorokhova, G.I. (1991): Dokl. Akad. Nauk SSSR 319, 873-878; [8] Khomyakov, A. P., Cámara, F., Sokolova, E. (2010): Can Min, 48, 291-300; [9] Della Ventura, G., Bellatreccia, F., Bonaccorsi, E. (2005): Eur. J. Mineral., 17, 847-851.
HT-study of carbobystrite, Na8[Al6Si6O24](CO3)•4H2O.
CAMARA ARTIGAS, Fernando
2010-01-01
Abstract
Cancrinite group minerals, [(Na,K)6(CO3,SO4)][(Na,Ca)2(H2O,Cl)2][Al6Si6O24], are modular microporous minerals, which are built up by a single module and may present increasing complexity structures depending on the number of layers and on the staking sequence [1]. Few studies on the thermal behavior of cancrinite and related phases are available. These have been undertaken up to date only on AB... stacking sequences: cancrinite [2,3,4], pitiglianoite [1,5], davyne and microsommite [6], and also very recently cancrisilite [4]. Non linear increase of cell parameters and eventual discontinuities have been described. Some have been related to dehydration [1,5], structure rearrangement [3] or even displacive phase transitions [6]. It is evident that these microporous materials have a non trivial thermal behavior and that additional studies may help to seed light. The next step in structural complexity of cancrinite group minerals is represented by bystrite [(Na,K)7Ca] [Al6Si6O24](S2-)1.5 • H2O (ABAC..., P31c) [7]. However, from a comparative point of view the presence of S2- anions characteristic of bystrite makes difficult comparison with AB... sequence minerals. Bystrite has two LOSOD cages, and two ϵ (cancrinite) cages per unit cell. Recently a new mineral of the cancrinite group minerals has been described: carbobystrite, ideally Na8[Al6Si6O24](CO3) • 4H2O [8], which is isostructural with bystrite. Its compositions is closely related to cancrinite - having (CO3) groups and H2O groups in the LOSOD cages and H2O groups in the ϵ cages - which makes it a perfect candidate for comparative work. Carbobystrite shows also Na-1K substitution in the LOSOD cages. A single crystal of carboystrite was studied by in situ high-T X-ray diffraction up to 525°C. Lattice parameters were collected each 25 °C steps and complete intensity data collections were performed at 25, 125, 225, 325, and 425 °C and then at room T after cooling down. Upon heating, the cell parameters increased in the range 25–300 °C, with the a lattice showing a very faint decrease at 75°C. After a couple of hours at 300°C the structure started to collapse of a 5 ‰ in volume. Upon heating the collapse progress down to ca. 1 %. Structure refinements show the starting of a dehydration reaction with loss of half a H2O group in the channels and migration of K from the LOSOD cages into the ϵ cages, with concomitant inverse migration of Na (as already observed in pitiglianoite by [5]). The process keeps going slowly and progressively involves also the total release of H2O from the ϵ cages. At 525 °C the crystal broke in smaller pieces compromising the collection of intensity data. Thus only lattice parameters were collected on cooling. Structure refinement of the larger fragment on a Bruker CCD at room-T reveals that the cracking of the crystal is due to the completion of dehydration (in agreement to what reported by [9] in pitiglianoite) by release of the remaining H2O groups from the LOSOD cages. No loss of (CO3) groups at the LOSOD cages is registered. After dehydration, Na and K in LOSOD cages occupy slightly different position at the top and lower windows formed by the 6-membered rings of tetrahedral and perpendicular to [001], showing an off-plane position. Same behavior is observed in the ϵ cages. References. [1] Bonaccorsi, E., Merlino, S. (2005): Rev Mineral & Geochem, 57, 241-290; [2] Ballirano, P., Maras, A., Caminiti, R., Sadun, C. (1995): Powder Diffr, 10, 173-177; [3] Hassan, I., Antao, S. M., Parise, J. B. (2006): Am Mineral, 91,1117–1124; [4] Ogorodova, L. P., Mel’chakova, L. V., Vigasina, M. F., Olysich, L. V., Pekov, I. V. (2009): Geochem Int, 47, 260–267; [5] Bonaccorsi, E., Della Ventura, G., Bellatreccia, F., Merlino, S. (2007): Microporous and Mesoporous Materials, 99, 225–235; [6] Bonaccorsi, E., Comodi, P., Merlino, S. (2005): Phys Chem Minerals, 22, 367-374; [7] Pobedimskaya, E.A., Terentieva, L.E., Sapozhnikov, A.N., Kashaev, A.A., Dorokhova, G.I. (1991): Dokl. Akad. Nauk SSSR 319, 873-878; [8] Khomyakov, A. P., Cámara, F., Sokolova, E. (2010): Can Min, 48, 291-300; [9] Della Ventura, G., Bellatreccia, F., Bonaccorsi, E. (2005): Eur. J. Mineral., 17, 847-851.
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