Thaumasite, Ca3Si(OH)6(CO3)(SO4)•12H2O, is a rare mineral with a hexagonal structure based on cylindrical columns of [Ca3Si(OH)6(H2O)12]4+ running parallel to the c axis, among which the SO42- and the CO32- groups are hosted. Thaumasite is unique because i) is the only known mineral stable at ambient P–T conditions that possesses silicon hexa-coordinated by hydroxyls; ii) is an indicator of sulphate attack in concretes (deterioration due to thaumasite formation especially occurs at subzero T). The thermal behavior of thaumasite has been investigated at LT and HT [1-3], and it was found that the thaumasite structure collapsed because of a severe dehydration above 417 K [3]. At all events, reports on the HP response of the thaumasite structure are lacking. In this work, the HP evolution of the thaumasite structure was investigated using synchrotron XRPD, up to 19.5 GPa. Although during the Rietveld refinements no symmetry changes have been observed, the unit-cell decreasing with P is characterized by two strong discontinuities in the 1.38–3.53 GPa and 7.40–15.02 GPa P-ranges, respectively. Hence, three distinct compression regimes can be defined, with the thaumasite structure that retains the same P63 space group. These isosymmetric transitions occur with an increasing of the unit-cell parameters, revealing that the thaumasite structure becomes stiffer after each discontinuity. Comparing the LT and HT data from literature [1-3] with those collected under HP conditions, it is interesting to underline that the variation of the axial ratio a/c as a function of the normalized cell volume V/V0 describes an ideal inverse relationship [4]. [1] Jacobsen et al. (2003) Phys. Chem. Miner. 30, 321-329. [2] Gatta et al. (2012) Am Mineral. 97, 1060-1069. [3] Martucci & Cruciani (2006) Phys. Chem. Miner. 33, 723-731. [4] Hazen & Finger (1984) Comparative crystal chemistry. Wiley, New York, pp 231.
High-pressure behavior and phase transitions of thaumasite
ARDIT, Matteo;CRUCIANI, Giuseppe;
2013
Abstract
Thaumasite, Ca3Si(OH)6(CO3)(SO4)•12H2O, is a rare mineral with a hexagonal structure based on cylindrical columns of [Ca3Si(OH)6(H2O)12]4+ running parallel to the c axis, among which the SO42- and the CO32- groups are hosted. Thaumasite is unique because i) is the only known mineral stable at ambient P–T conditions that possesses silicon hexa-coordinated by hydroxyls; ii) is an indicator of sulphate attack in concretes (deterioration due to thaumasite formation especially occurs at subzero T). The thermal behavior of thaumasite has been investigated at LT and HT [1-3], and it was found that the thaumasite structure collapsed because of a severe dehydration above 417 K [3]. At all events, reports on the HP response of the thaumasite structure are lacking. In this work, the HP evolution of the thaumasite structure was investigated using synchrotron XRPD, up to 19.5 GPa. Although during the Rietveld refinements no symmetry changes have been observed, the unit-cell decreasing with P is characterized by two strong discontinuities in the 1.38–3.53 GPa and 7.40–15.02 GPa P-ranges, respectively. Hence, three distinct compression regimes can be defined, with the thaumasite structure that retains the same P63 space group. These isosymmetric transitions occur with an increasing of the unit-cell parameters, revealing that the thaumasite structure becomes stiffer after each discontinuity. Comparing the LT and HT data from literature [1-3] with those collected under HP conditions, it is interesting to underline that the variation of the axial ratio a/c as a function of the normalized cell volume V/V0 describes an ideal inverse relationship [4]. [1] Jacobsen et al. (2003) Phys. Chem. Miner. 30, 321-329. [2] Gatta et al. (2012) Am Mineral. 97, 1060-1069. [3] Martucci & Cruciani (2006) Phys. Chem. Miner. 33, 723-731. [4] Hazen & Finger (1984) Comparative crystal chemistry. Wiley, New York, pp 231.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
