High‐Temperature, High‐Pressure X‐ray Investigation of Dicalcium Silicate

Abstract

Energy-dispersive X-ray powder diffraction experimentstemperature; 1-3,11 and finally ␣, the highest-temperature polyhave been investigated at high temperature and room presmorph whose structure is still uncertain and has trigonal 12,13 or sure, and at high pressure and room temperature, starting hexagonal symmetry. 14 The structures of all of these phases are from either ␥or ␤-Ca 2 SiO 4 . High-temperature studies built from isolated SiO 4 tetrahedra. The differences come from were performed up to 1980 K, using a versatile heating cell.the orientation of the SiO 4 tetrahedra and the movements of The high-temperature phase transformations previously calcium ions. described were reexamined. Volume and linear thermalDespite the numerous X-ray diffraction studies at elevated expansions were measured for each Ca 2 SiO 4 polymorph, ␥, temperature, few papers include the investigation of all of the ␤, ␣ L , ␣ H , and ␣. Volume thermal expansion increases Ca 2 SiO 4 polymorphs. Indeed, in most cases the starting material with increasing temperature except for ␣ H , whose thermal is either the ␥ 5,15 or the ␤ form of Ca 2 SiO 4 , 3,4 and because of the expansion tends to decrease at elevated temperature. Highdifficulty in reaching the highest temperatures, the ␣ phase pressure investigations were performed in the 0-15 GPa has scarcely been studied. 2,16,17 It also appears that the thermal pressure range, using a diamond anvil cell, with silicon oil as expansion data are not well-defined, in particular for ␣Ј H -and the pressure-transmitting medium. The value of the room-␣-Ca 2 SiO 4 . Authors generally report the unit-cell parameters or pressure bulk modulus K 0 , assuming a second-order Birchvolume data only at selected temperatures. 2-4 Furthermore, a Murnaghan equation of state with K 0 ‫؍‬ 4, is 140(8) GPa for great confusion still persists in the literature about the exact ␥-Ca 2 SiO 4 . The ␥ olivine form exhibits anisotropic comsequence of the pure Ca 2 SiO 4 phase transformations with pression, with the c axis as the most compressible. From increasing temperature. The scheme usually proposed is that in such in situ high-pressure X-ray investigations, the ␥-→ Fig. 1 but other possibilities have been reported. For instance, ␤-Ca 2 SiO 4 phase transformation induced by cold compresanother monoclinic form called ␣Ј m has been observed in the sion is clearly evidenced and extends from 2 to about 5 GPa.range 984-1252 K. 19 Again, Sarkar suggested that a "transient phase" between ␥ and ␣Ј L existed over the temperature range I. Introduction 1089-1183 K, 5 and the existence of a cubic phase at temperatures higher than 1873 K was also mentioned. 16 The characteris-

The crystal chemistry and the compressibility of silicate-carbonate minerals: Spurrite, galuskinite and tilleyite

“…The Fiquet et al (1994) and Remy et al (1997), respectively. Standard deviations on K 0 values of silicate-carbonate minerals are smaller than data points, while larnite with the error bars correspond to a w9% uncertainty.…”

Molecular Dynamics Simulation of the Thermodynamic and Structural Properties for the CaO-SiO2 System

“…3. 25) In Fig. 2, the temperature dependence of molar volume calculated from the g-Ca 2 SiO 4 shows the large volume decrease at about 1 750 K (g-a phase transition) and the large volume increase at about 2 420 K (melting temperature).…”

Fundamental studies on the chemical aspects of freeze linings

“…This temperature is well above α' → β and β → γ transformation temperatures of 630-680 and 500 °C respectively [108]. As a result, the freeze lining was stable during the experiment.…”

“…This is thought to be caused by a combination of the 12 vol.% increase associated with the β → γ transformation of the dicalcium silicate compound [108], and the thermal stresses caused by the high temperature gradients induced during quenching. This is similar to the behaviour of so-called 'self-dusting' or 'falling' slags [109,110] on cooling.…”