Papers by Keyword: α-Cristobalite

Abstract: This paper describes a new method developed to produce α-cristobalite from silica mine waste under lower-than-usual temperature, and presents XRD (X-ray diffraction) and morphologic analysis of the produced α-cristobalite. In the method, silica mine waste with average grain volume of 87.3 µm is firstly refined and processed to have a whiteness of 83.5 and a chemical composition of SiO₂ of 99.87%, Al₂O₃ of 0.15%, and Fe₂O₃ of 0.015% (by weight), before calcinating the processed samples. The experiment results demonstrate that, under oxidizing condition, addition of mineralizer and occurrence of iron oxides as impurity in the mine waste, facilitate lowering both crystallization temperature of amorphous silica and calcination temperature of cristobalite from 1470°C to 1100-1250°C, shortening formation time of cristobalite to 3 hours, raising transformation rate up to 93.7%-99.7%, and increasing whiteness up to 92.3%-96.7%.

Abstract: Analytical and Molecular Mechanics methods have been used to study the deformation mechanisms acting at the molecular level in the auxetic polymorph of crystalline silica (a-cristobalite). The analytical models indicate that a-cristobalite deforms by concurrent tetrahedral dilation and cooperative rotation when stretched along the x3 axis, and that a second phase is predicted to exist for this loading scenario, having a geometry similar to that of ‘idealised’ b-cristobalite. This is supported by preliminary Molecular Mechanics simulations, which also indicate that the cooperative rotation predicted for loading along x3 is not sufficient to describe the deformation mechanism for loading along x1. A negative hydrostatic pressure offset is observed to lead to a change in the sign of the predicted Poisson’s ratio from positive to negative, leading to improved agreement of the Molecular Mechanics model with experiment.