Thermal Decomposition of Fluorotetrasilicic Mica (KMg_2.5Si₄O₁₀F₂)

Abstract

In order to clarify the thermal property of fluorotetrasilicic mica (KMg_2.5Si₄O₁₀F₂), their thermal decomposition reactions in atmosphere were investigated by means of isothermogravimetric methods with specimens of large crystals and various grain sizes. X-ray diffractometry, scanning electorn microscopy and chemical analysis were used to examine the decomposition products.
Isothermal decomposition curves for each specimen (large crystal; 5 × 5 × 0.02 mm, 42-80mesh, 80-200 mesh and —325 mesh) at 1000-1100°C were all of the slowing-down type. The final weight loss value was 6.35% for each specimen, and the speed at which the final value was achieved becomes more rapid with decreased grain size and increased treating temperature.
The volatile component with thermal decomposition was mainly silicon tetrafluoride (SiF₄). The amount was 6.35%, determined in the same manner as used for the final weight loss value. The decomposition product was forsterite (Mg₂SiO₄); the amount was 35.0%. On the basis of these values, the reaction equation on the thermal decomposition of fluorotetrasilicic mica could be derived as follows:
KMg_2.5Si₄O₁₀F₂ → 0.25 SiF₄ + 1.0Mg₂SiO₄ + 1.0 amorphous matter (K-Mg-Si-O-F)
This reaction mechanism was best explained by the equation [F(α)=1-(1-α)¹/²=kt] which was based on a two-dimensional interface-controlled process. This suggests that the thermal decomposition reaction proceeded by the forward movement of the boundary between mother crystal and decomposition product. The apparent activation energy by the thermal decomposition was estimated to be 146 ± 16kJ/mol.