The thermal expansion of synthetic aluminosilicate-sodalites, M ₈(Al₆Si₆O₂₄)X ₂

Abstract

Thermal expansion data, determined by powder X-ray diffraction methods are presented for 11 members of the (Li,Na,K,Rb)₈(Al₆Si₆O₂₄)Cl₂ solid solution series, 3 members of the (Na,K)₈(Al₆Si₆O₂₄)Br₂ solid solution series and Na₈(Al₆Si₆O₂₄)I₂. Only the latter showed a discontinuity in its expansion curve at 810° C wigh a mean linear expansion coefficient of 22.0×10⁻⁶ °C⁻¹ below and 7.7×10⁻⁶ °C⁻¹ above the discontinuity. The mean expansion coefficients from 0° to 500° C decrease gradually over the range of room temperature cell edges from 8.4 to 8.89 Å, then increase up to a cell edge of 9.01 Å above which they decrease sharply and extrapolate to a zero coefficient at 9.4 Å. These variations may be related to the expansion characteristics of the bonds between the cavity cations and cavity anions in different sodalites. The aluminosilicate-sodalites which show a discontinuity in their thermal expansion curves are those with large cavity anions, I⁻ or SO ²⁻₄ ; the discontinuity is believed to occur at the point when the x-coordinate of the cavity cation becomes 0.25.