Rehydration properties of Na-rectorite from Makurazaki, Kagoshima Prefecture, Japan

Abstract

The mineralogical properties and rehydration behaviors for Na-rectorite from Makurazaki, Kagoshima Prefecture, Japan were examined.
X-ray diffraction analysis showed that the mineral has 25.3Å average basal spacing, which moved to 26.8Å by treatment with ethylene glycol. Chemical analysis proved a structural formula of (Na_0.81K_0.40Mg_0.07Ca_0.05) (Al_3.66Fe³⁺ _0.18Mg_0.06Ti_0.07) (Si_6.64Al_1.36)O₂₀(OH)₄, and the compositions of mica and smectite layers belong to those of paragonite and beidellite, respectively.
Ca-, Mg-, Na- and K-saturated specimens showed remarkable rehydration properties after heating at various temperatures. The d(001) spacings of the specimens heated below 400°C did not change on rehydration. After rehydration of the specimens heated above 500°C, the Ca- and Mg-saturated specimens showed contraction of the d(001) spacings, and the Na- and K-saturated specimens showed expansion of the d(001) spacings. In the case of exposure to air for the specimens heated at 400°C, the K-saturated specimen showed strong rehydration properties, while the Mg-saturated specimen did not rehydrate even after exposure to air for 2 days. These rehydration behaviors for the specimens heated below 400°C can be explained by rehydration mechanism for smectite group minerals. However, the rehydration behaviors of the specimens heated above 500°C are completely different from those of smectite group minerals. These behaviors are due to difference of positions of the interlayer cations after rehydration. In the case of specimens heated below 400°C, the interlayer cations migrated into hexagonal holes and returned to the interlayer space after rehydration. In the case of specimens heated above 500°C, on the other hand, the interlayer cations were fixed to the hexagonal holes, and keeping this rearranged configuration, water molecules were regained in the interlayer space. Therefore, the coordination states of water molecules to the interlayer cations changed. DTA and IR spectra for rehydrated specimens supported this consideration.