Enthalpies of hydration of low-temperature hydrotalcite(HT) metaphases have been measured by means of the adiabatic water-vapor absorption calorimeter. The metaphases were characterized by the Mg/Al ratio of 3 : 1 and CO₃²⁻ interlayer group. The hydration heat for HT-B (partially dehydroxylated HT with bidentate bonded CO₃²⁻) is 2-3 times larger than that of HT-D (dehydrated HT), if they are treated at the same dehydration temperature. Large absolute values of hydration heat, namely 398±4 and 508±10Jg⁻¹, were obtained for the HT-B at respective low dehydration temperatures, 70 and 110°C . This may promise the HT-B to be a new prospective heat exchanger usable at relatively low temperatures.

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The crystal structure of rengeite was analyzed with a single crystal of the type specimen; (Sr_3.62Ca_0.08Ce_0.03Nd_0.02Ba_0.01Pr_0.01)_Σ3.76(Ti_4.09Zr_0.84Al_0.04Fe_0.02Nb_0.02)_Σ5.01Si_4.11O₂₂, a=13.9830(10), b=5.6722(9), c=11.9960(10) Å, β=114.215(7)°, V=867.74(17) ų, Z=2. A refinement with space group P2₁/a showed strong correlations between the parameters of oxygen atoms related by pseudo-mirror planes, and gave non-positive mean square displacement parameters for the oxygen atoms. A refinement with space group P2₁/a and isotropic displacement parameters for oxygen atoms was converged to R1 [F_O>4δ(F_O)]=0.0523. On the other hand, a refinement with space group C2/m and anisotropic displacement parameters, in which several very weak observed reflections were ignored, was successfully converged to R1=0.0485. No significant differences in atomic parameter could be observed between the refinements with C2/m and P2₁/a, except for the x of O(1) site. Although the true space group of rengeite is P2₁/a, the space group can be regarded as pseudo-C2/m. Rengeite is isostructural with perrierite-(Ce). Zirconium in rengeite occupies preferentially one of three octahedral sites, and does not share the site with Ti. Therefore, the formula of rengeite is not Sr₄(Ti,Zr)₅(Si₂O₇)₂O₈, but Sr₄ZrTi₄(Si₂O₇)₂O₈. A re-indexing of the XRD data of strontio-chevkinite with reference to those of rengeite suggested that strontio-chevkinite might be an Fe-rich variety of rengeite.

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The molecular dynamics method was applied to aluminous perovskite solid solution in the system MgSiO₃-Al₂O₃. The two-body interatomic potential model CMAS94 (Matsui, 1994) was used for this purpose. The solid solution crystals (Mg_1−x,Al_x)(Al_x,Si_1−x)O₃ (0≤x≤1) accommodating Al³⁺ ions without the production of vacant site were realized by the following two methods:
(1) To replace Mg²⁺ and Si⁴⁺ ions with 2Al³⁺ ions at random disregarding the arrangement of Mg²⁺ and Si⁴⁺ in the crystal (Random substitution).
(2) To replace a pair of (Mg²⁺+Si⁴⁺) with 2Al³⁺ ions at the adjacent sites, so as to satisfy the local charge balance.
   Although the latter solid solution model gives slightly lower values of enthalpy, both models show virtually the same energetic and structural characteristics. Under atmospheric condition, both the unit-cell parameter c and the cell volume increase remarkably with the increase in the Al₂O₃ content. At around 40 - 60 GPa, however, the volume is almost insensitive to the Al₂O₃ content. Al₂O₃-containing perovskite is more compressible than aluminum-free perovskite. It is consistent with the results of recent high-pressure in-situ X-ray studies below 15 GPa. This compositional effect on compressibility becomes much smaller with increase in pressure.
   A positive excess enthalpy for the MgSiO₃-Al₂O₃ binary system is observed.

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In order to examine the phase relations of enstatite at high temperatures, high temperature X-ray powder diffraction experiments were performed with low clinoenstatite MgSiO₃ as starting materials. It was observed that upon heating low clinoenstatite transformed into protoenstatite at around 1100°C and protoenstatite persisted up to the melting point, indicating that protoenstatite is the high temperature stable phase for MgSiO₃ up to the melting point. Two extra peaks (d=3.074-3.079 Å and 3.323-3.331 Å at 1200°C) which could not be indexed as low clinoenstatite or protoenstatite, appeared along with protoenstatite at around 1150°C, but they disappeared at temperatures above ∼1400°C. These extra peaks did not reappear on cooling of protoenstatite. This indicates that these extra peaks originate from high temperature clinoenstatite metastably transformed from low clinoenstatite and that high temperature clinoenstatite has no stability field above and below that of protoenstatite.

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