Abstract
The crystal structure of rengeite was analyzed with a single crystal of the type specimen; (Sr3.62Ca0.08Ce0.03Nd0.02Ba0.01Pr0.01)Σ3.76(Ti4.09Zr0.84Al0.04Fe0.02Nb0.02)Σ5.01Si4.11O22, a=13.9830(10), b=5.6722(9), c=11.9960(10) Å, β=114.215(7)°, V=867.74(17) Å3, 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 [FO>4δ(FO)]=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.
