Crystal chemistry of Ca₂Al_3-pMn³⁺_pSi₃O₁₂(OH)piemontite: Mn³⁺ distribution and its effect on the crystal structure

Abstract

Mn³⁺ distribution in octahedral M1, M2 and M3 sites of piemontite and its effect on the structural changes were investigated in terms of X-ray Rietveld analysis of Ca₂Al_3-pMn_pSi₃O₁₂(OH)-piemontite synthesized by hydrothermal experiments at 200-350 MPa and 500-550 C. Piemontites were synthesized almost as single-phase from p=0.5, 0.75, 1.0 and 1.1 starting materials, whereas those crystallized from p=1.5 and 1.75 starting materials were associated with minor amounts of bixbyite and parawollastonite. The maximum Mn³⁺-content was 1.3 apfu. The site preference order of Mn³⁺ is M3>M1>>M2. Mn³⁺ enters M1 site even if M3 site is not filled by Mn³⁺ and Mn³⁺ occupancy of M1 site attains up to 0.30. K_D (=(Mn/Al) in M1/(Mn/Al) in M3) ranges between 0.024 and 0.120. If both Mn³⁺ and Fe³⁺ distribute in the octahedral sites, K_D of Mn³⁺ would become smaller. The a-, b- and c-dimensions vary with p as follows: a (A)=0.037p²-0.062p+8.879; b (A)=0.005p²+0.085p+5.585, c (A)=0.021p³-0.008p²-0.022p+10.155. The nonlinear variations of a- and c-dimensions with increasing p are caused by those of the M1-O distance (=0.115p²-0.081p+1.930 A) and O5-Si3-O6 angle (=11.322p²-10.165p+100.200 deg), respectively. The variations of the M3-O1 and M3-O2 distances reflect the distortion of M3 site, and that of the M1-O1 the M1 site deformation.