Distribution of Sc³⁺ at the octahedral sites and its effect on the crystal structure of synthetic Sc-bearing clinozoisite on the Ca₂Al₃Si₃O₁₂(OH)-Ca₂Al₂ScSi₃O₁₂(OH) join

Abstract

Synthetic Sc-bearing clinozoisite on the Ca₂Al₃Si₃O₁₂(OH)-Ca₂Al₂Sc³⁺Si₃O₁₂(OH) join was studied by single-crystal X-ray diffraction to understand better the distribution of Sc³⁺ among the octahedral sites, M1-M3, and its effect on the structure of epidote-group minerals. Oxide starting materials of Ca₂Al₂(Al_1-p)Sc_pSi₃O_12.5 composition with p = 0.5 and 1.0 were employed, and clinozoisite was successfully synthesized at P_H2O = 1.2–1.5 GPa and T = 700–800 °C. The Sc content in clinozoisite varies and attains 0.61 atoms per formula unit (apfu) from p = 1.0 starting material. Two Sc-bearing clinozoisite crystals from the product of p = 0.5 starting material (Run 20) were used for X-ray crystal structural analysis. The unit-cell parameters are a = 8.8815(4), b = 5.6095(2), c = 10.1466(5) Å, β = 115.318(6)º, and V = 457.0(1) ų for 20B, and a = 8.885(1), b = 5.6119(4), c = 10.153(1) Å, β = 115.27(2)º, and V = 457.9(4) ų for 20D. The resulting Sc³⁺ occupancies among the octahedral sites are ^M1Al_1.0^M2Al_1.0^M3(Al_0.684(7)Sc³⁺_0.316) for the former and ^M1Al_1.0^M2Al_1.0^M3(Al_0.629(6)Sc³⁺_0.371) for the latter, i.e., Sc³⁺ exclusively occupies M3. The mean ionic distance of < M3–O > increases with increasing Sc content at M3, but it tends to be slightly shorter than the expected value using the regression line based on the structural data of synthetic Ca₂(Al, Me³⁺)₃Si₃O₁₂(OH) clinozoisite. It is due to the reduced distortion of M3O₆ octahedra caused by the short M3–O1 and M3–O8 distances. Although the angular variance ends up at a similar value to the Al-Fe³⁺ epidote, the variation of ∠Oi–M3-Oi angles is different. The Sc-bearing clinozoisite has greater ∠O1–M3–O1’, but smaller ∠O2–M3–O2’ and ∠O2–M3–O4 relative to Al-Fe³⁺ series ones. Due to different local chemical surroundings, multiple peaks are present in the OH stretching region of Raman spectra. Three OH-stretching peaks, centered at 3342, 3382, and 3468 cm⁻¹ are assigned to the local configuration O10–H···O4–(^M1Al^M1Al^M3Sc³⁺) and O10–H···O4–(^M1Al^M1Al^M3Al), and O10–H···O2, respectively.