[S-CG45] Petrology, Mineralogy & Resource Geology

[SCG45-P03] Crystal chemistry of synthetic Sc-bearing clinozoisite on the Ca₂Al₃Si₃O₁₂(OH)-Ca₂Al₂ScSi₃O₁₂(OH) join

*Mariko Nagashima^1,2, Boriana Mihailova² (1.Graduate school of Sciences and Technology for Innovation, Yamaguchi University, 2.Fachbereich Erdsystemwissenschaften, Universität Hamburg)

Keywords:synthesis, scandium, clinozoisite, heflikite, crystal chemistry, Raman spectroscopy

This is the first report of the crystal structure of synthetic Sc-bearing clinozoisite, on the Ca₂Al₃Si₃O₁₂(OH)-Ca₂Al₂Sc³⁺Si₃O₁₂(OH) join to investigate the distribution of Sc³⁺ among the octahedral sites, M1-M3, and its effect on Sc³⁺ of the clinozoisite structure. Oxide starting materials of Ca₂Al_2-pSc_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 ºC. 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, beta = 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, beta = 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. Sc³⁺ only distributes M3. The mean ionic distance of 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 bond-length 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 differs. The Sc-bearing clinozoisite has greater O1–M3–O1’, and 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^M1M1M3(AlAlSc³⁺) and O10–H···O4^M1M1M3(AlAlAl), and O10–H···O2, respectively.