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

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(Al1-p)ScpSi3O12.5 composition with p = 0.5 and 1.0 were employed, and clinozoisite was successfully synthesized at PH2O = 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) Å3 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) Å3 for 20D. The resulting Sc3+ occupancies among the octahedral sites are M1Al1.0M2Al1.0M3(Al0.684(7)Sc3+0.316) for the former and M1Al1.0M2Al1.0M3(Al0.629(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–(M1AlM1AlM3Sc3+) and O10–H···O4–(M1AlM1AlM3Al), and O10–H···O2, respectively.