A crystal-chemical investigation of clinozoisite synthesized along the join Ca2Al3Si3O12(OH)-Ca2Al2CrSi3O12(OH)

Abstract

Cr³⁺-bearing clinozoisite along the join Ca₂Al₃Si₃O₁₂(OH)-Ca₂Al₂Cr³⁺Si₃O₁₂(OH) was synthesized using cold-seal pressure vessels at P_{H2O = 0.35 to 0.40 GPa and T = 500 °C and a piston-cylinder apparatus at PH2O = 0.8 to 1.5 GPa and T = 500 to 800 °C. Gel-starting materials of Ca2Al3–qCr³⁺qSi3O12.5 composition with q = 1.00, 0.75, 0.50, and 0.25 were employed to maximize the yields of clinozoisite. Mass fractions of clinozoisite in the experimental products with q = 0.50, 0.75, and 1 were about 70 to 90% along with lesser amounts of eskolaite, garnet, and quartz. Clinozoisite crystallized from the gel with q = 0.25 was associated only with zoisite. The crystal structures of clinozoisite in four runs, containing 0.28, 0.49, 0.50, and 0.62 Cr apfu were refined using X-ray powder diffraction data and the Rietveld method. The amount of Cr³⁺ at the octahedral M3 and M1 sites ranged from 0.37(1)–0.16(1) to 0.25(1)–0.12(1) apfu, respectively. Corresponding KD = (Cr³⁺/Al)^M1/(Cr³⁺/Al)^M3 values range between 0.57 and 0.73. The M2 site contained only Al. The KD values, and published results for intracrystal-line partitioning in epidote and piemontite, show that the preference of Cr³⁺ for M1 is stronger than that of Fe³⁺ and Mn³⁺ in spite of the fact that most Cr³⁺ is partitioned into M3. Unit-cell parameters of clinozoisite increase with increasing Cr³⁺. Variations in macroscopic unit-cell parameters can be related to variations in the local M3-Oi and M1-Oi distances.}