The structural state of Finnish Cr- and V-bearing clinozoisite: insights from Raman spectroscopy

Abstract

Severe structural damage in Cr- and V-bearing clinozoisite (Czo) from the Outokumpu copper mine, Finland, was discovered by Nagashima et al. (Eur J Miner 23:731–743, 2011). Single-crystal X-ray and electron diffraction patterns indicated moderate-to-very-poor structural coherency, suggesting a high density of faults in the translational symmetry. However, the poor crystallinity cannot be attributed to self-radiation due to negligible concentrations of radioactive elements. Annealing of Cr- and V-bearing Czo up to 750 °C only slightly improved crystallinity. To solve this enigma, polarized Raman spectroscopy was applied to gain further insights into the structural state of the Cr + V-bearing Finnish Czo. According to the parallel-polarized Raman spectra of Cr + V-rich (Czo-ts3), Cr + V-bearing (Czo-ts2), and Cr + V-free (Czo-ts1) Czo, the peaks broaden with the Cr + V content, indicating increasing density of structural defects. Spectra from a euhedral Cr + V-bearing Czo single-crystal evidence intact structural domains by strong polarization and orientation dependence. Thus, the structural state of Finnish Czo should be described in terms of defect-rich segmented domains with common crystallographic orientation. The incorporation of V and Cr into the clinozoisite structure strongly affects the Raman-active O–H bond stretching modes near 3343 cm−1 [(OH)a] and 3435 cm−1 [(OH)b], assigned to O10-H···O4 and O10-H···O2 hydrogen bridges, respectively. The H···O2 hydrogen bonding is strongly promoted when M3 is partially occupied by Cr + V, and as a result, the linkages between the M2O6- and (M1, M3)O6-chains are disturbed, leading to periodicity faults and size reduction of coherent structural domains. In the case of M3Fe3+ substitution for M3Al, the local stress imposed by the larger M3-site cation is relaxed within the distorted M3O6 octahedron. In the case of M3V3+ or M3Cr3+ substitution for M3Al, M3O6 octahedra are less distorted and local stress fields propagate farther than only to the first coordination sphere, thus disturbing the transitional symmetry. New high-resolution transmission electron microscopy (HRTEM) images indeed display nanocrystals of ca. 10 nm with common crystallographic orientation surrounded by amorphous regions. It is suggested that the segmentation of large coherent crystalline areas in Cr + V-rich Czo to nanocrystals may be triggered by a potential miscibility gap in combination with the sluggish diffusion behavior of Cr3+. Moreover, this study implies that nanostructures as commonly observed for metamict materials may also be caused by strain effects and/or exsolution phenomena precluded by tardy diffusion.