Severe structural damage in Cr-and V-rich clinozoisite: Relics of an epidote-group mineral with Ca _2Al _2Cr ~(3+)Si _3O _(12) (OH) composition?

摘要

Cr-rich clinozoisite with up to 0.89 Cr ~(3+) apfu and additional 0.23 V3+ apfu originating from the Outokumpu copper mine, Finland, was studied by electron microprobe analyses, laser ablation inductively coupled-plasma mass spectrometry, single-crystal X-ray structure refinement of natural crystals and after step-wise annealing to a maximum temperature of 750°C, by HRTEM and electron backscattered diffraction. Although Cr-rich clinozoisite appears optically of gemmy quality with sharp extinction behavior under crossed polarizers, its X-ray and electron-diffraction behavior is very poor and resembles that of almost amorphous materials as known for strong metamictization. Albeit the sample does not contain significant U or Th as potential source of radiation and destruction of lattice periodicity. Cr-and V-free clinozoisite, intergrown with Cr-and V-rich clinozoisite, displays good diffraction behavior. Cr-and V-rich clinozoisite of poor diffraction quality has larger unit-cell volume than expected for its composition. The volume decreases with increasing annealing but does not reach the range expected for the analyzed composition. Heating cycles slightly improve crystallinity but the diffraction quality remains unsatisfactory. Site occupancy refinements of single-crystal X-ray data collected for natural crystals after heat treatment could not reproduce the (Cr + V) concentration previously determined by analytical methods. We speculate that probably on the retrograde path nano-particles of a Cr-(and V-) rich amorphous phase were exsolved destroying the periodicity of the residual clinozoisite. Annealing led to substantial structural cure but the original (Cr + V) content could not be reabsorbed.