MORPHOLOGY, COMPOSITION AND STRUCTURE OF LOW-TEMPERATURE P4/nnc HIGH-FLUORINE VESUVIANITE WHISKERS FROM POLAR YAKUTIA, RUSSIA

Whiskers and needles of F-rich vesuvianite were found together with diopside in cavities of an altered magnesian skarn in the Tas–Khayakhtakh Mountains of Polar Yakutia, in Russia. The acicular crystals are strongly zoned and formed between two generations of diopside. The chemical composition of the vesuvianite whiskers is more homogeneous and resembles that of the outermost rim of the vesuvianite needles. In the last stage, fluorapophyllite, prehnite, titanite, calcite and quartz overgrew vesuvianite. Whiskers of vesuvianite crystallized at low activity of CO2 and P–T conditions corresponding to the prehnite– pumpellyite facies. Single-crystal X-ray refinements of the structure of three vesuvianite whiskers, for which electron-microprobe data also were collected, revealed P4/nnc space-group symmetry and (F, Cl) substitution at O(10) within disordered strings running parallel to the four-fold axis. In addition, there is partial substitution of F at O(11), usually occupied by OH in lowtemperature vesuvianite. The high symmetry (P4/nnc) in low-temperature (<350°C) whiskers of vesuvianite adds evidence that the degree of string order is determined not only by the temperature of crystallization, as hitherto assumed, but also by the prevailing composition of the fluid and the regime of crystal growth leading to substitutions that disturb intra-rod order and particularly long-range rod order. Long-range rod order leads to reduced symmetry (P4/n or P4nc), typical of vesuvianite crystallized at low temperature in rodingites. Vesuvianite whiskers formed in a kinetic regime where the growth rates were selectively influenced by surface-active substances poisoning the prism faces. Growth of faces in vesuvianite whiskers is explained by a tangential layer-by-layer mechanism without participation of a central screw dislocation.