HIGH-RESOLUTION FTIR SPECTROSCOPIC CHARACTERIZATION OF TESSIN QUARTZ; CANTON TICINO, SWITZERLAND

The spatial distribution of hydrous impurities within hydrothermal quartz provides insights into the temporal evolution of individual crystals as well as their host vein system. Currently, few specifics are known relating the effects of environmental variables (temperature, pressure, fluid composition) on observed characteristics in vein quartz. In the Swiss Alps, quartz crystals with tessin habit characterize the highest temperature fluid regime (>300°C; Mullis et al., 1994). The tessin morphology is identified by irregular prism face boundaries and thinning diameter up the c-axis. We present data from five tessin crystals collected from their type locality of Ticino, Switzerland. Concentrations of hydroxyl species AlOH, LiOH, and HOH were measured using micro-FTIR spectroscopy. Analyses include 100 µm aperture traverses across 1 mm thick polished wafers cut perpendicular to the c-axis. These data reveal the 3-dimensional distribution of impurities within each crystal. All five crystals show uniformly low impurity concentrations when compared to alpine quartz crystals from lower temperature regimes. The tessin crystals show significant decreases in impurity concentration toward their terminal rhombohedral faces but not toward their prism faces. The lack of observable diffusion profiles perpendicular to the c-axis is a new finding for hydrothermal quartz: we conclude that new material was added onto prism faces of tessin crystals throughout their growth, thus preventing formation of concentration gradients in impurities (as observed for active rhombohedral growth faces in all crystals). Only after cessation of growth can concentration gradients develop and migration of impurities out of the crystal occur, with diffusion an order-of-magnitude faster up the c-axis. This hypothesis offers an explanation for the characteristic morphologic features of tessin quartz; sector zones created by the faster growing rhombohedral faces of the terminus maintain a constant width, but continued growth on the prism faces leads to a progressively wider base offering an overall pyramidal shape to the crystal.