INDICATIONS OF C-AXIS POLARITY IN QUARTZ: DOES QUARTZ CRYSTALIZE WITH A HEXAGONAL PRISM?

Field observations of singly-terminated, euhedral quartz crystals from various locations in the United States suggest that quartz occurs as two morphological populations at each locality: one that appears hexagonal and one that appears trigonal. To investigate this observation, 320 individual quartz crystals from five localities were measured. On each crystal, the m-faces (prism) were measured at the base and near the termination as a ratio to quantify the extent of tapering along the c-axis. Those that had equant sides and a hexagonal appearance have a ratio near one and those with tapering sides and a trigonal appearance have a ratio greater than one. Analysis of ratio data plots show a pattern that can be classified into two populations of crystals; those with 6 prismatic faces (ratios near 1) and those with 3 prismatic and 3 tapered faces (ratios >1). To quantify the overall extent of tapering along the c-axis of each crystal, a plot of classified data reduced to a single ratio by grouping and averaging the two sets of prism measurements was made. This graph divides the data into two distinct populations that can be separated by a straight line. Quartz models in crystallography textbooks show doubly-terminated quartz crystals with a symmetrical hexagonal prism, in the absence of modifying forms. These same books typically show “odd” quartz morphologies that involve tapered crystals as singly-terminated crystals, without doubly-terminated analogues. Doubly-terminated, euhedral crystals from four of the five locations were also measured to quantify the extent of tapering along the c-axis at opposing ends of the crystals. The two poles on the individual doubly-terminated crystals correspond to the apparent hexagonal and trigonal populations measured. The occurrence of two populations of quartz morphologies that correspond to doubly-terminated crystals from the multiple localities suggests that, at least locally, quartz may crystalize as polar crystals that morphologically lack the 2-fold rotation axes. Further investigation could focus on the growth mechanisms responsible for these observations.