Canadian Rockhound: Weird and Wonderful Quartz - How Pseudocubic Quartz is Formed

In the Summer/Fall 1998 issue of the Canadian Rockhound, I presented an article titled Weird and Wonderful Quartz in Septarian Nodules from Alberta's Dinosaur Country. The article described unusual pseudocubic quartz, but at the time of publication in 1998, I was unable to provide a definitive explanation for its formation.

A follow-up with Ron Mussieux, the Curator of Geology at the Provincial Museum of Alberta, has unearthed an answer. Ron provided a copy of an article, "Burial metamorphic minerals in Upper Cretaceous strata, Alberta, Canada", by the University of Alberta's Drs. Roger Morton and Dorian Smith, published in Neues Jarbuch Miner. Abh. 157:147-55 (July 1987). The authors used SEM and X-ray diffraction on comparable crystals from the same geologic formation to establish that these are in fact quartz paramorphs after a-cristobalite. a-cristobalite is a polymorph of silica (SiO₂), i.e. it has the same chemical composition as quartz, but is a higher-temperature form not normally stable at atmospheric conditions. Although a-cristobalite is tetragonal in structure, its pseudocubic form explains the morphology of the crystals in question. SEM photographs at the micron scale show a structure composed of aggregates of well-crystallized, radiating, acicular quartz that have replaced the original a-cristobalite. Although a-cristobalite is apparently relatively common worldwide, cm-size crystals are apparently quite extraordinary! Morton and Smith propose formation from the silica-rich bentonite clay that is abundant in the area, at temperatures in the range 38 - 48°C. They suggest that growth of a-cristobalite was favored by the stabilizing effect of impurities, with subsequent inversion to quartz at slightly elevated temperatures, as burial depth increased within the sediments. The short wave UV fluorescence is apparently due to sub-micron sized organic inclusions.

There is one significant difference in occurrence between my crystals and the material analyzed by Morton and Smith. They describe crystals that are typically 1-2 mm, associated with petrified wood and dinosaur bone. I also found these mm-size crystals on quite widely distributed petrified wood, but collected my cm-size crystals (the largest is almost 1.5 cm) in only two very small areas, covering in total only a few square meters. In both areas they were associated only with chalcedony within ironstone nodules. There was neither petrified wood, nor any obvious features to distinguish these particular areas or nodules from others at the locality - this mystery remains!

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