How Do Carbonate Minerals Grow?

Jeanne Paquette
Earth & Planetary Sciences, McGill University, Montreal, QC, Canada

Eric tirelessly encouraged us to visualize geological structures in three dimensions. His interest in the interpretation of tectonic fabrics and diagenetic features across all scales nurtured my undergraduate curiosity in the zoning patterns revealed in unprecedented detail by cathodoluminescence. A M.Sc. thesis rewarded me with a deeper dual appreciation of my mentor and the complexity of calcite cements. I approached potential Ph.D. supervisors with a question: could we figure out how fluid chemistry modulates enigmatic fabrics like radial-fibrous calcite? Growing zoned calcite and using zoning-modelling software turned out to give us wonderful insights into the origin of zoning patterns. Crystal growth has held my attention since, and I want to show today why carbonate minerals are such a rich hunting ground. Morphology modelling software belongs in the toolbox of any carbonate petrographer. It is an eye-opener to realize that a modest changes in angle of section turns a zoned dogtooth calcite into a fair imitation of square-tipped aragonite. Or that growth topography distorts the true geometry of zoning patterns in sections through a crystal just as a rolling landscape modifies the dip of folded strata. Technology now enables us to study crystal growth in real time and down to the nano-scale: we can finally distinguish the influence of inhibitors from the role of strain in a crystal structure. Growing pure and magnesian calcites shows us that the scale of zoning in low-temperature carbonates is tremendously sensitive to fluid chemistry, with implications for the reactivity of the host minerals. Yet, one of Eric’s favorite question (“What does the rock tell you?”) draws me back to natural carbonates. The complexity of dolomite and calcite crystal surfaces still tells us that they grow with a diversity of mechanisms we have yet to replicate in the laboratory.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas