Dear Colleagues,

During the last three decades a series of innovative physical methods applied to calcareous biominerals resulted in considerable changes regarding the concept of biocrystallization. Far from a simple chemical reaction leading to ionic-type precipitation of Ca-minerals within dedicated crystallization spaces, specifically secreted organic components are now considered as key-factors in the biocrystallization process by establishing a physiological link between intra- and extracellular areas.

However, no sensible progress has been made in the explanation of this intriguing property of biominerals hypothesized by H.C. Urey in his seminal 1951 paper: “We may ask whether there is a vital effect”. More than half a century later, capability of living organisms to record environmental parameters in their skeletons each of them with a species-specific bias remains largely enigmatic. That this biologically driven crystallization mechanism directly impacts the incorporation of geochemical proxies is clearly assessed by high resolution measurements having established that within a given specimen built by distinct microstructural layers parallel series of environmental signals are simultaneously recorded, each of them precisely associated to a distinct crystallization area.

δ¹⁸O measurements made on the skeleton of the deep sea coral Lophelia offer demonstrative evidence of the practical importance of this differential crystallization mechanism. Actually, two vital effects are simultaneously running during growth of this calcareous skeleton, making obvious the direct and very precise control exerted by the polyp on the composition of its underlying skeleton. Similar observations can be conducted about various isotopic fractionations or chemical partitioning in any kind of calcareous biological structure.

From intracellular concentration of amorphous calcium carbonate to extracellular crystallization of the nanometer sized grains that built the crystal-like units, steps of these complex processes have to be deciphered as potential contributors to specificity of the skeleton compositions. Not only is an improved understanding of the biocrystallization process essential to a reliable interpretation of the recorded messages, but the use of ancient archives is also dependent of the stability of biominerals through time. Taking into account the complexity of the initial organo-mineral assemblages, there is no doubt that the diagenetic status of ancient biominerals (i.e., validity of the recorded proxies), cannot be assess through a simple mineralogical control.

Taking advantage of our unprecedented analytical capabilities the scope of this Minerals issue is to trigger an up-to-date attempt to address the vital effect question that is still a major hampering factor in the use of calcareous skeletons as environmental archives. From investigations dealing with cellular processes to experiments in which mineralogical and crystallographic properties of naturally or experimentally produced Ca-carbonate materials will be compared to their specific chemical or isotopic properties. This special issue dedicated to the prophetic and still unexplained Urey’s hypothesis will be a milestone in the field.

Prof. Dr. Jean-Pierre Cuif
Dr. Claire Rollion-Bard
Guest Editors

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• Biogenic carbonates
• Geochemistry
• Vital effects
• Ca-carbonate precursors
• Biocrystallisation
• Microstructures