WHY AND HOW DO PHOSPHATIC MINERALS REPLICATE SOFT TISSUES AT THE HIGHEST RESOLUTION? (Invited Presentation)

Authigenic phases such as silica, phosphates, pyrite, and aluminosilicate clays play a key role in the replication and mineralization of soft tissue anatomies in many exceptionally preserved fossil assemblages. Among these phases, phosphates (and perhaps silica) achieve the best preservational resolution; cellular and subcellular structures of soft-tissued organisms are often replicated by phosphates through coating and molding at micrometer and submicrometer scales. Our studies have shown that the nature of organic substrates, the timing and duration of degradation vs. mineralization, the size of authigenic minerals/particles, and diagenetic dissolution and recrystallization/reprecipitation are factors that control the quality and taphonomic resolution of exceptional preservation. Among these factors, mineral size is of paramount importance: the finest anatomical structures can only be replicated by the smallest minerals or particles, analogous to the achievement of the highest resolution by the smallest pixels in digital imaging. Our observation of phosphatized fossils shows that they are always replicated by apatite minerals of tens to hundreds of nanometers in size. How were these apatite minerals precipitated in such minute grain sizes? And, given their small size, why were they not dissolved or recrystallized during diagenesis? Answers to these questions are critical to a better understanding of fossil phosphatization processes. Experiments have shown that dissolution of phosphate minerals can be self-inhibited and even suppressed due to decreasing crystallite size approaching a critical value, which is in the range of tens to hundreds of nanometers, depending on undersaturation levels (Tang et al., 2004, Journal of Materials Chemistry, 14: 2341-2346). We hypothesize that the stability of phosphate minerals at nanometer sizes may endow them with their unusual capability to replicate soft tissues at the finest resolution.