MORPHOLOGICAL CHANGES ACCOMPANYING THE OPAL-A TO OPAL-CT SILICA PHASE TRANSITION IN SILICA SINTERS, FROM ORAKEI KORAKO AND TE KOPIA, TAUPO VOLCANIC ZONE, NEW ZEALAND

A progressive transformation from opal-A through opal-CT and opal-C to microcrystalline quartz + moganite is widely recognized in siliceous marine sediments, chert diagenesis, marine opals, wood petrification, silica residue, and silica sinter maturation. Samples of silica sinter from various locations within geothermal fields of the Taupo Volcanic Zone represent a range of environmental conditions, fabric types, diagenetic histories, microfacies and ages. The sinter samples display morphological modifications through the opal-A to opal-CT silica transition, as evidenced by scanning electron microscope (SEM) observations and progressive changes in X-ray powder diffraction traces. At the ultrastructural level, an initial morphological restructuring includes the formation of both small circular holes (< 0.1 to < 1.0 micron diameter) in opal-A microspheres (<1 to <3 microns diameter), and hexagonal platey silica structures. These initial textural developments precede any obvious changes in the XRPD patterns. The morphological transition to typical opal-CT bladed lepispheres occurs as the hexagonal platey silica structures become engulfed within developing lepispheres. At this stage, the X-ray scattering broad band of opal-A (centered at ~ 4.08 Å) starts to become overprinted by a broad, but sharp-peaked cristobalite diffraction line (centered at ~ 4.08 Å) which is, in some cases, associated with an incipient tridymite shoulder (centered at ~ 4.23 Å). Only with the appearance of numerous, well-developed lepispheres in the sinter matrix fabric is a typical composite broad opal-CT line seen in the X-ray diffraction trace. The successful recognition of such morphological changes that accompany the maturation of silica sinter is essential to deduce primary paleohydrological, paleoenvironmental, and paleobiological signatures in ancient hot spring deposits. Furthermore, sinter maturation studies provide a textural-mineralogic context for understanding silicification of microbial communities in geothermal settings, their subsequent taphonomic modifications, and the overall diagenesis of siliceous sinter deposits.