All samples of silica sinter, <2 y old taken from the discharge drain of the Wairakei geothermal power station and the Rainbow Terrace of Orakei Korako, consist of non-crystalline opal-A. This silica phase deposits directly upon the concrete drain wall and filamentous templets, extending from this wall, afforded by the microbial community present in the drain, whose nature was determined by a culture-independent strategy that entailed construction, fingerprinting and sequencing of a 16S clone library. The bacterial community is dominated by five major groups of organisms, present in approximately equal proportions, and which account for ~50% of the community. None of the 16S sequences from these dominant groups yielded a perfect match with 16S sequences for named organisms in the international databases. However one dominant group clusters with Hydrogenophilus thermoluteus, a thermophilic filamentous bacterium, and two cluster with putatively thermophilic members of the Cyanobacteria and green non-sulphur bacteria respectively. Initial opal-A deposits rapidly as agglomerations of silica nanospheres that, in turn, form chains of coalesced, oblate, microspheres <0.4×0.2 μm about the barbicel-like filaments, to produce a mat of fine woven strands. The majority of individual filaments are <8 μm long and 0.8 μm wide but may be up to 55 μm long by 1 μm wide. Where laminar flow dominates, most strands develop parallel to the drain current but some strands crisscross while others protrude above the mat surface. Where flow is turbulent, strands lack preferred orientation and some adopt a helical form. In general, following deposition, the values of the scattering broadband at half (FWHM) and three quarters (FWTM) of the maximum intensity decrease with increasing sample age. The behaviour of the band at one quarter maximum intensity (FWQM) is less consistent, but, in general, the youngest sinters possess the highest FWQM, FWHM and FWTM values that prove independent of fabric type. Opal-A silica matures following its removal from the parent fluid, especially where the sinter surface is filmed by water. A continual movement of silica is shown by a second generation of microspheres formed on the silica mat surface, by an increase in size of the initial microspheres, and by an increase in maximum intensity of the X-ray scattering broadbands. Similar silica aging behaviour occurs among young sinters developed upon microbial mats at Orakei Korako. The deposition and aging processes accord with the known behaviour of juvenile opaline silica in both natural and artificial systems whose pH, temperature and dissolved salt content are similar to Wairakei and Rainbow terrace: gelling of silica is favoured by the high pH (~8.3) and temperature (~60°C) of the Wairakei discharge fluid but the high dissolved salt content of the water (Na+ = 930 μg/g, Ca2+ = 12 μg/g, Cl = 1500 μg/g) and abundant microbial community facilitate rapid and copious flocculation of solid silica within the drain, in contrast to the slower accumulation on the natural sinter terrace at lower temperature (30–45°C) from less saline dilute bicarbonate-chloride waters (Na+ = 180 μg/g, Ca2+ = 0.2 μg/g, Cl = 400 μg/g, pH = 8.1).

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