| Abstract |
Samples of silica sinter, <2 years old taken from the discharge drain of the Wairakei Power Station and the sinters terraces of Orakei Korako, all consist of non-crystalline opal-A. Initially silica deposited in the Wairakei drains forms a mat woven from strands ( 3 6 p long, 0.4 mm diameter) of smooth non-porous opal-A in which most strands are aligned parallel to the current but some protrude above the mat surface. The silica of the strands rapidly forms chains of oblate, coalesced microspheres <0.4x0.2 pm. Following deposition the width of the opal-A x-ray scattering broadband at quarter (FWQM), half (FWHM) and three quarter (FWTM) of the maximum intensity decreases slightly. Samples younger than 0.1 years display the highest FWQM, FWHM and FWTM values. Opal-A silica continues to precipitate and mature following removal from the parent fluid, so long as the sinter surface is filmed by water. A continual movement of silica is shown by a second generation of microspheres formed on the mat surface, by an increase in size of the initial microspheres, and by an increase in maximum intensity of the x-ray scattering bands. These changes 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 Orakei Korako e.g. gelling of silica is favoured by the high pH and temperature of the Wairakei discharge fluid with the high dissolved salt content of the water essential for solid silica to accumulate within the drain. |