| Emerging hot spring waters commonly precipitate opal-A (SiO2· nH2O), resulting in the formation of different types of siliceous sinter with variable microstructures and water contents. The patchy and inhomogeneous transformation of opal-A to its more stable phase, opal-CT, takes place through a dissolution-reprecipitation pathway. Fabrics indicative of dissolution and reprecipitation are most common in sinter composed of opal-CT. The total water content, determined using electron microprobe (EMP) analysis and Fourier transform infrared (FTIR) spectroscopy, of opal-A deposits (6-9 wt %) are generally higher than opal-CT (3-6 wt %). The total water content (molecular water + silanols) in opal-A and opal-CT is largely formed of molecular water. In the transformation of opal-A to opal-CT, silanols are lost before the molecular water. These trends are common in sinters from the Taupo Volcanic Zone, New Zealand and Geysir, Iceland, indicating the transformation of opal-A to opal-CT must follow analogous pathways despite variable environments. |
