Abstract

Opal-A (SiO₂·nH₂O) and its diagenetic derivative, opal-CT, are found in environments that range from the deep ocean floor to terrestrial spring/geyser systems. Although the loss of water may be a critical factor in the transition from opal-A to opal-CT, analytical difficulties in determining the distributions and quantities of molecular water (H₂O_mol) and silanols (H₂O_SiOH) has hindered a full understanding of the processes involved with this diagenetic transition.

Electron microprobe (EMP) and micro–Fourier transform infrared (FTIR) analyses of opal-A and opal-CT collected from geyser discharge aprons on Iceland and the North Island of New Zealand produce compatible and reliable derivations of the total weight % water in opal-A and opal-CT, provided that they contain < 1 wt % of other elements such as Al. EMP analyses produce excellent results where microscale variations in total water content are required but it cannot determine if that water is formed of molecular water (H₂O_mol) or silanols (H₂O_SiOH). In contrast, FTIR analyses yield total water, molecular water, and silanol contents of the opal. Microscale variations, however, are commonly masked because the scale of FTIR analysis is coarser than that for EMP analyses. Analyses of sinters from Iceland and New Zealand show that the opal-A contains 2.1–12.1 (average 7.3) wt % total water whereas the opal-CT contains 1.0–8.9 (average 5.5) wt % total water. Based on FTIR analyses, the opal-A contains 1.2–9.8 (average 6.8) wt % molecular water and 0.4–2.4 (average 1.0) wt % silanols compared to opal-CT, which contains 3.8–8.2 (average 6.7) wt % molecular water and 0.1–0.7 (average 0.2) wt % silanols. The dual usage of EMP and FTIR analyses provide important information on the distribution of different types of water in opal-A and opal-CT, irrespective of the environments where they formed.