Siliceous spicules, typically ≤3 cm high and ≤5 mm in diameter, are common just above water level alongside hot-spring pools and channels in the Waiotapu geothermal area of New Zealand, which contain waters of 50–80°C and pH of 1–3. These structures are formed of irregular sequences of silicified Cyanidium mats, silicified Alicyclobacillus mats, and homogeneous opal-A laminae. Allochthonous microorganisms (e.g., diatoms) form a minor part of the silicified biota. Lamina boundaries are discontinuities that reflect pauses in opal-A precipitation. Barite, gypsum, and kaolinite crystals were precipitated on the spicule surfaces during these periods.
Opal-A precipitation on subaerial spicules occurs when SiO2-rich fluid is delivered by (1) splashing water and spray, (2) temporary inundation by surging spring water, wind-driven waves, or local changes in the water table following periods of heavy rain, (3) steam condensate, and/or (4) capillary action. These abiogenic processes work in conjunction with biogenic processes controlled by microbial mats, which become temporarily established on spicule surfaces when conditions permit. These mats mediate spicule construction as they become silicified, providing substrates for opal-A cement precipitation. Nevertheless, stromatolite morphology cannot be attributed to a particular taxon because different localities have different microorganisms involved with spicule growth.