High-temperature (>90°C) calcite precipitation at Waikite Hot Springs, North Island, New Zealand

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Abstract
The Waikite Hot Springs emit waters (most >90°C) with a pH of 7.2–8.9, high SiO 2 (150–170mgl −1 ) concentrations, but low Ca (−1 ) and Mg (−1 ) concentrations. Although amorphous silica (sinter) is the dominant precipitate, calcite is forming around some springs. At Springs WS-2 and WS-11, ledges and 'lily-pads' in the shallow waters around the pool edges and outflow channels, have rims formed of trigonal calcite prisms and interiors formed of interlaminated calcite and amorphous silica. At Spring WS-5, water ejected at 92–98°C has cooled to 72°C, 2 m downslope from the orifice. The soft calcite that covers the discharge apron is composed of disorganized micrite or composite fibrous calcite crystals. Although extracellular mucus is common, no microbes were found. Near the orifice, ooids up to 4 mm in diameter, were found in the soft micrite that forms the floor of a shallow pool where the water temperature is 92°C. Mucus, generated by thermophilic bacteria, mediated the formation of some cortical laminae. Calcite is being precipitated at these springs because the constant supply of Ca 2+ and CO 2– 3 compensates for the low Ca content of the water. Its crystal form is controlled by the supply rate of Ca + and CO 2– 3 and mucus generated by the thermophilic bacteria.