AbstractFrom 2 to 28% opal-cristobalite was isolated from the 2–20 µm fraction of rhyolitic and andesitic tuffaceous pyroclastics from the Island of Honshu, Japan, where it had been formed in hydrothermal springs at temperatures of ∼25–170°C as calculated from the oxygen isotopic ratios (18O/16O). Three of the isolates gave X-ray powder diffractograms with strong peaks at 4.07 Å. Two of these also had very weak peaks at 4.32 Å indicative of the presence of traces of tridymite. The fourth isolate had a strong 4.11 Å cristobalite peak and a very weak 4.32 Å peak. The morphology, determined by the scanning electron microscope, varied with the formation temperature indicated by the oxygen isotopic ratio (δ18O), from spheroidal and spongy for the opal-cristobalite formed at ∼25°C (δ18O = 26.0‰) in contrast to angular irregular plates and prisms for that formed at ∼115°C (11.9‰), ∼135°C (7.9 ‰) and ∼170°C (6.8 ‰). The differences in δ18O values are attributed to variation in hydrothermal temperature, but some variability in oxygen isotopic composition of the water is possible. The field-measured temperatures related roughly with the calculated fractionation temperatures except in one site, while the contrast in cristobalite morphology related well to calculated low and high fractionation temperatures. Low-cristobalite of hydrothermal origin in New Zealand (δ18O = 9‰) had characteristic rounded grains with some evidence of platiness. Co-existing quartz grains (δ18O = 10‰) showed more subhedral and irregular prismatic morphology.