Abstract
Oolitic opal (siliceous oolite) and massive opal from Tateyama Hot Spring Shin-yu, Toyama Prefecture, Central Japan, have been investigated based on mineralogy, geochemistry, and SEM-observations.
Both oolitic opal and massive opal are composed of amorphous spherical silica particles. Individual oolitic opals ups to their diameter to 10mm have cores of rock or mineral fragments. Each silica particles of oolitic opal or massive opal increased their diameter by combinations of smaller ones that assimilated each other by changing their forms. Oolitic opal and massive opal are ultimately the same.
The water of the hot spring showing about 70°C, pH=3 and over-saturated with silica, contains abundant small silica particles. Oolitic opal and massive opal are precipitating at the Tateyama Hot Spring Shin-yu now.
Some of siliceous oolites found in Japan would be formed through the same mechanism and circumstances such as Tateyama Hot Spring Shin-yu.
