Cristobalite and opal are contained in the wall-rocks (tuff and agglomerate) of the native sulphur and iron sulphides ore deposits, and in the disseminated limonite (goethite) ore of the Abuta Mine in south-western Hokkaido, Japan.
These are not found in the massive iron sulphides ore and moss-limonite (goethite) ore that always contains quartz as silica minerals. The x-ray diffraction data show that the cristobalite is mostly α-(low) type. Its hexagonal platy crystal can be found in the electromicrograph.
It is considered that the cristobalite has been formed as a secondary mineral from opal which is one of the hydrothermal alteration minerals-alunite, kaolin, montmorillonite, anatase?, pyrite, marcasite, native sulphur, etc. Cristobalite is also found in the wall-rocks of the native sulphur ore deposits of the Tokusyumbetu Mine and in the clay under the moss-limonite (goethite) ore deposit of the Kuttyan Mine.
