The titanian esseneitic diopside in Al-skarn from Myojin and Mutsuki islands in Seto Inland Sea, Japan

Abstract

The reinvestigation of so-called titanian fassaite from Myojin island, Seto Inland Sea of Japan, has revealed that the minerals are titanian-esseneitic-diopside by chemistry. The diopsides with similar chemical compositions were found in Mutsuki island.
These diopsides occur as dark crystalline aggregates of nodule in saccharodial limestone. The dark fragments in this area are recrystallized skarns, which underwent Al-Ti-Fe concentration composed of titanian esseneitic diopside, potassic-magnesiosadanagaite, hercynite, ilmenite, and aluminian titanite, as accessory phlogopite, apatite, and perovskite. These minerals almost all are rich in Ca-Al-Ti-Fe.
The electron microprobe analysis shows that the diopsides in this area have high Al₂O₃, TiO₂ and Fe₂O₃ contents. Al ≥0.894, Ti ≥0.183, and Fe³⁺≥0.276 on the basis of O=6 and total cations=4. The diopsides have high molecule contents of esseneite, Ti-Tschermaks, and Ca-Tschermaks diopsides.
It is suggested that occurrence of the titanian esseneitic diopsides rock from Myojin and Mutsuki islands are from the sediment such as laterite or terra rossa, which is rich in the Al, Ti, and Fe, through the metamorphism with high temperature which is 600°C or more.