Geochemical studies on the color and chemical composition of Rainbow stone in Kita and Minami Daito Islands

抄録

Rainbow stone, or colored limestone consists of four portions which differ in its color and chemical composition. They are black colored portion due to manganese dioxide (micro-manganese nodules), reddish brown portion due to iron oxides, pale yellowish pink portion due to aluminosilicate and some other compounds, and white portion due to calcite and/or proto-dolomite. The color of Rainbow stone is controlled mainly by Fe₂O₃ and MnO₂ contents. Electron probe micro analyses (EPMA) clarify the existence of micro-manganese nodules in the black portion with iron oxides in its nucleus. Rhombohedral crystals of dolomite are observed with calcite. NaCl crystals, which are confirmed by X-ray diffraction analysis and EPMA, are incorporated into proto-dolomite almost regardless of its color. This shows that proto-dolomite on Daito Islands could have been formed through the evaporation of concentrated sea water.