ABSTRACT

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.