A Spectrographic Interpretation of the Shock-Produced Color Change in Rhodonite (MnSiO_3): The Shock-Induced Reduction of Mn(lll) to Mn(II).

Samples of rhodonite (MnSiO_3-pyroxenoid from Franklin, New Jersey) have been shockloaded to pressures up to 496 kilobars. Optical spectral studies of four recovered samples show a decreasing Mn^(3+) content upon recovery from successively higher shock pressures; after shock-loading to 496 kbar, the Mn^(3+) has essentially disappeared. No corresponding change in the optical spectrum results from heating rhodonite to 1250°C for 3.5 hours in a reducing atmosphere. Rhodonite heated to 1360° under the same conditions melts incongruently to manganese-rich glass and silica with disappearance of the 540 nm Mn^(3+) absorption band. The color change in the shocked rhodonite arises from irreversible reduction of Mn3+ during high shock pressures and possible high shock temperatures. It is suggested that Mn^(3+) is reduced to Mn^(2+) by water present in the sample during the shock event