A Spectrographic Interpretation of the Shock-Produced Color Change in Rhodonite (MnSiO3): The Shock-Induced Reduction of Mn(III) to Mn(II)

Rex V. Gibbons, Thomas J. Ahrens, George R. Rossman
California Institute of Technology
Seismological Laboratory
Pasadena, CA  91109


Samples of rhodonite (MnSiO3-pyroxenoid from Franklin, New Jersey) have been shock-loaded to pressuresup to 496 kilobars. Optical spectral studies of four recovered samples show a decreasing Mn2+ content upon recovery from successively higher shock pressures;after shock-loading to 496 kbar, the Mn3+ 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 Mn3+ 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 Mn3+ is reduced to Mn2+ by water present in the sample during the shock event.

spectrum rhodonite
Rhodonite spectra. The spectra of three samples recovered after being shock-loaded to 210, 39O, and 496
kbars are compared with the spectrum of an unshocked rhodonite (A) showing the loss of the Mn3+ band at 540 nm
in the more intensely shocked samples. These spectra have been normalized to represent the absorbance of a sample
of 0.15 mm thickness but have been drawn with vertical displacement in absorbance for purposes of contrast and clarity.

last updated: 27-Jul-2023