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

Abstract

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.

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.