Shock-induced color changes in nontronite: implications for the Martian fines.

Shock recovery experiments performed on Riverside nontronite cause a marked color change in the clay, from olive-yellow to strong brown upon recovery from shock loading to peak pressures of 180 to 300 kbar. This color change spans the color range observed in the fine material at the Viking lander sites. The change in color is attributed to a shift in the O(2-)-Fe(3+) charge transfer bands to longer wavelengths, which causes the material to become both redder and darker. The observations that the color change is associated with the dehydroxylation of the clay and the Moessbauer spectra that reveal a lowering of the coordination number of the Fe(3+) as the OH(-) is removed from the structure are seen as suggesting that the movement of the absorption bands into the visible is caused by the formation of 4-fold or distorted 5-fold-like Fe(3+) within the octahedral layers of the clay.