Both shock recovery and thermal annealing experiments have been carried out on Riverside nontronite, an iron-rich smectite clay that closely matches the composition, but neither the color nor magnetic properties, of the Martian fines. Nontronite was recovered from peak shock pressures between 7 and 32 GPa, with calculated peak mean bulk temperatures ranging from 125¿ to 1100¿ C. Other nontronite samples were heated in air for one hour at temperatures ranging from 400¿ to 1000¿ C at 100¿ C increments. Both shocked and heat-treated samples were analyzed by magnetic measurements, X ray diffraction, and M¿ssbauer spectroscopy. Nontronite powders shock-loaded to peak calculated temperature of 650¿ C to 750¿ C show a dramatic increase in ferromagnetic saturation magnetization (Ms). A similar increase in magnetization in the same temperature range is observed in isothermally annealed powders at atmospheric pressure. The X ray diffraction and M¿ssbauer studies show, however, that for samples that experienced temperatures of 600¿ C or more, the shocked material has little structural similarity to the isothermally annealed nontronite. This suggests that the ferromagnetic properties may be due to a minor phase. There is evicence from M¿ssbauer spectra for metallic iron in the shocked samples and for superparamagnetic Fe2O3 in the annealed samples. In addition to magnetic and structural changes, the shock and heating experiments altered the color of the nontronite from olive-yellow toward reddish-brown, yielding material that matches the observed properties of the Martian fines much more closely than the starting material does. If nontronite is the major component of the Martian fines as some workers suggest, some mechanism for altering its properties is necessary, the most likely being repeated impacts onto the surface of Mars.