A palagonitic soil (HWMK1) from Mauna Kea, Hawaii, has Marslike spectral and magnetic properties. Reflectivity spectra of <20 μm size separates resemble spectra for Martian bright regions, and reflectivity spectra of coarser size separates have some spectral characteristics in common with spectra for Martian dark regions. The bulk soil is magnetic with a saturation magnetization of 0.98 A m2/kg of sample (~10 A m2/kg of Fe) at 296 K. Petrographic observations show that the soil contains separate populations of black and orange tephra particles which are relatively unaltered fragments of tachylite (merocrystalline basalt) and extensively weathered (palagonitized) fragments of sideromelane (clear, brown basalt glass), respectively. Although both types of particles contain plagioclase and olivine microphenocrysts, their iron-oxide mineralogies are substantially different, as documented by Mossbauer and static-magnetic measurements. A strongly magnetic, opaque iron-oxide (titanomagnetite) is associated with black particles and is responsible for the magnetic nature of the soil. A relatively weakly magnetic, superparamagnetic ferric-oxide and minor hematite are associated with orange particles and are responsible for the color. The more highly weathered state (higher state of oxidation) of orange particles is a consequence of the faster weathering (palagonitization) rate for their sideromelane precursors than for tachylite. Increasing state of oxidation, decreasing saturation magnetization, and increasing near-IR reflectivity with decreasing particle diameter of bulk soil are manifestations of the increasing proportions of palagonitic material as particle diameter decreases. ¿ American Geophysical Union 1990