Tucson is a unique iron meteorite with Si--Cr bearing metal and about 8 vol.% of silicates that are almost free of Fe, Cr, Mn, and alkalis. The silicates are mainly forsterite (66.4%) and enstatite (30.2%) with minor diopside (2.7%), anorthite plus feldspathic glass (0.7%), and traces of Mg--Al spinel and brezinaite. 'Enstatite' contains 0.5 to 21% Al2O3 and 'diopside' from 5 to 18%. Al2O3 generally increases away from early formed forsterite. The pyroxene is the most aluminous known in nature and contains close to the theoretical limit of Mg-Tschermaks solution. These compositions crystallized metastably because of the difficulty of nucleating feldspar in the rapidly cooled assemblage. The preferred hypothesis is that the present forsterite-enstatite silicate assemblage is similar to the precursor silicate assemblage, with little or no high temperature metal-silicate interaction. The metal and silicates were turbulently impact mixed at temperatures high enough to produce melt and forsterite (about 1500 ¿C), with volatilization of Ga, Ge, and As from the metal and Na, K, and Mn from the silicates, and then rapidly cooled so that silicates formed metastable aluminous pyroxenes and anorthitic glass. The precursor forsterite-enstatite assemblage is a previously unrecognized new type of chondrite or achondritic highly reduced assemblage related to the enstatite meteorites.