Aqueous alteration in the matrix of the Vigarano (CV3) carbonaceous chondrite

The matrix of Vigarano, a meteorite which belongs to the reduced subgroup of the CV3 chondrites, contains small amounts (<10%) of ferrihydrite and smectite. These hydrous minerals occur together as fine fibrous intergrowths between anhydrous silicate and oxide grains. Coarser crystals of ferrihydrite fill fractures that cut matrix minerals, and smectite also lines narrow channels within olivine grains. These channels may have formed by preferential alteration of olivines along (100)-parallel defects. Formation of ferrihydrite and smectite in the matrix of Vigarano was the result of mild aqueous alteration in a low-temperature (<150 degrees C), oxidising parent body environment. Partial equilibration of matrix olivines indicates that alteration was followed by thermal metamorphism with a peak temperature of 400-500 degrees C. Mineralogically similar alteration products, which also were formed by parent body processes, have previously been described from the matrices of four CV meteorites: Ball, Grosnaja, Kaba and Mokoia, all of which belong to the oxidised subgroup. This discovery of the products of oxidative aqueous alteration in Vigarano has important consequences for understanding the chemical and thermal history of the CV class of meteorites.