AQUEOUS ALTERATION IN THE MATRIX OF THE VIGARANO (CV3) CARBONACEOUS CHONDRITE

The matrix of Vigarano, a meteorite which belongs to the reduced subgr oup of the CV3 chondrites, contains small amounts (<10%) of ferrihydri te and smectite. These hydrous minerals occur together as fine fibrous intergrowths between anhydrous silicate and oxide grains. Coarser cry stals of ferrihydrite fill fractures that cut matrix minerals, and sme ctite also lines narrow channels within olivine grains. These channels may have formed by preferential alteration of olivines along (100)-pa rallel defects. Formation of ferrihydrite and smectite in the matrix o f Vigarano was the result of mild aqueous alteration in a low-temperat ure (<150 degrees C), oxidising parent body environment. Partial equil ibration of matrix olivines indicates that alteration was followed by thermal metamorphism with a peak temperature of 400-500 degrees C. Min eralogically similar alteration products, which also were formed by pa rent body processes, have previously been described from the matrices of four CV meteorites: Ball, Grosnaja, Kaba and Mokoia, all of which b elong to the oxidised subgroup. This discovery of the products of oxid ative aqueous alteration in Vigarano has important consequences for un derstanding the chemical and thermal history of the CV class of meteor ites.