  The rare CI carbonaceous chondrites are the most aqueously altered and
chemically primitive meteorites but due to their porous nature and high
abundance of volatile elements are susceptible to terrestrial weathering. The
Ivuna meteorite, type specimen for the CI chondrites, is the largest
twentieth-century CI fall and least affected by terrestrial alteration. The
main mass of Ivuna (BM2008 M1) has been stored in a nitrogen atmosphere at
least since 2008 and is the most pristine CI chondrite stone. We report the
mineralogy, petrography and bulk elemental composition of BM2008 M1 and a
second Ivuna stone (BM1996 M4) stored in air. Both Ivuna stones are breccias
consisting of multiple rounded, phyllosilicate-rich clasts formed through
aqueous alteration followed by impact processing. A polished thin section of
BM2008 M1 analysed immediately after preparation was found to contain
sulphate-bearing veins formed when primary sulphides reacted with oxygen and
atmospheric water. A section of BM1996 M4 lacked veins but had sulphate grains
on the surface recently formed (last 6 years). Differences in the extent of
terrestrial alteration recorded by BM2008 M1 and BM1996 M4 probably reflect
variations in the post-recovery curation history of the stones before entering
the NHM collection, and indicate that where possible pristine samples of
hydrated carbonaceous should be kept out of the terrestrial environment in a
stable environment to avoid modification. The bulk elemental composition of the
two Ivuna stones show some variability due to their heterogeneous nature but in
general are similar to previous analyses of CI chondrites. We combine our
elemental abundances with literature values to calculate a new average
composition for the Ivuna meteorite, which is in good agreement with existing
compilations of CI chondrites and the most recent solar photospheric
abundances.
