PHYLLOSILICATES IN THE MATRIX OF THE UNIQUE CARBONACEOUS CHONDRITE LEWIS-CLIFF-85332 AND POSSIBLE IMPLICATIONS FOR THE AQUEOUS ALTERATION OF CI CHONDRITES

The fine-grained matrix of the unique, unequilibrated carbonaceous cho ndrite Lewis Cliff (LEW) 85332 has been studied by scanning electron m icroscopy (SEM), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). Compositionally, LEW 85332 has a matrix tha t is more Fe-rich than typical CI chondrites but has elemental abundan ce ratios that appear to be closer to CI matrices than to CM or CR cho ndrites. The mineralogy of the matrix is dominated by phyllosilicate p hases that are predominantly interlayered Fe-rich serpentine/saponite; anhydrous silicate phases such as olivine and pyroxene are rare. Mino r magnetite, troilite and ferrihydrite also occur associated with the phyllosilicates. Despite the high degree of weathering in LEW 85332, t he phyllosilicates appear to have an extraterrestrial origin, but the highly variable Mg/Fe ratios of saponite may be the result of partial terrestrial oxidation of Fe-rich saponite to a more Mg-rich saponite a nd ferrihydrite. Alternatively, some of the ferrihydrite may have form ed as a result of terrestrial weathering of Fe-Ni metal. The compositi onal and mineralogical data suggest that the matrix of LEW 85332 may r epresent a very early stage in the type of aqueous alteration experien ced by the CI chondrites, although it is improbable that LEW 85332 was a precursor to the CI chondrites because of its high abundance of cho ndrules. The absence of carbonates, the high-Fe content of the matrix and phyllosilicate phases and relatively low abundance of magnetite al l indicate that the degree of oxidation and leaching of LEW 85332 matr ix was significantly less than that experienced by the CI chondrites. The absence of clear evidence for alteration of chondrules suggests th at either the formation of the hydrous phases in the matrix occurred p rior to accretion or that alteration occurred on a parent body and inv olved limited amounts of fluid, such that the reactions took place pre ferentially and exclusively within the fine-grained (anhydrous?) matri x materials.