At 1:30 P.M., a carbonaceous chondrite fell in the Federated SSR, USSR. Only two stones with a combined weight of 1,165.6 g were recovered, while three other small stones were reportedly destroyed. This CM chondrite is unusual in exhibiting a brecciated texture over a large field of view. Olivine and pyroxene in Boriskino are essentially Fe-free, occurring as forsterite and enstatite. According to Ohnishi and Kazushige (2003), enstatite has been aqueously altered to serpentine by relatively low pH fluids that are enriched in Fe and depleted in Na and Si (smectite is produced at higher pH values and higher Na contents as in the CV group). Iron is present mainly within the serpentine-like phyllosilicates, with a minor amount present in magnetite. It is thought that magnetite was produced by the oxidation of FeS during low-temperature aqueous alteration processes, though some may have a nebular origin (Hyman and Rowe, 1983). Since Boriskino has one of the lowest magnetite contents of any CM chondrite, it probably experienced a relatively moderate degree of parent body alteration.
Boriskino contains several FeNi-sulfide/phosphide phases, some of them unknown before. A significant proportion of these phases is P-rich, but may also contain K and Cr, each of these condensing in a non-typical chalcophile manner. These phases are thought to have formed by the sulfidation of kamacite in the nebula under reducing conditions at temperatures below 427°C, or alternatively, during sulfidation of presolar FeNi-carbide grains (Nazarov et al., 1997, 1999). A secondary mineral assemblage consisting of mackinawite (FeS), carbonate, magnetite, and FeNi-metal is associated with low-temperature aqueous alteration processes on the CM parent body (Boctor et al., 2002, 2004).
Similar to other CM chondrites, a presolar nanodiamond phase is present in Boriskino that contains the noble gases Xe and Ar, which were implanted at various energies corresponding to different interstellar events. A new noble gas component with intermediate characteristics has recently been identified in nanodiamond fractions from Boriskino (Fisenko et al., 2002). The photo above shows the interior of a 0.146 g fragment of Boriskino.
