re: impact lapilli
Interesting, beds of mm-sized lapilli in a breccia matrix are being recognized on earth as an indicator of ancient giant impacts.
Here is an example from the Chicxulub impact.
I'm not sure that's relevant in this case, the fine layering and the even texture of the matrix at Meridiani doesn't sound like an impact deposit.
However, it *is* a non-biologic process by which you can make non-igneous spheriods, in case the APXS data show that this isn't basalt or something obviously volcanic.
http://www.geosociety.org/fieldForums/01rpt1.htm
http://www.ehu.es/~gpplapam/congresos/bioeventos/claeys.html