>Lat. ka- -> (c^a ->) French s^a-
>(spelled cha-). There is to my knowledge no phonological explanation
>for this.
I refer you to Rebecca Posner:
"The French /a/ is pronounced further forward in the mouth than in the
other Romance languages: it may be this fronting of /a/ that induced the
palatalization of preceding /k/ and /g/, a phenomenon found only in French
and Rhetian... The fronting tendency of French is often put down to the
influence of the pre-Roman inhabitants of France, the Gauls...."
Peter