The Williams driver, who has competed in more than a third of all the championship grands prix ever held since the first in 1950, tangled with the Spaniard at the Bus Stop chicane as a sudden shower made conditions slippery.
Barichello retired on the spot while Alonso made his way back to the pits and was in 17th place after seven of the scheduled 44 laps, having made two pitstops already.
Alonso had started the day in fifth place overall, 20 points behind Red Bull's overall championship leader Mark Webber with six races remaining after Sunday.
Webber puts Red Bull on pole in Belgium
Webber wins in Hungary to take F1 lead
Vettel takes pole for home German GP
Webber wins British Grand Prix for Red Bull
Schumacher handed grid penalty, Mercedes fined