March 16 (Reuters) - Canada’s main stock index opened lower on Tuesday, as both energy and materials stocks tracked weaker commodities, while data showed that foreign investment in Canadian securities was the lowest in six months. * At 9:30 a.m. ET (13:30 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 5 points, or 0.06%, at 18,949.75. (Reporting by Amal S in Bengaluru; Editing by Shailesh Kuber)