July 30 (Reuters) - Canada’s main stock index opened lower on Friday, weighed by weakness in energy and mining stocks, while official data showed the nation’s economy most likely expanded in June. * At 9:30 a.m. ET (1330 GMT), the Toronto Stock Exchange’s S&P/TSX composite index was down 78.19 points, or 0.38%, at 20,233.59. (Reporting by Amal S in Bengaluru; Editing by Shailesh Kuber) Our Standards: The Thomson Reuters Trust Principles.