April 23 (Reuters) - Canada's main stock index opened

marginally lower on Tuesday, pulled down by materials and energy

stocks, tracking a decline in commodity prices, while investors

looked forward to an earnings packed week in the U.S., and at

home.

At 9:30 a.m. ET (13:30 GMT), the Toronto Stock Exchange's

S&P/TSX composite index was down 16.19 points, or

0.07%, at 21,855.77.