* TSX down 0.27 percent at 13,570.79
* Suncor slips 0.88 percent to C$39.21
* Oil trade volatile, gold and copper see losses
By Julie Gordon
TORONTO, May 19 (Reuters) - Toronto's main stock index turned negative on Thursday, as heavily weighted energy and materials issues reversed course and fell along with volatile commodity prices.
"Canada is underperforming a bit today," said Francis Campeau, broker at MF Global Canada. "Mostly on some weakness after such a strong day (on Wednesday) in oil, gold and copper."
The energy sector was down O.38 percent after starting the session higher. Suncor Energy (SU.TO), down 0.88 percent at C$39.21, was one of the biggest drags on the sector and broader market.
U.S. crude oil futures extended losses and Brent crude turned negative as investors digested U.S. jobless claims data that raised concerns about the labor market in the world's largest oil consumer. [O/R]
The materials index was down 0.54 percent, with the mining sector skidding with lower metal prices. Copper prices fell. Gold pared some losses, but was expected to close lower. [GOL/] [MET/L]
Goldcorp shares (G.TO) fell 0.42 percent to C$47.
The financial index also turned negative, as Royal Bank of Canada (RY.TO) slipped 0.59 percent to C$59.46.
At midmorning, the S&P/TSX composite index was down 0.27 percent at 13,570.79, while the blue-chip S&P 60 index .TSE60 slipped 0.28 percent to 778.11.
"Over the last three sections, S&P 60 rallied nearly three percent," said Campeau. "So it looks like a pause before some key economic data."
Statistics Canada is scheduled to release consumer price index data for the month of April on Friday. ECONCA
BlackBerry maker Research In Motion RIM.TO was the top net loser on the index, falling more than 2 percent to $43.21, following two days of gains.
($1=$0.97 Canadian) (Editing by Jeffrey Hodgson)