TORONTO, Sept 13 (Reuters) - Canada's main stock index closed up slightly on Friday, helped by a sharp rise in Potash Corp shares on hopes that a partnership between Uralkali and Belaruskali might be rekindled, which would support prices for the crop nutrient. The Toronto Stock Exchange's S&P/TSX composite index unofficially closed up 22.35 points, or 0.18 percent, at 12,723.40. It declined 0.76 percent over the week.