(Updated to include second day stock movement.)A report that Brazilian mining giant Vale ( VALE) is considering a bid for Mosaic ( MOS), to the tune of a possible $25 billion, has continued to push shares of that fertilizer producer higher on Friday. Vale's move was reported by a Sao Paolo newspaper on Thursday. The purported price tag would value the Minnesota company at $56 a share. Mosaic's stock gained another 8% Friday before cooling off a bit in afternoon trading. They were moving recently at $51.93, up $1.95, or 4%, on heavy volume. On Thursday, the stock closed at $49.98, up 12%. Vale's New York-listed American Depositary Receipts, meanwhile, were trading basically down 11 cents to $18.05. Both companies declined to comment on the report. At least one stock analyst, Anthony Rizzuto of Dahlman Rose, said Vale's Australian rival BHP Billiton ( BHP) could be compelled to make a counter bid for Mosaic. Mosaic is basically a miner of potash and phosphate; Vale already extracts potash in Brazil -- and has phosphate and potash properties under development elsewhere -- though not nearly enough to sate Brazil's overall demand for the fertilizer, of which it is a major importer. Mosaic was formed in 2004 when Cargill combined its fertilizer assets with that of IMC Global. Cargill, the second biggest private company in the U.S., owns about 64% of Mosaic.