In what now seems to be a regular announcement, Rio Tinto revealed on Friday that it has again extended its takeover offer for Africa-focused coal miner Riversdale Mining by a further two weeks.The extension was taken in the hope that it would give minority shareholders additional time to jump on board before the target is delisted from the Australian Stock Exchange.Just two weeks ago, the mining giant announced that it had pushed the deadline back to 20 May (from 6 May), but has now prolonged it until 3 June, "unless further extended," the statement said.Since the last extension, Rio has been able to raise its stake in Riversdale by 0.11 percentage points to 73.39%, meaning that the free-float of Riversdale shares not already controlled by either itself or Tata Steel (which holds a 27% stake) has fallen from 0.44% to 0.33%.---BC