Mining giant Rio Tinto is extending the acceptance period for its A$3.9bn bid for Australian coal miner Riversdale by three days.The extension applies both to the acceptance period, which now runs to 6 April, and the deadline by which more than 50% of Riversdale shareholders must accept the offer in order for the higher offer price of A$16.50 to kick in; this period now runs to 28 March.The extension has been granted to enable some market participants to unwind equity swaps and settle trades that have occurred during the offer period.The total of the Riversdale shares in which Rio has a relevant interest, plus shares subject to Institutional Acceptance Facility instructions, was 34.94% of the Riversdale shares in issue as at 9.00am (Sydney time) on Monday, 21 March. That's up from 33.04% last Friday.