Mining giant Rio Tinto has again pushed back the deadline for its takeover offer for Africa-focused coal miner Riversdale Mining, giving minority shareholders more time to jump on board before the target is delisted from the Australian Stock Exchange.On Tuesday, Rio announced extended the offer period to Friday 6 May, but has now decided to prolong the deadline for another two weeks, until Friday 20 May.The firm has continued to receive acceptances to the offer and now holds a relevant interest in Riversdale of over 73%, following the sale by Brazilian steel firm Cia Siderúrgica Nacional of its 19.9% stake.Consequently, the number of Riversdale shares trading on the market - not controlled by either Rio Tinto or Indian steel company Tata Steel (which holds a 27% stake) - has dropped to just 0.44% of all shares in issue. ---BC