A potential tie-up between Rio Tinto Group and Glencore Plc would rank among the largest transactions ever attempted in the mining sector. The combined company would be valued at roughly $260 billion and would control a broad mix of iron ore, copper, and other industrial metals at a point when supply growth across several markets is slowing. The structure of the two companies explains why the idea continues to resurface. Rio’s iron ore business generates steady and predictable cash flow. Glencore, by contrast, has spent the past decade building…