Rio has been floated as a buyer for the US$4.8 billion, 32% stake in SQM that PotashCorp (CN:POT) must sell because of a merger.
The major diversified miner has no producing lithium operations of its own and the Jadar project in Serbia is years away.
Gait's argument hinges on his thesis lithium is headed for oversupply and lithium carbonate prices will not climb much further from Bernstein's $16,000 per tonne estimate for September.
"We are drawn to the, from our perspective worrying, conclusion that for Rio Tinto this acquisition would be, in essence, purely a move on the expectation of higher future lithium prices," he said.
"As we have made clear in our recent work, we do not see this as a sensible expectation."
Gait has forecast a 16% compound annual growth rate in lithium carbonate supply between 2015 and 2030, although this assumes all planned projects get built.
He said battery production will never be limited by lithium because there will always be enough to meet the needs of the available cobalt and nickel.
This analysis is disputed by other analysts, who say lithium should be treated like a chemical product where processing is a major limiting factor.
In any case, Gait said Rio could just be keen on a peek into SQM's operations.
"Of course, there are other benefits that can accrue from putting oneself into a bid process of this kind, such as the exposure to the information available within the data room for the acquisition process," he said.
"Information in such a process would presumably give detailed insight into SQM as a company and its place within the global lithium industry, and therefore would be particularly pertinent to Rio's own thinking around the Jadar project."