The initiative became more acutely needed after an inflow of cheap dollars fuelled a boom in the BRICS for a decade and then reversed to a sharp outflow last year.
"We have reached an agreement that, in current conditions of capital volatility, it is important for our countries to have this buffer in addition to the International Monetary Fund," Siluanov said.
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But the framework agreement to be signed in Brazil will not include any direct commitments, which are due to come later when the central banks sign agreements.
A senior Brazilian official who participates in the negotiations said the pool could become operational as soon as in 2015.
According to the agreement, the cash will continue to be held in the reserves of each BRICS country, but it can be transferred if needed to another member to soften volatility in its foreign exchange market.
China, holder of the world's largest foreign exchange reserves, will contribute the bulk of the contingency currency pool, or $41 billion.
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Brazil, India and Russia will chip in $18 billion each and South Africa $5 billion.
"It is to be a mechanism that could react swiftly to capital outflow by offering swap operations .. in dollars," Siluanov said.
If a need arises, China will be eligible to ask for half of its contribution, South Africa for double and the remaining countries the amount they put in.
"Some countries may put in less, but their needs are also greater, proportionally," Siluanov said.
A BRICS member would be able to immediately get 30 percent of its eligible share and the remaining 70 percent only with a stabilisation programme from the IMF, Siluanov said.
- By Reuters