Stock index provider S&P Indices has announced Atlas Iron and Equinox Minerals have joined the ranks of the top 100 Australian companies, while five other Western Australia-based firms have been elevated to the S&P/ASX 300.

West Perth-based Sandfire Resources has been added to the S&P/ASX 200 index after Standard and Poor's removed Centennial Coal from all of its indices.

Perth companies Kagara and Mermaid Marine Australia have been added to Standard and Poor's ASX 200 index, as part of the ratings agency quarterly rebalance of its indices.

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Miner Kagara and mining services company NRW Holdings have been removed from the S&P/ASX 200 index while two Western Australian companies with an African focus have been added.