April 9 (Reuters) - MP Materials ( MP ) said Australian

billionaire Gina Rinehart's mining firm has taken a 5.3% stake

in the company, adding to her rare earth investments.

In a regulatory filing last week, the Las-Vegas based miner

said Hancock Prospecting, owned by Australia's richest person

Rinehart, holds about 8.8 million shares of MP Materials ( MP ).

MP Materials ( MP ), the biggest producer of rare earths in the

United States, had ended talks with Australia's Lynas Rare

Earths ( LYSCF ) in February over a potential merger.