(Changes dateline, adds Hancock response in paragraph 4, and...

(Changes dateline, adds Hancock response in paragraph 4, and details in paragraph 5)

MELBOURNE, April 10 (Reuters) - U.S. rare-earths producer MP Materials MP.N said Australian billionaire Gina Rinehart's mining firm had taken a 5.3% stake in the company, expanding her investments in the mineral ore sector.

In a regulatory filing last week, the Nevada-based miner said Hancock Prospecting, owned by Australia's richest person Rinehart, holds about 8.8 million shares of MP Materials.

MP Materials is the second-biggest producer of rare earths outside China after Australia's Lynas Rare Earths (LYC) . Lynas ended talks with MP Materials over a potential merger in February.

Hancock declined to comment on Wednesday. The iron ore miner has been building stakes in critical minerals projects, including rare earths. Hancock's investments include a 6.17% stake in Brazilian Rare Earths (BRE) and a 9.14% holdings in Arafura Rare Earths (ARU) , according to LSEG data.