MELBOURNE, April 10 (Reuters) - U.S. rare-earths

producer MP Materials ( MP ) 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 ).

MP Materials ( MP ) is the second-biggest producer of rare earths

outside China after Australia's Lynas Rare Earths ( LYSCF ).

Lynas ended talks with MP Materials ( MP ) 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 and a

9.14% holdings in Arafura Rare Earths ( ARAFF ), according to

LSEG data.