Mrima Hill, a sacred site about 65km (40 miles) southwest of Mombasa on Kenya’s Indian Ocean coast, is emerging as a new front in the US-China rivalry for critical minerals.US-led and Chinese bidders are competing for rights to develop the site, which is rich in rare earths and niobium, a metal used to make steel stronger and lighter.American officials have offered to support Kenya’s development of its domestic critical minerals processing industry as Washington seeks to challenge Beijing’s...