The government will invite bids for 20 critical mineral blocks in next two weeks, a top official said on Tuesday.The 20 critical blocks include lithium and graphite mines, Mines Secretary V L Kantha Rao told reporters..Last month, the Centre had approved royalty rates of three% each for lithium and niobium and one% for Rare Earth Elements (REEs).Critical minerals have become important for the nation's economic development and national security.Lithium and REEs have gained significance keeping in mind the country's commitment towards energy transition and achieving net-zero emission by 2070.