NEW YORK, Feb. 27, 2025 /PRNewswire/ -- Entegris Inc. (NASD: ENTG) will replace Arcadium Lithium plc (NYSE: ALTM) in the S&P MidCap 400 effective prior to the opening of trading on Thursday, March 6. Rio Tinto plc (ASX: RIO) is acquiring Arcadium Lithium in a deal expected to be completed soon, pending final closing conditions. Following is a summary of the changes that will take place prior to the open of trading on the effective date: Effective Date Index Name        Action Company Name Ticker ...