Magnum Hunter Resources Corp said on Tuesday it will sell a stake in a natural gas gathering unit in U.S. Marcellus and Utica shale fields to the infrastructure investment unit of Morgan Stanley, giving the two co-ownership in the business valued at $1 billion. In addition, Morgan Stanley Infrastructure will also buy about 41 percent in Eureka Hunter from a unit of ArcLight Capital Partners LLC, a U.S. private equity company, for an unspecified amount.