Run an Echoes Battleground, making sure to find the hidden yellow-bar boss within the mission and kill it before you complete it. At the end you'll get a "beacon" (don't remember the exact name) that go into your inventory. Take that to the Watcher's Grave on Nessus and use it at the big Conflux to the right of where you spawn in, summoning 4 giant yellow-bar Vex. Kill all of them and you should get a 400g chunk of Radiolite. I'm pretty sure this can only be done once a week, though.