The booming and controversial solar industry in China’s far-west Xinjiang region has weathered many storms.Despite tariffs, rising production costs and human rights sanctions, it pumps out almost half of the world’s polysilicon, the shiny material that lets solar panels turn sunlight into electricity.The region has almost everything it needs to do the job as fast and as cheaply as possible, from mines to factories to labour – except for one crucial raw material that exists in just a handful of…