Most of the so-called Bayan Obo fine-grained dolomite marbles collected from the main and east orebodies show a microporphyritic texture, namely the microphenocrysts are set in a very fine-grained matrix, although nearly all of them have undergone recrystallization caused either by deformation or alteration. The texture seems likely to have maintained the original features. It is known that one of the most characteristic textures of volcanic rocks is the porphyritic texture, and the microporphyritic texture is a variety in which both the phenocrysts and the matrix are only distinguishable with the microscope. Therefore, the dolomite marbles in the main and east orebodies may be related to the extrusive carbonatites. In addition, there also occur some carbonatite sills and dykes with different textures at Bayan Obo. Thus, the Bayan Obo carbonatites are polyphase intrusive and extrusive carbonatites.