Radially arranged nepheline crystal aggregates can be found in some of Nyiragongo´s lavas. The very low silica content of the volcano´s magmas is the reason why they have such a low viscosity, making them some of the fastest moving and hence deadliest lavas on earth. Minerals crystallising from such magmas are hence also silica poor, for example the white feldspathoids shown in this image. (Photo: Ingrid Smet)