The Crystallography of Pyrite Framboids

Single crystal X-ray diffraction analyses of even the most perfectly organized framboids show ring patterns indicative of randomly oriented particles. Therefore, framboids are not mesocrystals or extreme skeletal varieties of single crystals. Electron backscatter diffraction shows that the microcrystals within a framboid are not crystallographically aligned. Around half of the microcrystals in organized framboids have crystallographic orientations rotated 90º. The results of single crystal XRD and framboid EBSD studies clearly show that the microcrystals are self-organized rather than being the result of a crystallographic template or preexisting structural control. The pre-formed framboid microcrystals which are initially randomly organized throughout the framboid volume then, in some cases, begin to wholly or partly self-order. This is effected by rotation of the microcrystals until an ordered array is produced. The consequence of this rotation must be that the microcrystals are initially packed loosely enough for rotation to occur. The processes involved in the rotation could include forces intrinsic to the microcrystals themselves, such as surface forces, or forces imposed from outside the framboid, such as Brownian motion. The fundamental driving force for microcrystal rotation and the development of organized microcrystal arrays in framboids is entropy maximization.