Microcrystal Morphology

Framboids are constituted by microcrystals with approximately log-normal size-frequency distributions, and 95% of framboidal microcrystals are between 0.1 and 3.1 μ‎m. Nanocrystals are not generally observed in framboids. Packing efficiencies vary between close-packings in which the microcrystals occupy up to 74% of the framboid volume and random packings with a 56% volume of microcrystals. The ratios between framboid diameter and microcrystal size show a clear bimodal distribution which reflects the populations of close-packed ordered framboids and randomly organized framboids. Framboids may contain up to 500,000 microcrystals. The average numbers of microcrystals in both disordered and ordered framboids are similar, which suggests that the organization of microcrystals is the result of an additional process. Minerals that do not commonly produce equant crystals forms are unlikely to display the framboidal texture. Framboid microcrystals are essentially limited to isometric minerals like pyrite which produce equant crystals. Pyrite displays the greatest variety of crystal shapes among the common minerals. This means that pyrite is able to approximate forbidden fivefold symmetries such as the pentagonal dodecahedron, but with asymmetric pentagonal faces, and the icosahedron, again with different-sized triangular faces, as a combination of the octahedron and pyritohedron.