Hematite in porous red-clouded feldspars: Evidence of large-scale crustal fluid–rock interaction

Transmission electron microscopy shows that red-clouded alkali feldspars from granitic rocks contain numerous open pores up to several hundred nanometres in cross-section, and that almost every pore contains rosettes or needles of crystalline hematite. In the samples described here, the hematite forms at the reaction interfaces at which plagioclase feldspar is replaced by alkali feldspar. These observations, together with the fact that the origin of the porosity can be explained by the mechanism of sub-solidus mineral–fluid replacement processes, indicate that hematite is a direct product of fluid–rock interaction and is not a solid state exsolution product from the feldspar.