Octahedral versus tetrahedral coordination of Al in synthetic micas determined by XANES

We used the JUMBO monochromator at SSRL to measure the Al K-edge X-ray absorption spectra of synthetic micas having variable Al content and occupancy, from 0 to 2/3 in the octahedral M positions, and 0 to 2/4 in the tetrahedral T positions. The measured Al K edges differ markedly, but the differences may have a common explanation: (1) Micas containing 1/3 Al in M or 1/4 Al in T have K edges that differ in the energy and intensity of the first two features, which are related to interaction of Al with its first-shell nearest neighbors (O and OH or F). They are nearly identical to the K edges of reference minerals such as albite (tetrahedral Al only) or grossular (octahedral Al only). (2) Micas containing Al in both M and T have K edges that can be interpreted as a weighed combination of the simple edges.