The golden iridescence observed in orthoamphiboles from the Nuuk district, southern West Greenland has been shown by analytical transmission electron microscopy to be the result of the interference of light reflected from alternate, coherent gedrite and anthophyllite exsolution lamellae parallel to (010). The lamellae are periodic with an average spacing of ∼180 nm; lamellae with this spacing would give rise to yellow iridescence. It is concluded that the exsolution lamellae are the origin of the features previously seen on cleaved surfaces of the amphiboles in the atomic force microscope (AFM). The suggestion is that the 0.75° difference in the cleavage angle for the two phases produces corrugations that are imaged by AFM. The morphology of the lamellae is consistent with an origin by spinodal decomposition.
