Exsolution of hornblende and the solubility limits of calicum in orthoamphibole

Exsolution between orthorhombic and monoclinic amphibole has been postulated for many years on the basis of crystal-chemical and structural differences. Transmission and analytical electron microscope examination of calium-rich gedrite specimens from southwestern New Hampshire has now revealed evidence for exsolution of calcic clinoamphibole (hornblende) from ferromagnesian orthoamphibole. Analytical electron microscopy data suggests that calcium has a low solubility limit in the orthoamphibole structure. The hornblende lamellae range from only a few unit cells in thickness to about 80 nanometers. The formation of the calcic amphibole lamellae resulted from heterogeneous nucleation and growth along pre-existing (100) stacking faults. Thus, the deformation processes producing the stacking faults played a key role in preparing the sites for exsolution.