Exsolution in clinoamphiboles

Ten amphibole specimens from a variety of metamorphic rocks such as talc schists, eclogites, and metamorphosed iron formations contain lamellae of a second amphibole oriented parallel to (101) or (100), or both, of the host. Tremolites, actinolites, and hornblendes commonly have lamellae of a calcium-poor clinoamphibole with P21/m space-group symmetry, or lamellae of citmmingtonite with C2/m space-group symmetry. Likewise ciimmingtonites and P21/m clinoamphiboles commonly contain lamellae of calcium-rich C2/m amphiboles such as tremolite. Results of x-ray diffraction, electron-probe, and microscope studies indicate that most lamellae result from unmixing of a homogeneous amphibole. The P21/m clinoampliibole is analogous to the clinopyroxene pigeonite in agreement with the results of M. G. Bown.