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Science 8 March 1968:
Vol. 159. no. 3819, pp. 1099 - 1102
DOI: 10.1126/science.159.3819.1099
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Articles

Exsolution in Clinoamphiboles

Malcolm Ross 1, J. J. Papike 1, and Paul W. Weiblen 2

1 U.S. Geological Survey, Washington, D.C. 20242
2 Department of Geology and Geophysics, University of Minnesota, Minneapolis

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 ([unknown]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 cummingtonite with C2/m space-group symmetry. Likewise cummingtonites 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 clinoamphibole is analogous to the clinopyroxene pigeonite in agreement with the results of M. G. Bown.


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