In recent years several attempts have been made to arrange in a series the characteristics of non-aluminous amphiboles. Sundius1 and Winchell2 succeeded fairly well in representing refraction, birefringence and specific gravity as a continuous function of the molecular percentages of iron and manganese. Unfortunately, the properties of Lake Superior grünerite, an important link in this series was hitherto practically unknown. A reëxamination of this amphibole3 revealed new aspects which seem worth while to be taken into consideration.

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