Nine new chemical analyses of members of the cummingtonite-grunerite series are presented. The material is from the high grade metamorphic, Precambrian, Wabush Iron Formation, Labrador, Canada. These analyses and additional analyses from the literature are recalculated in terms of the components Mg7Si8O22(OH)2, Fe7Si8O22(OH)2 and Mn7Si8O22(OH)2. The solid solution within this ternary system is found to contain 35 to 100 mole per cent Fe7Si8O22(OH)2 and 0 to 34 mole per cent Mn7Si8O22(OH)2. Variation in optical properties, unit cell dimensions and density within this solid solution series is presented graphically in binary and ternary variation diagrams. Completely indexed powder diffraction patterns are given for three chemically dissimilar members of the cummingtonite-grunerite series.

The γ index and Z∧c angle are the most easily obtainable optical parameters for determination of chemical compositions in the ternary system. The b cell dimension shows an appreciable and systematic variation with composition, whereas a sin β and c vary little. Manganoan cummingtonites (those containing 10 mole per cent or more Mn7Si8O22(OH)2) have physical properties which are so similar to those of the tremolite-actinolite series that the presence of manganese must be established chemically.

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