Four pairs of associated calcic and sodic amphiboles from blueschist facies metamorphic rocks were analyzed with the electron microprobe and studied by single-crystal X-ray diffraction techniques. Except for ranges in the ratios Mg/(Mg+Fe) and Fe3+/(Fe3++Al+Ti), the sodic amphiboles are similar in chemical composition. The amount of calcium in the M(4)-site ranges only from 0·18 to 0·21 ion per formula unit. The calcic amphiboles, in addition to a range in Mg/(Mg+Fe), vary in Na/(Na+Ca) ratio (0·29–0·48). Three of the calcic amphiboles contain less than 1·5 calcium ions per formula unit, indicating a significant solid solution of sodic amphibole components in the calcic amphibole phase. The a and b unit-cell parameters of the calcic amphiboles decrease with increased content of the sodic component.