Abstract
This study gives electron probe analyses and optical descriptions for twenty-two amphibole pairs, which include, actinolite-hornblende, actinolite-glaucophane, hornblende-glaucophane, hornblende-hornblende, and several pairs which are intermediate in composition between hornblende and glaucophane. One pair is quoted from the literature.
In actinolite-hornblende pairs the hornblende always has the lower Mg/(Mg+Fe) ratio. The one hornblende-hornblende pair shows a similar fractionation. In actinolite-glaucophane pairs glaucophane has lower Mg/(Mg+Fe) as well as lower Si/(Si+Al) ratios. In four out of six hornblende-glaucophane pairs the glaucophane has a smaller Mg/(Mg+Fe) ratio than the hornblende, whereas in two pairs this is reversed. The Si/(Si+Al) fractionation between these two highly aluminous phases is very irregular. Four pairs, which consist of glaucophane and hornblende-glaucophane compositions, have Mg/(Mg+Fe) and Si/(Si+Al) fractionation patterns very different from those observed in the other pairs.
In a few of the 2-amphibole assemblages homogeneous, separate grains of the two amphiboles coexist in random intergrowth, but the majority of textures show complex zonation and patches of the two amphiboles in composite grains. The contact between the two amphiboles has been found to be optically and chemically sharp in every 2-amphibole assemblage, except one. These 2-amphibole assemblages are evidence for the probable existence of miscibility gaps in the system actinolite-hornblende-glaucophane at relatively low temperatures. The majority of the rocks in which these assemblages occur are part of the greenschist or epidote amphibolite facies, and the glaucophane schist facies, all of which were formed at relatively low temperatures.
