Abstract

Three new stilpnomelane analyses are reported together with their physical data. One of the minerals was separated from quartzose schists and the other two, which exhibited zonary relationship, had crystallized in a glaucophanic mineral assemblage. Using unit cell dimensions determined by Gruner, tentative empirical cell contents for these stilpnomelanes have been calculated, and a figure of 30 oxygens per unit cell appears most probable. The compositions of the minerals are intermediate to those previously recorded and the optical and chemical data determined allow preparation of somewhat better composition/refractive index curves than those recorded earlier (Hutton, 1938, 1945). The modes of occurrence in both instances in plumose aggregates and vein-fillings that uninterruptedly traverse foliation, microfolds, and other structural features, point to a very late period of crystallization.

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