Abstract

Tschermak, in 1871, showed that the then supposedly distinct species of the pyroxene group, acmite and aegirite, are chemically alike, that is, essentially Na2O·Fe2O3·4SiO2. Both names have persisted. Brownish, slightly pleochroic crystals are usually called acmite, while those that are greenish and are decidedly pleochroic are called aegirite. Entirely homogeneous crystals, composed of only the brown or the greenish variety, are rare, and some of the megascopically brown acmite is colorless when seen in thin section. If both varieties occur in the same crystal the arrangement is generally zonal, most often the brown or colorless material forming the exterior: less often the distribution is irregular. The term aegirite-augite, proposed by Rosenbusch1 to denote pyroxenes intermediate in chemical and optical characters between aegirite and augite occurs frequently in petrographic Iiterature. Other names2 have not met with general acceptance.

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