. Bulletin of the Geological Society of America. Geology. PETROLOGY 323 Chemical Analysis of Hornblende from Camptonite (J. G. Dinwiddie, analyst) Si02 Al2Oa Fe203 - FeO MgO CaO Na20 K20 H20 Ti02 MnO As will be observed, the ferrous iron and soda in this analysis are too low for arfvedsonite or barkevikite. The hornblende is zonally built, as shown in figure 4. It is usually quite fresh, but where alteration occurs, pale green chlorite and magnetite are noted as the secondary products. Twinning is common, the composition plane
. Bulletin of the Geological Society of America. Geology. PETROLOGY 323 Chemical Analysis of Hornblende from Camptonite (J. G. Dinwiddie, analyst) Si02 Al2Oa Fe203 - FeO MgO CaO Na20 K20 H20 Ti02 MnO As will be observed, the ferrous iron and soda in this analysis are too low for arfvedsonite or barkevikite. The hornblende is zonally built, as shown in figure 4. It is usually quite fresh, but where alteration occurs, pale green chlorite and magnetite are noted as the secondary products. Twinning is common, the composition plane being in most cases 100. Sometimes several lamellae occur in the same individual. Figure 3 shows a case of abnormal twinning. Augite is developed as an important con- stituent in all the thin-sections studied, but is much less abundant than hornblende. It occurs in short, stout, well formed crystals, with the faces 100, 110, 010, and often 011, and in crystal aggregates. It is almost col- orless except in the border zones, which are sometimes grass green. The inclination of c to z is about 44°. Zonary growth is com- mon (figure 4), the colorless augite of the centra] zone of some crystals passing into an outer zone of grass green aegirite, while in others the entire crystal is colorless and normal Twinning in Hornblende o\ Camptonite the zones can be detected only by differences m optical orientation. In the latter case the outer zone sometimes shows an extinction angle 2 to '-\ degrees larger than that of (he inner /one. In some crystals, where there are numerous concentric zones, a wave o( extinction passes from the center to (he margin of the crystal. In other eases extinction starts midway between I he center and margin and moves. Figure :?.—sketch showing ah-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1890
