97 Olivine

The Geology and Petrography of Crater Lake National Park, 1902






Olivine is a constant but very fluctuating ingredient. It appears to be particularly abundant in those rocks where hypersthene is either missing or at least not abundant. No. 157 well illustrates this fact. It is a nearly holocrystalline basalt, with abundant augite and olivine, but with only two or three hypersthene individuals visible in a thin section. The olivine appears either colorless or slightly yellow, but it is frequently stained blood red along the irregular cleavage cracks.

In a few cases a slight serpentinization has started in. The olivine occurs usually in granular form or in clusters of grains and only exceptionally in roughly defined crystals. It is the largest constituent of these basalts and is the only one that may be classed as a phenocryst. In No. 152 the olivine appears to have suffered to some extent from magmatic resorption. This is indicated by a border of opaque ore grains. This black border does not occur on all individuals. It is best seen on the smaller ones. It may occur on one end of a crystal and be missing on the other end. A few of the smaller crystals show only a core of unaltered olivine while the greater part of the crystal has been thus altered. In this specimen the small individuals of olivine are unusually well defined and show quite distinctly the crystal shape. In such cases the granular mass of ore has retained the original crystal form of the olivine. The olivines show occasional inclosures of magnetite and also once in a while of brown glass.

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