56 Accessory Minerals

The Geology and Petrography of Crater Lake National Park, 1902





Quartz does not appear to form an essential ingredient in these andesites, except as it may form part of the allotriomorphic colorless material to be seen in most of the holocrystalline varieties. With this exception this mineral has been certainly identified in only two cases (46, 79), and in these two cases it does not occur at all evenly distributed, but only in a few irregular grains that may easily be accounted for by secondary or accidental causes.

Tridymite was recognized in only one case, an andesite of a decidedly dacitic type (79), about 1-1/2 miles south of the camp ground.

Hornblende.—A very few yellowish-brown and strongly pleochroic crystals of this mineral, mostly in fragments, are to be seen in specimen No. 44, from near the camp ground, and specimen No. 45, from Cathedral Rock. The larger grains have the black resorption rims of magnetite so common in andesites. In the smaller individuals the resorption is complete and only a black mass of granular magnetite remains (see Pl. XVI, B). Also one small crystal is seen in No. 48, a rock transitional between andesite and dacite. This mineral, which is very common in the dacites of Crater Lake, is otherwise entirely wanting in the andesites.

Olivine occurs very sparingly as an occasional rounded grain or fragment, more particularly among the nests of older crystallized minerals. In nearly all the andesites it is entirely wanting. It is most abundant in No. 49, where it is seen in well-rounded grains that have marked resorption rims of magnetite and, apparently, of augite. In No. 43 there are one or two forms which resemble olivine crystals and which are filled with a yellowish polarizing substance, presumably serpentine.

Magnetite forms an always present but rather sparingly developed ingredient among the minerals of the first generation and occurs frequently inclosed in plagioclase, hypersthene, and augite. In the groundmass it is perhaps more abundant, and then occurs as small octahedral crystals or as grains; also in the andesites with a brown, glassy base, as a very fine powder. It is likewise very abundant as inclosures in the augite microlites of the groundmass.

Hematite appears to occur occasionally either as a reddish stain or as minute brownish-red hexagonal scales (65).

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