70 Hornblende

The Geology and Petrography of Crater Lake National Park, 1902





This is a very characteristic ingredient of the Crater Lake dacites. While it is almost entirely wanting in the andesites, it is conspicuously present, although in only occasional crystals, in the dacites, or, at least, in most of them. Out of twenty-eight thin sections prepared from different rock specimens, hornblende could be found in all but five. True, it is to be seen in a few of the thin sections in very small crystals, and in none of the rocks does it assume a prominent role as far as quantity is concerned, but it is always very easily recognized by its peculiar color and characteristic pleochroism. It occurs always in long slender prisms, several to many times as long as thick. The prism invariably, and sometimes also to a slight extent the clinopinacoid, is strongly and sharply developed. Terminal faces are often wanting, and when present appear to be the usual flat basal or pyramidal faces. They are frequently minute and are rarely over 1.5 millimeters in length. These hornblendes may all be classified as belonging to the brown variety, although there is really a great range in color. They may be further divided into, first, brownish-green; and second, brownish-red varieties.

The first, or brownish-green hornblende, is much more common than is the second variety. It may be studied in Nos. 102, 117, and 118. Pleochroism is very strong. c = dark olive green with usually a trace of brown, b = dark greenish brown, a = lemon yellow to greenish yellow. At times the rays vibrating parallel to c are almost a pure deep olive green. In any event brown is most conspicuous in the rays vibrating parallel to b. This hornblende has a very unusual absorption. in that the absorption parallel to b is greater than that parallel to c, thus, b>c>a. In some cases there appears to be very little difference in color and absorption between b and c, but wherever there is a marked difference the absorption is as given above. Professor Rosenbuscha refers to an observation by A. Osann of a hornblende phenocryst in an andesite from Hoyazo, Cabo de Gata, which has not only the same absorption but also nearly the same colors as have these hornblende phenocrysts from Crater Lake. The pleochroism as given is as follows: a, light greenish yellow; b, greenish brown; c, dark greenish yellow; and b>c>a.

aMikroskopische Physiographie, 3d edition, Vol. I, 1892, p. 558.

The extinction angles for these hornblende crystals are very small, 6° or 7° being the maximum observed. Further optical properties, as far as could be observed, appear to correspond with those of the similar hornblende that forms a large part of some of the so-called secretions to be described later in this paper. Twinning parallel to the orthopinacoid is very common.

The dark brownish-red variety of hornblende is to be seen in Nos. 104, 110, 112, and 114; it also occurs less abundantly elsewhere. In form it does not differ from the above-described variety. The pleochroism is fully as marked, but the colors are very different, c = dark brownish red, b = reddish brown, a = yellow, with c>b>a. The absorption is normal. The deep brownish-red color of the rays vibrating parallel to c are very striking and characteristic. This color is sometimes almost blood red and reminds one strongly of the color of hematite in very thin scales. The extinction angle does not differ materially from that of the first variety.

In one or two cases both of these varieties of hornblende may be seen in the same thin section (104). They are both of them usually perfectly sharply crystallized and unaltered. Only in rare cases can a partial resorption with development of black rims be noted (124), and, from the very few sections observed, it is not possible to state whether both of the varieties may have such resorption rims, but it appears as though this were the case. As far as can be seen, there appears to be no difference in the relative ages of these two varieties. They are the youngest of all the phenocrysts. The greenish-brown variety, at least, contains inclosures of plagioclase, hypersthene, and augite. The hornblende occasionally appears with other minerals in the form of nests and then is not as apt to occur in slender prisms as in more irregular grains.

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