94 Feldspar

The Geology and Petrography of Crater Lake National Park, 1902






The feldspar is entirely plagioclase, which, on account of its great abundance, dominates the structure of the rock. It has the rather long lath form common to basalts of this class. In most cases the laths are rather irregular in outline, but where a glass base is more in evidence, as is usually the case, the edge of these lath forms may be quite sharp. In such cases the ends are cut off squarely, as though by a pinacoidal face. Wherever the absence of a glass base prevents the free development of the terminal faces the long sides of the laths are more perfectly formed than are the ends. Their length is usually several times their width. The general shape of these plagioclase crystals is tabular, as the square cross sections so characteristic of plagioclase in microlitic form are conspicuously absent.

The different thin sections of these basalts present not a little variety in the size and uniformity of the feldspars in the same specimen. No. 161 will serve to illustrate one in which the feldspars are unusually uniform in size. Their average length is 0.2 millimeter. Nos. 152 and 157 show much greater variation in the size of the feldspars, those of No. 157 having an average length of 0.3 millimeter, with a maximum of 0.7 millimeter and a minimum of 0.1 millimeter: the feldspars of No. 158 have an average of 0.15 millimeter with a maximum of 0.3 millimeter and a minimum of 0.04 millimeter. Most of the plagioclase laths disclose sharply defined polysynthetic twinning, the smaller and more slender laths having two or three and the larger ones half a dozen or more stripes. In each thin section, however, there are not a few individuals that do not appear to be twinned. There is no good reason to infer that any of these represent a monoclinic feldspar. Usually such untwinned individuals have much broader forms, such as one would expect to see in a tabular plagioclase cut parallel to the largest face. The supposition that this is the case is strengthened by the frequent appearance of well-defined cleavage cracks that correspond in direction to the basal cleavage of plagioclase. These larger apparently untwinned sections not infrequently also show a well-defined zonal structure, which structure may also be seen at times on the larger crystal grains that are cut so as to show the twinning. As is generally the case in zonal plagioclases the zonal banding is best seen on the brachypinacoid. The extinction angles observed indicate a very basic plagioclase, namely, anorthite. Measurements made on sections at right angles to the brachypinacoid—that is, on sections that give equal extinction angles to the right and to the left of the trace of the twinning plane—gave maximum angles varying between 30° and 36° (the last-named angle was observed in No. 152). These measurements were made on crystals of different sizes, not only on the largest ones. In cases where the individuals were too small to allow one to determine the direction of cutting the very large extinction angles commonly obtained also point to a very basic feldspar.

With the exception of an occasional apatite needle and a little black dust, these feldspars are free from inclosures. An exception to this statement may, perhaps, be taken in that where the dust is conspicuous there is not unlikely also a small amount of glass inclosing it. Part of this black dust appears to be opaque and may be considered to be magnetite, but other particles, when highly magnified, lose their apparent opacity as well as their blackness. These last are undoubtedly globulitic matter. Such dust-laden glass is quite similar to the small amount of glass base that at times may unmistakably be detected between the plagioclase laths of these and of other basalts of this region. In a few cases (155) a small number of somewhat larger and thicker plagioclase crystals are to be seen that may possibly be considered as phenocrysts and as belonging to an older generation, but they bear little resemblance to the more characteristic phenocrysts of the andesitic basalts described further on in these pages.

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