69 Feldspar

The Geology and Petrography of Crater Lake National Park, 1902





While phenocrysts of feldspar are fairly abundant in all these dacites, and in some specimens very numerous indeed, in no case could orthoclase be detected. Not only is it true that the feldspars turn out to be plagioclase, but plagioclase of a decided basic variety, usually labradorite. In form they do not appear to differ materially from the similar phenocrysts in the andesites. There are to be seen comparatively large, stout crystals with the basal pinacoid and brachypinacoid, the two half prisms, and an additional dome or pyramid; also crystals that are more nearly rectangular in cross section and usually smaller in size. The stout crystals with more elaborate forms are probably older than the others, at least they appear to have greater extinction angles and to belong to a more basic plagioclase. These crystals are usually not numerous enough in any one thin section to allow the accurate determination of the maximum extinction angles; the extinction angles given below, therefore, although they are the maximum observed, probably do not indicate feldspars as basic as those actually present. Sections cut perpendicular to the twinning plane, and therefore showing symmetrical extinctions, gave the following maximum extinction angles, viz: 30°. 101, 31°. 104, 30°. 197, and 33° in No. 103. All four of these specimens came from the dacite of Llao Rock. No. 101 is a vitrophyric dacite from the southern edge of the flow, and is almost identical with No. 102, of which a chemical analysis will be found on page 140. No. 104 is a spherulitic variety of a vitrophyric type, and Nos. 197 and 103 are approximately holocrystalline types. These measurements indicate a plagioclase at least as basic as labradorite, and probably more so. The above extinctions represent, however, not the whole crystal, but the inner part. The outer shell often gives much smaller angles. For instance, in No. 101 the margin gives an extinction angle of 24° is six degrees less than at the center, but even this is not too small an extinction for labradorite. The measurements given for the plagioclase phenocrysts of the Llao Rock flow do not differ materially from the observed extinction angles in the other dacite masses.

Zonal structure is very strongly developed in these larger plagioclase phenocrysts, and is particularly conspicuous in sections cut approximately parallel to the brachypinacoid. In such sections the zonal banding, as seen in polarized light, indicates that the crystals in the earlier stages of growth had simpler forms. For instance, in a section parallel to the brachypinacoid, showing externally traces of the basal pinacoid, prism, and two domes, the central core shows only the basal pinacoid and one of the domes almost at right angles to the first-named form. In such sections the zones of different extinctions usually shade gradually into each other, so that from the center outward the extinction angle becomes less and less oblique to the trace of the basal pinacoid. Almost always, however, there are to be seen one or more quite sharply defined shells with rather abrupt difference in extinction. Furthermore, this abruptness of change from one shell to the next is not infrequently accentuated by the fact that the extinctions do not change regularly from the center outward, but oscillate more or less. In other words, the plagioclase consists of concentric shells that alternate between less acid and more acid feldspars. As far as observed, the actual center is nearly always the most basic portion of the feldspar, but the shell immediately surrounding this center may be more acid than the next succeeding one. The alternation of more and less acid shells is not usually sharp enough to admit of positive measurement. In the two following cases the measurements were sharp enough to justify recording.

In No. 102 occurs a section of plagioclase that is cut approximately parallel to the brachypinacoid and that shows the alternation of zonal shells very clearly. This may be seen illustrated in fig. F of Pl. XIV (p. 76). The three most conspicuous zones are marked 1, 2, and 3, from the center outward. The crystal forms that could be identified by means of the cleavage cracks are the basal pinacoid (001) and the prism (110). Two other plagioclase crystals are grown into this one—one almost at right angles, to be seen on the left side of the figure; the other, in the upper left corner of the figure, appears almost to continue the outlines of the main crystal. These two crystals do not appear to be in twinning relationship to the other. The extinction angles, as measured to the trace of basal pinacoid, as well as the corresponding percentage of the anorthite molecule, are given below for the three zones:

1 = -12° per cent An = basic andesine.
2 = -21° per cent An = labradorite.
3 = -5° per cent An = basic oligoclase

The percentages of An are given as corresponding to extinction angles measured on a section exactly parallel to the brachypinacoid. That this is not quite true of this section is proved by the fact that the angle of equal illumination for the three zones is +29° the required angle of +34°. As these extinction angles do not indicate quite as basic a plagioclase as do the extinction angles on symmetrical sections, it is probable that a section cut exactly parallel to the brachypinacoid would give still larger extinction angles than are here indicated. But, at least, these measurements suffice to prove that in this case the inner portion of the crystal is not as basic as is the intermediate zone.

In No. 101 was seen another section of plagioclase, cut similarly to this one, and also showing similar extinctions. Like the example given above, 1 is the center and 4 the margin.

1 = -22° labradorite.
2 = -14° basic andesine.
3 = -16° basic andesine.
4 = -5° basic oligoclase.

A comparison of the refractive powers of the plagioclase phenocrysts in the rocks containing these two crystals shows that they have higher refractive power than has the adjacent Canada balsam, i. e., higher than 1.540. This would indicate that they are more basic than oligoclase, even at the edge of the crystal.

The plagioclase phenocrysts are frequently broken and the fragments scattered through the glassy groundmass. They also show frequent corrosion, but, unlike the andesitic plagioclases, they do not often contain abundant glass inclusions; at least, the crowding with glass inclusions and their distribution in an intermediate zone is not characteristic. An exception must be taken to this statement, however, in favor of the dark-colored secretions that receive special treatment further on in these pages. Inclusions of slender apatite needles are common; also zircon crystals in short colorless or slightly brownish prisms may be seen, but very sparingly.

The plagioclase that belongs more particularly to the groundmass seems to be oligoclase, on account of the very small extinction angles. A more detailed description of the groundmass feldspars will be found in connection with the description of the different dacite flows.

Orthoclase and quartz are also entirely confined to the groundmass and will be discussed later. Tridymite is not common. It occurs in the customary clusters, apparently filling small cavities (121) also to some extent in the fluidally arranged lithoidal dacites.

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