The Geology and Petrography of Crater Lake National Park, 1902
The hypersthene phenocrysts usually occur in well-developed crystals that appear in short to long prismatic habit, seldom in very slender prismatic habit. The forms nearly always present are the brachypinacoids and macropinacoids (100) (010), and the prism (110). The pinacoids are equally developed, and nearly always are more prominent than the prism. This gives to the cross section either a squarish form with truncated corners or an octagonal form. The terminal faces are not so easily determined, but they appear to be a flat pyramid or equally flat domes. In some cases this mineral may occur in more or less irregular grains. This is not apt to be the case when it occurs isolated, but it occurs in this manner when it forms nests, either alone or with augite and magnetite (2). Usually the hypersthene is older than augite, but at times the two appear to have been formed simultaneously.
In not very thin sections hypersthene appears strongly pleochroic, and is then distinctly greenish parallel to the vertical axis and reddish or brownish red at right angles to this axis. These rocks are so fine grained that they require rather thin sections, so that the hypersthenes show much paler colors; but even in the very thin nest sections one can recognize the distinctive pleochroism, as rays vibrating parallel to the vertical axis are always greenish, while those vibrating at right angles to the vertical axis are faintly reddish or yellowish. The three elasticity axes agree with the three crystal axes, as follows: a=a, b=b, c=c. In convergent light sections cut at right angles to the vertical axis give a positive bisectrix and large optical angle, while sections cut parallel to the macropinacoid give a negative bisectrix and a much smaller optical angle. In longitudinal sections the parallel cleavage lines are usually well developed, as is also a cross fracture, but the customary prismatic and pinacoidal cleavages are not often sharply defined in horizontal sections unless the section is unusually thin or the crystals larger than common.
Mineral inclosures in hypersthene are confined to magnetite and apatite, the former being very common and the latter rare. Glass inclusions, however, are very common and characteristic. They are almost always to be seen in the larger and sometimes also in the very smallest crystals, and vary in number from one to twenty or more in case the crystal is unusually large. They are usually roundish or oval, but may be irregular in shape, or they may have polygonal forms resembling that of the crystal. They are usually colorless or have a light-brown shade, but never the deep brown to be seen in many of the feldspars. The color of these glass inclusions does not seem to depend on the color of the glass base nor even upon the presence of such base. Nearly always each of these inclosures may be seen to contain a gas bubble, the size of which does not bear any relationship to the size of the inclosing glass. However, one can often find glass inclosures without gas bubbles in the same crystal in which most of the glass inclusions contain bubbles. The size of such inclusions varies from about 0.01 millimeter to very minute microscopic dimensions. In fig. C of Pl. XV is presented a hypersthene crystal with such inclosures of glass and bubbles.
Hypersthene is almost invariably perfectly fresh. It may at times show a slight trace of serpentinization through the development of a yellowish, fibrous, polarizing substance.
As stated above, hypersthene is to be ranked as one of the oldest crystallizations, but although it occasionally occurs inclosed in the plagioclase phenocrysts it is not always older than this mineral, as the plagioclase sometimes impresses its form on the hypersthene.
In addition to phenocrysts, not a few of the Crater Lake andesites also contain very much smaller hypersthene crystals with a slender lath-like habit. It is not always clear whether these are to be considered as belonging to a later generation and as forming a part of the groundmass or not. In a few cases, at least, of decided glassy varieties (9, 13, 16), this conclusion seems to be justified. In such rocks the groundmass consists of a brown glass containing magnetite grains, a few feldspar microlites, and abundant slender, almost microlitic prisms of augite and hypersthene. The hypersthene is quite uniformly about 0.05 millimeter in length and about one-fifth to one-tenth of that amount in width. The augites are sometimes the same size, but usually not more than half as large. These hypersthenes of the groundmass are sharp, and they frequently show flat terminations. They contain magnetite and also glass inclusions with bubbles. They are mostly very distinct from the hypersthene of the first generation in size and habit and are very much more abundant.