96 Hyersthene

The Geology and Petrography of Crater Lake National Park, 1902

 PART II.

BASALTS.

INTERSTITIAL BASALTS.

MINERAL COMPONENTS.

HYERSTHENE.

Hypersthene is a characteristic but not always abundant constituent of these basalts. It is very fluctuating in amount. Although it may be more abundant than augite, as above noted (152), it is generally not so abundantly represented, and in one case appears to be entirely absent (161). It occurs almost invariably in small, fairly well-defined prisms that are two or three times as long as wide. In the more distinctly holocrystalline varieties this mineral is almost granular, the prismatic habit being very roughly developed. On the other hand, where the structure is less crystalline and the glass base distinctly recognizable, the characteristic habit of hypersthene can readily be made out. In such cases the forms are brachypinacoid and macropinacoid with subordinate prism, terminated by a flat pyramid.

In color and pleochroism they exactly resemble the hypersthenes of the andesites. In the very thin sections, however, necessary for the study of these rather fine-grained rocks, it is not easy always to distinguish between augite and hypersthene. In such cases the general form, parallel extinction, and lower double refraction are usually sufficient. In the finer grained specimens the hypersthene does not differ greatly from the plagioclase laths in size, being perhaps, on the average, somewhat smaller; in the coarser rocks they are apt to be decidedly smaller than the feldspar laths. They are nearly free from inclosures, magnetite octahedrons and an occasional apatite prism being the only observed exceptions. As stated above, they are older than the augite grains, and seem also in most cases to be older than the plagioclase.

Parallelism of growth between augite and hypersthene is a common occurrence. This is more noticeable in the basalts that are poor in augite. In such parallel growths the augite invariably appears as a slender strip on each side of a prismatic hypersthene prism, the two strips on the opposite sides of the crystal extinguishing together. Very rarely may augite be seen on the end of a hypersthene crystal. It is worthy of note that this parallelism of growth between these two pyroxenes is not universal, that is, not all of the hypersthenes in any one thin section are thus bordered by augite.

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