55 Augite

The Geology and Petrography of Crater Lake National Park, 1902






The phenocrystic augites are, like the hypersthenes, usually sharply crystallized, but they are not so persistently idiomorphic. They not infrequently occur in hypidiomorphic or in granular forms. This, of course, is naturally the case where they form nests either with or without hypersthene. In color they show in thin sections a nearly uniform pale green, the depth of color varying with the thickness of the slide. They rarely show any appreciable pleochroism, and this property, together with the stronger interference colors and the oblique extinction, nearly always suffices to distinguish them from the hypersthenes. Twinning parallel to the orthopinacoid is very common. This is true not only of simple twins, but also of crystals with repeated or polysynthetic twinning.

The inclosures in augite are exactly the same—even to the glass inclusions with bubbles—as are to be found in hypersthene. The crystal form is also very closely analogous to that of the orthorhombic pyroxene. These are the prism with brachypinacoids and macropinacoids and a flat terminal form, presumably a pyramid. Usually, however, the prism and the pinacoids are about equally developed.

Parallel growths of augite and hypersthene are occasionally to be seen (11,171), in which the augite forms more or less irregular shells around the older hypersthene (see fig. E of Pl. XIV, p. 76); but this phenomenon is by no means as common as with the basalts of this region. A more detailed description will be found under the basalts.

As a groundmass constituent augite is very characteristic and is universally present. In proportion to the development of hypocrystalline groundmass, this mineral is to be seen in more and more granular form. In the great majority of specimens, however, this mineral assumes well-defined slender prismatic form. In the hyalopilitic varieties the form is very sharp indeed. The crystals measure two or three or even five times as long as wide. In exceptional cases the length may be proportionately greater still. Usually in sharply developed crystallites the width varies between 0.001 and 0.004 millimeter and the length between 0.01 and 0.03 millimeter. In the holocrystalline varieties the augites are not only granular in shape, but usually much larger. Both microlitic and granular individuals customarily inclose minute magnetite in octahedral and granular forms.

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