77 Spherulitic Dacite

The Geology and Petrography of Crater Lake National Park, 1902






A specimen of spherulitic dacite, No. 108, was collected on Llao Rock in 1883. The field label accompanying the specimen does not state the exact location. In the hand specimen this is seen to consist of a nearly black glass very thickly crowded with brownish spherulites that measure from 3 to 5 millimeters in diameter. So thickly crowded are these spherulites that they often interfere and make up much more than one-half of the mass. The spherulites consist of two, sometimes three parts. The center is of a dark-gray color, and has a very dense, felsitic texture. Around this is a ring or zone about a millimeter wide, of a less dense or even of minutely porous material, and of a brownish, reddish-brown, or light grayish-brown color. Outside of this again occurs usually, but not always, the third part, dense like the central portion, and either gray in color like the center or of a deeper brown than the intermediate zone. Many of the spherulites show gapping cracks that seem to be confined to the middle zone. There is, however, no trace of lithophysal development. The inside of these cavities is rough, and of about the same color as the middle zone of the spherulites in which they lie. They do not appear to contain crystals. Upon cross fracture these spherulites have a distinct radiated appearance. The customary phenocrysts occur quite indifferently in the glassy portion as well as in the spherulites.

Under the microscope the glassy groundmass is almost identically the same as that of the vitrophyric dacite described above. The same colorless augite microlites of about the same size, 0.03 to 0.05 millimeter long by 0.003 millimeter wide, may be seen. A very slight distinction may be noted in that these augite microlites are not perfectly clear, but often have a small amount of black, opaque, dusty matter—probably magnetite—either adhering to the outside or inclosed within. Curved, black microlites were not observed. Most of the spherulites show two or three periods of growth that correspond to the different colored zones noticeable in the hand specimen. One or two appear to have had but one period of growth—that corresponding to the inner portion of the others. This inner part has a dirty-brown or grayish-brown color, lets through but little light, and shows a distinctly fibrous radiating structure. The fibers are very fine, and hardly distinguishable from each other. They polarize light feebly, and have a positive extension. Owing to the partial opacity of this central portion, the usual black cross is hardly discernible.

The portion of the spherulites that belongs to the second or intermediate zone does not seem to be as porous as the rather rough appearance in the hand specimen would indicate. It is, in fact, mostly quite solid. It appears in a light-brown color, very much lighter than the central part, and is composed essentially of distinct shreds of a colorless mineral diverging from the center outward and branching at low angles. These shreds are coarse enough to show extinction angles often quite oblique to their longer axes. They have invariably a positive extension. In polarized light they appear to continue the finer fibers of the central portion. Between the arms of the branching positive shreds occurs frequently unindividualized matter that shows feebly negative polarization—considering this substance to be also fibrous; but all the more distinctly recognizable shreds are positive.

These coarse branching shreds so closely correspond to the feldspar of the spherulites in the obsidian of Obsidian Cliff in the Yellowstone National Park, as described by Iddings,a and to that of the spherulites from the region of Rosita and Silver Cliff, Custer County, Colo., as described by Cross,b that, after studying thin sections prepared from spherulites from the latter place, the writer has no hesitancy in pronouncing these in the Llao Rock dacite as belonging to feldspar also. In their positive character they correspond to the feldspar in many of the spherulites from Custer County, Colo. The brownish color of this intermediate portion is due to the presence of brown, yellowish, and reddish ferritic matter in the form of dust particles, and occasionally in the form of minute scales. A radial arrangement of this ferritic matter is not marked.

aSeventh Ann. Rept. U. S. Geol. Survey, 1888, pp. 276-278.
bConstitution and origin of spherulites in acid eruptive rocks: Bull. Philos. Soc. Washington, vol. XI, 1891 pp. 411-440.

The outer portion of these spherulites when seen in thin sections appears to be identical with the central core. It is often entirely missing, and when present does not usually envelop the whole spherulite, but appears as irregular lobes or prolongations of the same.

The strings of augite microlites that accentuate the fluidal structure of the glassy part of the rock pass uninterruptedly through these spherulites, and the phenocrysts lie embedded in them as well as in the remainder of the rock, as is universally the case in such bodies. Part of the ferritic matter may be seen to arise from the further oxidation or hydration of the ore particles that adhere to the augite microlites.

The phenocrysts observed in this rock are plagioclase hypersthene, hornblende and augite, with accessory apatite and magnetite. The order of crystallization is (1) magnetite and apatite, (2) plagioclase, (3) hypersthene, (4) augite and hornblende, (5) the spherulitic forms; to which may be added augite and feldspar microlites of the groundmass, which belong between 4 and 5. The hornblende is very sparingly developed, as is usual, but occurs in both the brownish-green and in the brownish-red varieties.

This spherulitic dacite should be compared with the very similar occurrences in the Cloud Cap flow.

in the thin section of this rock may also be seen a few small inclusions of identical nature with the inclusions of older secretions to be found in No. 102, described immediately below.

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