99 Hypersthene, Apatite and Pseudobrookite Crystals in Basalt

The Geology and Petrography of Crater Lake National Park, 1902





In a specimen of basalt of this type collected on the east base of Red Cone (156) occur numerous flattish cavities that measure from one-quarter to one-half inch in greatest diameter. These cavities are lined with minute crystals of four different kinds; first, hypersthene; second, pseudobrookite; third, apatite; fourth, unknown white mineral.

The crystals of hypersthene occur in very brilliant though minute tablets, transparent, and of a fine deep-brown color. When examined under the microscope these crystals give the properties of hypersthene as seen in section parallel to the brachypinacoid. Pleochroism is marked with greenish color in the direction of the vertical axis and a decided brown at right angles to this direction. The extinction is parallel and the vertical axis the direction of least elasticity. No optical image can be obtained on the large crystal face.

Owing to the extreme brilliancy of these hypersthene crystals and to the fact that they appeared to present interesting crystal forms, pieces of the rock containing these cavities were sent to Dr. C. Palache, of Harvard University for further identification and measurement. The writer is extremely indebted to Dr. Palache for working out the forms of these crystals as well as for the identification and measurement of the pseudobrookite crystals. The result of this painstaking work is given below. As will be seen by comparing his measurements the supposition that these brown, pleochroic tablets are hypersthene is abundantly proved.

The cavities also contain a very few black and not well-defined crystals which, under the microscope, appear to have a deep reddish-brown color and to show no pleochroism. Although the writer is not certain that these black crystals are the same as those identified by Dr. Palache as pseudobrookite, the probability is in favor of that supposition.

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