. Bulletin of the Department of Geology of the University of California. 186 University of California. [Vol. i. shown in Plate 11, Fig. 2. More rarely they are united on a face oi the prism, as in Fig. 2. In basal sections the prismatic cleavage is very clearly marked, giving the rhomb-shaped network so char- acteristic for amphibole. Twinning has not been Figure 2.—Showing the forms of cross sections and grouping of prisms of the blue amphibole. In c the center of the prism is occupied by albite. Optical Characters.—The optical investigation of the amphibole is rendered difficult b
. Bulletin of the Department of Geology of the University of California. 186 University of California. [Vol. i. shown in Plate 11, Fig. 2. More rarely they are united on a face oi the prism, as in Fig. 2. In basal sections the prismatic cleavage is very clearly marked, giving the rhomb-shaped network so char- acteristic for amphibole. Twinning has not been Figure 2.—Showing the forms of cross sections and grouping of prisms of the blue amphibole. In c the center of the prism is occupied by albite. Optical Characters.—The optical investigation of the amphibole is rendered difficult by the intensity of the pleochroism in all sections. The plane of symmetry is the plane of the optic axes, and an axis emerges very excentrically in orthopinacoidal sections. The posi- tion of the acute bisectrix, and, consequently, the optical sign of the mineral, was not determined. Repeated measurements of the extinction in sections cut parallel, or approximately parallel, to the clinopinacoicl were made with the Bertrand ocular, and the maxi- mum values thus obtained were as follows:— I3°3. 13°, I2°5> I2°-2- I2°- 11 °-5; Il0» Io°-9- We may thus consider the direction of extinction as making an angle of about 130 with the chief crystallographic axis. The axis of elasticity corresponding to this direction of extinction was deter- mined by means of the quarter-undulation mica plate as the axis of maximum elasticity, tt. This orientation of the elasticity axes appears to be the most important optical character for the deter- mination of this mineral. It places it in close relation to riebeckite, which alone of all the amphiboles possesses this optical orientation. Although the observation could not be made in this mineral, it. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original University of Cali
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