Hornblende

This hornblende is from Lake Poway Park in Southern California. This hornblende is dramatically pleochroic. If a single linear polarizing filter is oriented with the vibration direction at right angles to the long axis of the grain then the grain is nearly colorless. If the polarizer in oriented with the vibration direction paralled to the long axis, as is the case here, the grain is very dark green.

Transmitted Single Linear Polarized Light Oriented North-South

Definition/Function:

Hornblende is an amphibole series with the general chemical composition indicated as
Na_0-1Ca₂(Mg,Fe_3-5Al_2-0) [(Si_6-7Al_2-1)O₂₂](OH,F)₂. It is a very common mineral in plutonic igneous and metamorphic rocks and sediments from those rocks.

Significance in the Environment:

Characteristic Features:

Hornblendes belongs to the monoclinic crystal system and are optically positive. Its refractive indices are 1.615-1.705, 1.618-1.714, and 1.632-1.730. Its birefringence ranges from 0.014 to 0.028. The density of hornblendes range from about 3.0 to 3.5 grams/cc. Small grains are brown to green with the density of the color varying with iron content. Hornblende is pleochroic with alpha:light, beta:darker, gamma:darker to much darker.

Associated Particles:

References:

Deer, W. A., R. A. Howie, and J. Zussman, AN INTRODCUTION TO THE ROCK-FORMING MINERALS, ISBN 0-582-30094-0, pp. 248-58, 1992.