Sand, Tourmaline Beach, San Diego, CA

There are three tourmaline grains in this image. The two at the lower left are darker than the one in the upper center of the field of view because they are higher in iron content. With darkfield dispersion staining the light blue color of the upper center grain is the omega direction color and indicates an omega refractive index of about 1.65. The two grains on the lower left show a similar blue color but they are oriented so that the epsilon dispersion color is showing, 1.65 is near the epsilon refractive index.

Transmitted Linear Polarized Light, Darkfield Dispersion Staining Illumination Mounted in 1.662 Refractive Index Medium

Definition/Function:

Tourmaline has the general formula of (Na,Ca)(Mg,Fe,Mn,Li,Al)₃(Al,Mg,Fe)₆[Si₆O₁₈](BO₃)₃(O,OH)₃(OH,F). It is a common mineral in some granites or metamorphic deposits.

Sand is a large accumulation of small rock and/or mineral particles in the size range of from 20 micrometers to 2,000 micrometers in diameter. Sand may include small shells or shell fragments and oolitic deposits. Sand is a size and density segregated material that is the result of air or water action.

Significance in the Environment:

This is an example of a dense sand where wave and current action has removed most of the lighter minerals, such as quartz and feldspars, from the beach. (Click here for more images of Ocean Sands) (Click here for more images of Sand in general)

Characteristic Features:

Tourmaline belongs to the trigonal crystal system and is optically negative. Its refractive indices are 1.612-1.650 for epsilon and 1.633-1.671 for Omega. Its birefringence ranges from 0.017 to 0.035. The density of tourmaline ranges from about 2.9 to 3.22 grams/cc. Small grains may be colorless to black. Their pleochroism is strong if they are strongly colored. The absorption is always strongest along omega, the highest refractive index. The long axis of the grains tends to be the epsilon direction.

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. 130-7, 1992.