This sand appears green because of the olivine grains. Olivine has the general chemical composition of (Mg,Fe)2
It is a very common mineral in igneous rocks and sediments from those rocks. The magnesium end member is the mineral Forsterite and the iron
end member is Fayalite. As the amount of iron increases the names chrysolite, hyalosiderite, hortonolite, and ferrohortonolite are used.
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Significance in the Environment:
Olivines belong to the orthorhombic crystal system and may be optically positive or negative, the iron rich members being optically negative.
The refractive indices range from 1.635-1.827 for alpha, 1.651-1.869 for beta, and 1.670-1.879 for gamma. Its birefringence ranges from 0.035
to 0.052. The density of hornblendes range from about 3.22 to 4.39 grams/cc. The 2V ranges from 82 to 134 degrees. Small grains tend to be
colorless to green or yellow. Hornblende is pleochroic with alpha and gamma:pale yellow and beta:orange- yellow to riddish brown, depending on
Deer, W. A., R. A. Howie, and J. Zussman, AN INTRODCUTION TO THE ROCK-FORMING MINERALS, ISBN 0-582-30094-0, pp. 3-13, 1992.