Silicate of magnesium and iron, consisting of mixtures in variable proportions of the two silicate end-members; Fe2SiO., (fayalite) and lvlg2SiO., tforsterite). which are hardly ever found pure.

Crystal system Orthorhomblc.

Appearance In iron- and magnesium-rich intrusive rocks it usually occurs as anhedral crystals: but in effusive rocks. such as basalts, it more often has a prismatic appearance. The color is a yellowish (olive) green, a stronger. almost bottle green, or pale yellow tinged with green. It has unexceptional, vitreous luster, increasing its resemblance to olive oil.

Physical properties Olivine has a hardness of 6.5-7. The density varies a great deal according to the composition of the specimen: in gem quality stones it is about 3.32-3.35 g/cma. although the mineral itself ranges from 3.25 to 4.35 g/cm3. It has strong birefringence with indices of about nu 1.654, ny 1.690. It has indistinct prismatic cleavage and. often. conchoidal fracture.

Genesis Very widely distributed in iron- and magnesium rich igneous rocks, it is found as large crystals on the island of Zebirget in the Red Sea. and in basalt in the United States and in certain lavas on the Hawaiian islands. It also occurs in Burma and is known in the volcanic bombs of Vesuvius. Italy and the Eifel region of Germany.