448 



MINERALOGY 



the waves after storms. It was also obtained from the Arabs of 

 Egypt, but the exact locality from whence it was gotten is unknown. 

 Gem material is found near Fort Defiance, Arizona. 



Chrysolite is a yellow variety resembling very touch the yellow 

 topaz when cut and polished. 



Olivine occurs as an essential constituent of many igneous rocks, 

 which are low in silica and rich in the alkali earth metals. Its 



composition is variable, 

 depending upon the 

 proportion of mag- 

 nesium, calcium, iron, 

 or manganese, present 

 in the magma. It is 

 found in such rocks as 

 peridotite, gabbros, 

 basalts, nephelites, and 

 leucites, and many 

 lavas, while dunite is 

 almost entirely olivine. 

 It appears less often in 

 andesites and trachytes. 

 In rock sections it is 

 colorless or pale, with 



crystalline outline, or more often granular or irregular. The two 

 cleavage directions are well marked, that parallel to b more so than 

 that parallel to a. Extinction parallel. The index of refraction 

 being high, the relief is marked, with all cracks distinct. Inter- 

 ference colors of the second and third order. The plane of the 

 optic axes is parallel to 001 and the optical character is plus, with 

 the acute bisectrix a, when the ferrous oxide is below 12 per 

 cent. ; when above 12 per cent., the acute bisectrix is b and the 

 optical character is minus. Inclusions are not characteristic, but 

 spinel, chromite, apatite, and hypersthene appear ; also glass and 

 slag in the lava occurrences. Pleochroism is marked only when 

 the iron content is high. 



In its alteration olivine readily forms serpentine, the alteration 

 following the fractures or cleavage cracks in the crystal, with the 

 serpentine fibers lying crosswise. The iron at the same time 

 separates as oxide and is deposited along the cracks, or where the 

 specimen is rich in iron as layers interlaminated with the serpen- 

 tine; carbonates, as magnesite and calcite, and also opal, quartz, 



FIG. 486. Olivine Crystals from France. 



