446 MINERALOGY 



of the constituent oxides ; but as a rule some flux, as calcium 

 chloride, must be added to lower the fusing point to a tempera- 

 ture at which the formation of garnets is possible. 



OLIVINE GROUP 



The olivine group is composed of isomorphous orthosilicates of 

 the general formula R // 2 SiO 4 , in which R" is Ca, Mg, Fe", Mn, Zn, 

 or mixtures of these metals ; they are of orthorhombic symmetry 

 and members of the didigonal equatorial type, crystallizing usually 

 in combinations of the three pinacoids with the unit pyramid and 

 a dome, or pyramidal in habit; at times tabular, parallel to b 

 more often than to a. They have two well-developed cleavages 

 at a right angle. 



The following table will serve to show their relations both opti- 

 cally and chemically. 



OLIVINE 



Olivine. Chrysolite ; Magnesium iron orthosilicate ; MgFe- 

 Si0 4 ; MgO = 49.19, FeO = 10.54, SiO 2 = 39.85 ; a : b : c = .4656 : 

 1: .5865; 110 A 100 = 24 58'; 001 A 101 = 51 33'; 001 A 011 = 

 30 24' ; Common forms, a (100), b (010) ; c (001), m (110). s (120), 

 d (101), e (111), k (021); Twinning plane, Oil rare; Cleavage, 

 b distinct, a less so ; H. = 6.5-7 ; G. = 3.27-3.37 ; Color, shades 

 of green to red or brown ; Streak, white or pale ; Luster, vitreous ; 

 Transparent to opaque; a = 1.653; p = 1.670; y = 1.689; 

 y-a = .036 ; Optically () ; Axial plane = 001 ; Bx a = a or b ; 

 2 V = 86 89'. 



B.B. Dark-colored specimens fuse to a magnetic slag, while 

 the light-colored specimens whiten and fuse with difficulty. Gelat- 

 inizes with HCl. With the fluxes reacts for iron. 



General description. Crystals are usually small and nearly 

 equidimensional or tabular, parallel to a or b ; more often granu- 

 lar, friable masses, in which form it is often found in large rock 

 areas, as the dunites of Georgia, North and South Carolinas, where 

 it is associated with corundum. Chrysolite and peridote are 

 names often applied to olivine, but more particularly to the clear 

 transparent varieties, which are used as gem stones. Peridote is 

 leaf-green in color, and for a long time was gathered along the shore 

 of the Red Sea, where the water-worn pebbles were thrown up by 



