442 MINERALOGY 



eral very similar in physical and crystalline properties, as well as 

 associations, but contains carbonates in its composition. 



In sections, nepheline when crystalline is either square or hex- 

 agonal in outline, colorless, with no relief ; cleavage is not marked 

 except when decomposition has begun. Inclusions are not charac- 

 teristic except that in some occurrences it may be filled with dust- 

 like inclusions of glass and gas bubbles, in concentric bands or 

 zonal. Interference color very low grays of the first order, and 

 extinction parallel to the cleavage or symmetrical. 



Nepheline is a primary mineral of many igneous rocks, especially 

 those rich in alkalies and low in silica. It seems to form in those 

 magmas in which the sodium is in excess of that required to pro- 

 duce feldspars, separating from the magma in most cases just 

 before the feldspars and directly after the sodalite group, with 

 which it is frequently associated, as in the lavas of Vesuvius. At 

 Litchfield, Connecticut, it is associated with cancrinite in an elseo- 

 lite syenite. In syenites the massive elseolite is characteristic, 

 lending its name to the group, elaeolite syenites. 



In weathering, zeolites, especially natrolite, result ; but the forma- 

 tion of sodalite by the addition of chlorine may be the first step 

 in this reaction. 



Artificially nephelite has been produced by a fusion of its con- 

 stituent oxides at a low temperature, from which crystals easily 

 separate. 



GARNETS 



Garnets are orthosilicates of the general formula R/'aR/'^CSi 

 in which R" may be Ca, Mg, Fe", Mn, and R'" = Al, Fe'", Cr : 

 Isometric; Type, Ditesseral Central; Common forms, d(110), 

 n(211), s (321) ; Twins rare; Cleavage, dodecahedral sometime 

 distinct; Brittle; Fracture, conchoidal ; H. = 6.5-7; G. = 3.15- 

 .4.3 ; Color, all colors ; Streak, white or pale ; Transparent 

 opaqus; Luster, vitreous to resinous; n = 1.7-1.8. 



B.B. Fuses at three, except uvarovite which fuses at six. 

 After fusion gelatinizes with HC1, otherwise not much affected by 

 acids. Those containing much iron become magnetic after fusion 

 in O. F. 



General description. Crystals are common and in some 

 instances very large, up to a foot in diameter; in habit either 

 rhombic dodecahedrons or tetragonal trisoctahedrons or combina- 



