THE NONMETALLIC MINERALS. 



313 



9. GADOLINITE. 



This is a basic orthosilicate of yttrium, iron, and glucinum, though 

 with frequently varying- amounts of didymium, lanthanum, etc. The 

 formula as given by Dana is Gl 2 FeY 2 Si 2 O 10 , silica 23.9 per cent, 

 yttrium oxides 51.8 per cent, iron protoxide 14.3 per cent, and glu- 

 cina 10 per cent. Actual analyses yielded results as below: 



(I) Ytterby, near Stockholm, Sweden; (II) Llano County, Texas. 



The mineral is sometimes found in form of rough and coarse crystals, 

 but more commonly in amorphous, glassy forms. Hardness 6.5 to 7; 

 specific gravity 4 to 4.47. Color brown, black and greenish black, 

 usually translucent in thin splinters and of a grass green to olive green 

 color by transmitted light. No true cleavage; fracture conchoidal or 

 splintery like glass, and with a vitreous or somewhat greasy luster. 

 Through oxidation and hydration the mineral becomes opaque, brown, 

 and earthy. Hence masses are not infrequently found consisting of the 

 normal glassy gadolinite enveloped in a brown red crust of oxidation 

 products. (Specimen No. 62780, U.S.N.M.) On casual inspection 

 the mineral closely resembles samarskite and the dark, opaoue varie- 

 ties of orthite, but is easily distinguished from the fact that before the 

 blowpipe it glows brightly for a moment and then swells up, cracks 

 open, and becomes greenish without fusing. Some varieties (the nor- 

 mal anisotropic forms) swell up into cauliflower-like forms and fuse 

 to a whitish mass. Like orthite, it gives a jelly when the powdered 

 mineral is boiled in hydrochloric acid. 



Localities and mode of occurrence. The mineral occurs mainly in 

 coarse pegmatitic veins associated with allanite, and other allied 

 minerals. The principal locality in the United States thus far described 

 is some five miles south of Bluffton on the west bank of the Colorado 

 River, in Llano County, Texas (Specimen No. 62780, U.S.N.M). The 

 region is described * as occupied by Archaean rocks with granite, and 

 occasional cappings of limestone. 



'American Journal of Science, XXXVIII, 1889, p. 474. 



