Hess and Wells — Occurrence of Struverite. 433 



Occurrence. (F. L. H.) 



The mineral is found in considerable abundance as an orig- 

 inal constituent of the granite pegmatite dike on which the 

 Etta claim is located, one and a half miles south of Keystone, 

 in the Black Hills of South Dakota. While on a reconnais- 

 sance trip for the United States Geological Survey in Septem- 

 ber, 1908, I visited the claim and collected specimens. 



Since the early 1880's the Etta dike has been famous as a 

 storehouse of rare minerals. The claim which is located upon 

 it was first worked for mica, and while being thus operated 

 cassiterite was discovered. Out of the discovery grew the tin 

 excitement of the Black Hills lasting through the late 80's 

 and early 90's. Cassiterite did not prove to be in sufficient 

 quantity to pay for mining, and latterly the dike has been 

 worked for spodumene, which is used as an ore of lithium. 

 The Etta dike is in some ways one of the most remarkable 

 pegmatites known, and although it has been described in geo- 

 logical literature a number of times, some of its features merit 

 attention at this time. It has a roughly oval outline, and is 

 about 150 by 200 feet in horizontal dimensions. Some of its 

 component minerals are gigantic. The crystals of spodumene 

 are probably unequaled in size by any other known occurrence, 

 single crystals reaching 42 feet in length with a cross section 

 of approximately 3 by 6 feet. It is said that 37 tons of spodu- 

 mene were mined from one crystal. Cassiterite has been found 

 in masses weighing from 50 to 60 pounds each,* and irregular 

 aggregates of columbite weighing 600 pounds.f 



In parts of the dike are finer-grained masses predominantly 

 composed of honey-yellow muscovite and white feldspar, both 

 microcline and albite. The microcline shows some crystal 

 faces from one-half inch to several inches across, and is 

 partly flesh-colored. The albite is pure white and occurs in 

 thin plates which reach an inch or more in breadth. The 

 muscovite is in flakes ranging from minute scales to plates 

 three-fourths of an inch across. Through these masses are 

 mixed other minerals in greater or less profusion, — white beryl, 

 small spodumene crystals, cassiterite in small particles, quartz, 

 secondary opal, and struverite. 



Columbite occurs in considerable quantity, in most places as 

 individual crystals ranging from small ones up to those weigh- 

 ing several pounds, with some larger aggregates such as those 

 mentioned above. The crystals are tabular, in many speci- 

 mens from one-third to one-half longer than wide, though some 



* Blake, W. P. Tin ; Min. Ees. of the United States for 1883-4. Geol. 

 Surv. Washington, 1895, p. 607. 

 f Personal communication, A. M. Lane, Keystone, S. D. 



