THE MINERALS OF IDAHO 341 



TOURMALINE (426) 



Complex borosilicate, with the general Rhombohedral, 



formula: 12Si0 2 .3B 2 3 .9(Al 2 3 .3RO) Hemimorphic. 



.3H 2 0. 



Tourmaline is in reality a group of minerals represented by the 

 above generalized formula and varying greatly in composition by 

 variation between alumina and the bases represented by RO in the 

 above formula and also in the character of the latter which may be 

 ferrous iron, manganese, lime, magnesia, potash, soda, lithia, etc., 

 giving rise to several varieties which have received separate names. 

 With the single exception of the Boise County occurrence, the 

 tourmaline thus far found in Idaho localities is all of the black iron- 

 bearing variety which passes under the varietal name schorl or 

 schorlite. This variety has no commercial use or value. The occur- 

 rences are described in detail as far as known in the following: 



BLAINE COUNTY 



Ordinary iron tourmaline in coarse imperfect black crystals was 

 found in the pegmatitic rock occurring in blocks in the talus at the 

 head of the cirque on the south flank of Mount Hyndman, by the 

 writer while assisting Prof. L. G. Westgate in the mapping of the areal 

 geology of the Hailey quadrangle for the United States Geological 

 Survey. This tourmaline presented no unusual features. 



A specimen sent to the National Museum for identification by 

 B. A. Smith, of Martin, Idaho, consists almost entirely of tourmaline. 

 The specimen is from a claim belonging to Mr. Smith in the Lava 

 Creek district 20 miles west of Arco. The tourmaline forms radial 

 fibers aggregated into spherulitic masses up to 5 mm. across and is 

 greenish to brownish-black in the hand specimen with a silky luster. 

 A little quartz is associated with the tourmaline in making up the 

 rock. Under the microscope the mineral appears as slender prisms 

 of negative elongation and parallel extinction which are intensely 

 pleochroic in colorless to pale violet-brown parallel to the elongation 

 and deep blue-gray to brownish-blue across the elongation. The 

 refractive indices are e= 1.647, to = 1.671. 



BOISE COUNTY 



A specimen of a bluish gray mineral sent to the United States 

 Geological Survey for identification from the Boise Basin has been 

 identified as tourmaline by Dr. Waldemar T. Schaller. The mineral 

 is in flat blades, which have the optical properties of the elbaite 

 variety of tourmaline. It is easily fusible before the blowpipe and 

 gives the characteristic boron flame. The tourmaline blades are 

 associated with and contained in a rusty limonite stained sericite 

 such as is common accompanying veins in the Boise Basin region. 



