BENJAMINITE^ A NEW MINERAL SHANNON 



green to white or pale brown and much of it has a more or less pearly- 

 luster. It is probably all of the margarodite variety of muscovite, 

 low in potash and correspondingly high in basic hydrogen. Opti- 

 cally it is negative with the acute bisectrix perpendicular to the plates, 

 2E estimated at 60° to 70°, a = 1.562, ^= 1.597, 7= 1.602, r>v weak. 

 Other specimens from the mine consist entirely of masses of scales 

 of mica, some of which contain scattered crystals of fluorite an rare 

 masses of benjaminite. 



Pyi-ite occurs as cubic crystals which are rare in intimate associa- 

 tion with the benjaminite. Other quartz specimens which contain 

 no benjaminite show cubic crystals up to 3 cm. in diameter isolated 

 in the quartz which also contains 

 mica. The pyrite crystals are 

 greatly shattered and show dis- 

 tinct cubic cleavage. 



Molybdenite occurs as a graph- 

 itic slickensided smear on frac- 

 tures and also as foliated scales 

 and small rosettes of scales. It 

 is occasionally interleaved with 

 mica. Some of the rosettes reach 

 a diameter of 5 mm. Occasion- 

 ally these have a sharp hexagonal 

 outline and are made up of six 

 triangular sectors with a twinning 

 line down the center and stria- 

 tion on either side like the A 

 structure of mica. As seen in 

 polished sections the molybde- 

 nite is older than the benjamin- 

 ite and its folia have been con- 

 torted, separated, and bent by 

 the later fractures, although the 

 cracks do not cross the folia but 

 pass around their ends. The relations of the molybdenite to the 

 other minerals are shown in the drawings (figs. 2 and 3) . 



Fluorite occurs especially as isolated crystals in the granular 

 masses of mica where it forms crystals up to 4 cm. in maximum 

 size. The color varies from deep purple to purplish red, yellow and 

 colorless. In the mica fluorite occurred in the same specimen with 

 benjaminite but only one specimen shows fluorite in quartz and this 

 does not contain any benjaminite. The fluorite forms a skeleton 

 cube apparently developing by metasomatic replacement of the 

 quartz. 



Chalcopyrite occurs only as minute, almost microscopic, grains 

 and was introduced with the late quartz which fills fine cracks in 



\,'/i'i'/'>,f\ ln.i4bde^.t. 



^enjaTninift. 



ChoUopifHta 



Fig 3.— Sketch of Polished Surface Show- 

 ing Relations of Benjaminite, Molyb- 

 denite, Chalcopyrite and Quartz. 



