330 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



Its occurrence in this association is of interest as it forms a link 

 connecting such lime-silicate contact phenomena with the formation 

 of zeolite-bearing veins in which prehnite is commonly found else- 

 where. A single specimen from Kootenai County contains prehnite 

 associated with zeolites. 



CUSTER COUNTY 



J. B. Umpleby reports prehnite as occurring in the ore of the Phi 

 Kappa mine in Phi Kappa Canyon, tributary to upper Lost Kiver 

 in the Hailey quadrangle. 56 He also found it in specimens from the 

 Basin group on the mountain nearly due north of and across Park 

 Creek from the point where the Ketchum-Mackay road crosses the 

 Trail Creek divide. Specimens from the latter locality have been 

 examined in detail. The prehnite occurs in a rock largely composed 

 of brown garnet as the filling of small cavities up to 3 cm. in diameter, 

 which are lined with transparent green prismatic epidote crystals 

 up to 5 mm. long, the prehnite evidently being the youngest mineral 

 of the aggregate. Occasional isolated grains of sphalerite and galena 

 occur in the prehnite. 



The prehnite is colorless to white and transparent to translucent in 

 the hand specimen. It does not form free crystals but is in bladed 

 cleavable masses. Close inspection is necessary to detect the mineral 

 as the cleavage is not conspicuous and the prehnite closely resembles 

 quartz to the naked eye. Under the microscope in powder the 

 mineral is colorless and transparent and is biaxial positive with 2V 

 medium and moderate dispersion, r>v. It exhibits the confused 

 twinned structure and anomalous optical behaviour characteristic 

 for the species. Sections perpendicular to an optic axis show no 

 extinction but abnormal interference colors. The refractive indices, 

 though somewhat variable, average a = 1.626, j8 = 1.629, 7 = 1.652, 

 all ±.003; Birefringence, 7- a = 0.026. 



A sample, purified by means of heavy solutions, was analyzed with 

 the following results : 



Analysis of prehnite, Basin prospect 



(E. V. Shannon, analyst) 



Per cent 



Silica (SiO a ) 42. 26 



Alumina (A1 2 3 ) 24. 65 



Ferric oxide (Fe 2 3 ) 2. 10 



Lime (CaO) 26. 86 



Magnesia (MgO) 1 . 04 



Barium oxide (BaO) None. 



Water (H 2 0) above 110° C 4. 72 



Water (H 2 0) below 110° C None. 



Total 100.63 



The composition shown by the analysis is that of normal prehnite. 



« J. B. Umpleby. Personal letter 1923, to be published in report by the U. S. Geological Survey on the 

 Hailey quadrangle. 



