276 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



Cleavage plates lying on 010 show the acute bisectrix, and yield an 

 extinction angle of Z' Ac= 15° ± . 



LABRADORITE 



LIME-SODA FELDSPAR 



Sodium-calcium aluminium silicate, inter- 

 mediate between albite (NaAlSiaOs) and 



anorthite (CaAl 2 Si 2 8 ), ranging in pro- Triclinic. 



portions from AbiAm to AbiAn3. 



Labradorite is a common feldspar in intermediate igneous rocks 

 as diabases, basalts, and gabbros, and as a constituent of such rocks 

 it is known from many localities in Idaho. The only occurrence 

 noted which is of interest to the mineralogist is the following: 



FREMONT COUNTY 



The occurrence of andesine in the lava of Crystal Butte has been 

 described above. A similar black porphyritic igneous rock makes up 

 Mac's Butte in the same region, and a specimen from this Butte, also 

 collected by Mr. Stearns, contains abundant pale yellow transparent 

 tabular feldspars in a dense groundmass of streaked black and red 

 lava. While the feldspars of the specimens average only about 5 mm. 

 in length, an occasional costal reaches a length of 2.5 cm. The 

 largest are glassy, transparent, and pale yellow in color and show 

 fine twinning lines to the eye. Under the microscope, in powder, the 

 feldspar is colorless and transparent, and properly oriented grains 

 show albite twinning lamellae. It is biaxial, positive with 2E large, 

 dispersion pronounced, r>v. The indices of refraction measured are 

 a= 1.560, j8= 1.562, 7=1.565. These indices indicate the feldspar 

 to be a sodic labradorite of about the composition Ab 40 An 60 or 

 Ab:An = 2:3. This feldspar is entirely similar in appearance to the 

 andesine last described, and it is probable that study of a number of 

 specimens from the same series of lavas would show a gradation of 

 the plagioclases between these two examples. 



THE PYROXENE GROUP 

 Of the relatively large number of minerals included in the pyroxene 

 group only diopside, hedenbergite, and augite will be mentioned as 

 occurring in Idaho. Probably others occur, but they have not as 

 yet been given detailed mention in the literature nor have specimens 

 showing features of interest been available for description. The most 

 common pyroxenes in the State are those of the diopside-hedenbergite 

 series. The end members of this series are: 



Diopside Calcium-magnesium pyroxene CaO.Mg0.2Si0 2 



Hedenbergite Calcium-iron pyroxene CaO.Fe0.2SiOo 



