THE MINERALS OF IDAHO 



Measurements of olivine from Minidoka 



311 



SCAPOLITE GROUP (386) 



Calcium-aluminium silicates, etc., variable. Tetragonal. 



The scapolite group includes a number of silicates of varying com- 

 position. These are commonly regarded as consisting of isomorphous 

 end members, marialite and meionite, having the following com- 

 positions : 



Meionite 4Ca0.3Al 2 03.6Si02.(C0 2 ,S0 3 ). 



Marialite 3Na 2 0.3Al 2 3 . 18Si0 2 .2NaCl. 



Several intermediate members of the series have received names 

 as listed below. While the optical properties naturally vary some- 

 what, with the variation in proportions between the C0 2 and S0 3 in 

 the meionite molecule, the various intermediate isomorphous mixtures 

 of the two end members may be identified by the optical properties 

 which are as follows: 



The composition of scapolites from Idaho has not been determined 

 by actual analysis and the classifications given below are based upon 

 optical determinations. 



ADAMS COUNTY 



Scapolite, probably mizzonite (dipyre), occurs in the metamorphic 

 deposits adjacent to granodiorite contacts in the Seven Devils district, 

 and has been especially noted at the Helena mine. It appears to be 

 for the most part confined to the impure shaly portions of the 

 limestone. 34 



34 D. C. Livingston and F. B. Laney. Idaho Bur. Mines and Geol., Bull. 1, p. 63, 1920. 



