Ford and Bradley — Study of a Ldbradorite. 151 



Aet. XIII. — Chemical and Optical Study of a Ldbradorite; 

 by W. E. FoKD and W. M. Bkadley. 



Through the courtesy of Dr. George F. Kunz the Brush 

 Mineral Collection received some time ago a suite of water- 

 worn pebbles of a perfectly clear and almost colorless mineral 

 wliich on investigation proved to be a labradorite. The 

 locality from which the specimen came is given by a corre- 

 spondent of Dr. Kunz as the Altai Mountains, in Mexico, just 

 across the international boundary from New Mexico. The 

 specimens offered ideal material for chemical and optical study, 

 and as plagioclase feldspars of this composition are nut very 

 frequent in occurrence, it was thought that a brief description 

 of the mineral would be of interest. 



The analysis by Bradley gave the following results : 



I II Average Molecular Eatios 



SiO, 51-27 51-21 51-24 -8483 



AljOg 30-89 30-79 30-84 -3017 



CaO 13-52 13-65 13-59 -2422 



Na„0 3-74 3-78 3-76 -0605 



K,0 0-18 0-16 0-17 -0018 ' ^®^^ 



Fe.Og 0-71 0-75 0-73 



Ign .- 0-24 0-15 0-24 



100-55 100-59 100-57 



Molecular Albite Anortliite 



Eatios Molecule Molecule 



SiO, -8483 -3738 (or 6-00) -4745 (or 2-00) 



Al.Og -3017 -0623 (or 1-00) -2348 (or 1-009) 



Na,0 -j- K^O -0623 -0623 (or 1-00) 



CaO -2422 -2422 (or 1-021) 



Ratio of albite molecule to anorthite molecule, 1: 1-918. 



The small amount of water is considered to be hydroscopic 

 and the 0-7 per cent of ferric oxide is assumed to be due to 

 small inclusions of hematite as explained in a later paragraph. 

 On calculation the analysis gives, as is shown above, the ratio 

 of the albite molecule to that of anorthite as 1 : 1-918. This 

 places the mineral in about the middle of the labradorite 

 series. 



The specific gravity of the material analyzed was found to 

 be 2*718. The theoretical value for the specific gravity of a 

 mixture corresponding to Ab,An,gj8 obtained by using the 

 values of the specific gravities for the end members of the 



