32 S Penfield and Sperry — Mineralogical Notes. 



Our knowledge of the barium feldspars is confined to analy- 

 ses of the mineral from a few localities. For the sake of com- 

 parison we quote the following : 



I. Binnenthal, Switzerland, analysis by Stockar-Escher.* 

 II. Jakobsberg, Sweden, analysis by L. J. Igelstrom.f 

 III. Unknown locality, analysis by Pisani.J 



I. II. * Hi. 



Si0 2 52-67 



A1 2 3 21-12 



Fe 2 3 



CaO -46 



MgO -04 



BaO 15-05 



Na 2 2-14 



K 2 7-82 



Ign -58 



Ratio. 





Ratio. 





Ratio . 



878 



53-53 



•892 



55-10 



•917 



205 



23-33 



•226 



23-20 

 •45 



•225 

 •003 



•008") 







1-83 



•0331 



001 | 



3-23 



■081") 



•56 



•014 | 



098 y 



7-30 



•047 



730 



•048 y 



034 1 



083 J 





r 



7-45 



•120 | 



11-71 



1-25 J 



•83 



•009J 









3-72 





99-88 . 99-10 100-44 



The ratios in the three analyses are as follows : 



Si0 2 :Al 2 3 RO 

 I. -878: -205: 224=4: 0-93: 1-02 

 II. -892: -226: 253=4: 1-01 : 1-13 



III. -917: -225: 224=4: 0-98: 0-98 



All of these analyses have a ratio of Si0 2 : A1 2 3 : RO=4 : 1 : 1 

 and a formula HAl 2 Si 4 12 ; but one thing is very noticeable, 

 that in all, the RO is very variable. In I, which is that of the 

 purest hyalophane, the ratio of BaO + (Ca, Mg)0 : K 2 0-f-Na 2 = 

 1:1, and some have proposed that the feldspar is a mixture of 

 K 2 Al 2 Si 6 16 and a Ba feldspar like anorthite, BaAl 2 Si 2 8 , simi- 

 lar to Tschermak's theory for the plagioclase feldspars. If, 

 however, LI and III are to. be referred to hyalophane, we can- 

 not accept this last hypothesis, but must accept for the formula 

 of the species RAl 2 Si 4 12 , E=Ba, Ca, Mg, K 2 , Na 2 . 



Going back to our own analysis, it seems best to regard the 

 feldspar as a mixture of 35*23 per cent of albite, 51*15 per cent 

 of orthoclase and 13*17 per cent of hyalophane, BaAl 2 Si 4 12 . 

 The two latter are monoclinic, and have either grown together 

 as isomorphous mixtures or, if in any other way, the optical 

 properties are such that the relation to the two minerals can 

 not be told. It must be noted also that in making this suppo- 

 sition it is necessary to regard the hyalophane as a pure barium 

 feldspar ; we see, however, no other way to interpret our analy- 

 sis. If we regard our barium feldspar as united with potash 

 to form a hyalophane like that from the Binnenthal, then the 

 excess of potash cannot be united with A1 2 3 and Si0 2 to form 



* Kermgott, Uebersicht der Min. Forsch., 1856-57, p. 107. 

 f Bull. Soc. Min. de France, vi, 139. 

 % Bull. Soc. Min. de France, i, 84. 



