224: W. T. Schaller — Dumortierite. 



The ratios of the last analysis are : 



Si0 2 5-94 or 6X '99 



A1 2 3 8*09 8X1*01 



B () 3 -80 1 X -80 



H" 2 ]-22 IX 1*22 



Approximately they agree with the formula. 



The only question of which there seems to be any doubt is 

 whether the water should be 1 or 1|- molecule. As, however, 

 the best analyses show but 1 molecule and their evidence is 

 considered as of much more value than a large number of 

 inferior analyses, there is no question in the writer's mind but 

 that the formula for dumortierite is 



8Al 1 O i .lB 1 1 .lH i 0.6SiO s . 



This may be written (Si0 4 ) 3 Al(A10) 7 (BO)H, which is the same 

 as the formula given by Groth except that for (BO) he puts 

 (AlO). His formula is (Si0 4 ) 3 Al(A10) 8 H. 

 This may be written in the following form: 



Si0 4= (A10) 3 



/ 

 Al — Si0 4 ==(A10) 



\Si0 4 — AlO 

 ~^BO 



This formula is similar to that of andalusite, which is written 

 .Si0 4 = Al 

 Al — Si0 4 E=Al 

 \si0 4 = (A10) 3 . 



Dumortierite alters to muscovite. The change can be very 

 well shown by these formulae and seems to be entirely in accord- 

 ance with the group of minerals to which it is related. 



Dumortierite. Muscovite. 



/ Si0 4 = (AlO), / Si0 4 ==; Al 1 



Al — Si0 4 = ( A10) 3 = Al — Si0 4 = Al y + alumina and boric 

 \siO_A10 * \SKWK I acid set free. 



4 ^BO ^H 2 J 



^H 



The writer's thanks are due to Mr. John A. Thoman of San 

 Diego, the owner of the California dumortierite property, and 

 to Mr. K. C. Xaylor of San Diego, for many courtesies and 

 favors. 



