442 N. L. Bowen — Cacoclasite from Quebec. 



avoiding concave faces, angular values for cacoclasite 

 were obtained that are believed to be fairly reliable. 

 The values are very close to those for sarcolite with 

 which they are compared below. The relationship of 

 cacoclasite to sarcolite in angles was noted by Lewis. 



Sarcolite. Cacoclasite. 

 o r— 51° 27' 51° 29' 



o e = 41° 35' 41° 46' 



a r=z56° 26' 55° 43' 



The great similarity in angles and the identity in 

 habit make it necessary to regard cacoclasite as a pseu- 

 domorph after sarcolite. Sarcolite has the composition 

 (Ca,Na 2 ) 3 Al 2 (Si0 4 ) 3 and in its typical occurrence Ca: 

 Na 2 = 9:1 nearly. The change from sarcolite to grossu- 

 larite Ca 3 Al 2 (Si0 4 ) 3 is, therefore, not a great one, indeed 

 it might be termed paramorphism without seriously 

 stretching the truth. A marked decrease of volume is 

 involved, however. Sarcolite of the composition of its 

 typical locality (G = 2-9 Eammelsberg) would give a 

 volume of grossularite (G = 3-5) approximately equal to 

 75 per cent of its own volume. It is worthy of note, 

 therefore, that the "best" cacoclasite of Genth contains 

 approximately 75 volume per cent grossularite. This 

 correspondence suggests that cacoclasite was formed by 

 the alteration of sarcolite to grossularite without change 

 of volume, the resulting voids being occupied by calcite 

 and apatite. 



Summary. 



Chemical, microscopic, and crystallographic evidence 

 all point to the fact that cacoclasite is a pseudomorph 

 (essentially a paramorph) of grossularite after sarcolite 

 with calcite and apatite filling the voids produced by the 

 reduction of volume involved in the change. 



Mineralogy Department, 

 Queen's University. 



