378 PROCEEDINGS OF THE NATIONAL MUSEUM. voi . 54. 



or a sulfate? Dana^ placed it in a "concluding division" of sili- 

 cates. Penfield and Pratt ^ not only accepted its interpretation as 

 a silicate, but even wrote the following constitutional formula with 

 silicon as linking element: 



HO— Ca— Ov .0— C— O— Ca— OH 

 Si 

 HO-^ \0— S— O— Ca— OH 



o o 



Now that it has been discovered that sulfates played an im- 

 portant role in the zeolite deposits of the Watchung Mountain 

 region, which yield far more thaumasite than all other localities put 

 together, the view suggests itself that this mineral is a sulfate, a 

 derivative of anhydrite, as expressed in the structural formula 

 below : 



O 



,0. .0— Ca— O— Si— OH 

 Ca S 



^0-^1^0— Ca—O—C —OH 



This formula agrees with the following facts: 



1. Thaumasite is derived, in the Watchung Mountain region, by 

 the action on anhydrite (CaS04), (or on the calcium sulfate por- 

 tion of glauberite (CaS04-[-Na2S04) ), of solutions capable of de- 

 positing calcium carbonate (calcite) and silicates (zeolites). 



2. It contains 15 molecules of water, but, as has been shown by Dr. 

 H. E. Merwin,^ 14 of these go off as the temperature is increased 

 without a break in the dehydration curve, and must be regarded as 

 " water of crystallization ; " the last one is driven off only at red heat. 

 The formula given shows that two hydroxyl (OH) groups are pres- 

 ent, joined to different elements, which accounts for the high tem- 

 perature needed to cause tliem to unite and liberate water (HgO). 



It is, of course, not to be inferred that in the crystalline mineral 

 the atoms are actually arranged in the manner indicated, for from 

 I'ecent work in crystallography, especially the application of X-rays 

 to the study of crystal structure, it is known that the atomic arrange- 

 ment in crystals is based on geometrical rather than chemical rela- 

 tionships. Such a structural formula means, therefore, merely that 



1 System of Mineralogy, ed. 6, 1892, p. 698. 

 " Amer. Journ. ScL, ser. 4, vol. 1, 1896, p. 229. 

 8Journ. Wasb. Acad. Sci., vol. 4, 1914, p. 49G. 



