282 UNIVERSITY OF VIRGINIA PUBLICATIONS 



are listed by Dana as probably altered zircon, and 10 analyses of these 

 several altered forms of the mineral are quoted which show the percentage 

 of water to range* from 0.95 to 9.53. Clarkef also states that the only 

 alteration of zircon that has been described is that of hydration, producing 

 hydrous zircon (malacon), which is tetragonal in crystal form and has a 

 specific gravity of 3.905. According to Van HiseJ the change from hydra- 

 tion involves an increase in volume of 24.05 per cent, and the reaction is 

 written by him as follows: 



3ZrSi04 + H2O = HaZraSiaOis. 



Kemp§ remarks that the zircons in coarse pegmatites near Port Henry, 

 New York, "have in instances undergone alteration to a greenish outside 

 crust, much more brittle than the fresh mineral." 



In 1898 HussakU concluded from his study of the zircon oxide (not 

 baddeleyite) favas° composed of almost pure zirconia (over 97 per cent) 

 and found in large quantities in the gravels of the augite syenite region 

 of the Serra de Caldas Minas Geraes, Brazil, were of secondary origin, 

 derived probably from zircon by loss of silica. After describing the micro- 

 structure of the f avas Hussak says : 



The whole structure shows that these favas are a secondary product of decomposi- 

 tion, and so far as concerns the occurrence of the great zircon crystals, it is most prob- 

 able that the mother mineral was zircon which through the loss of SiOo had been con- 

 verted into pure zirconia, as is partly the case in malaeonitization (Malaconitisirung). 

 At least, this is a more probable hypothesis than to assume the presence of the very 

 rare silico-zirconiates (Silicozirkoniaten) in the augite syenite mass found on the 

 spot. 



As the water content of the zircon favas is very small, they cannot be designated 

 as hydrated (hydratisirte) zirconia, but the product of metamorphosis may at first 

 coincide with the baddeleyite. 



Similar zircon favas have been reported from the diamond sands of 

 different localities in Brazil. 



Later Hussak and Reitinger® from a very careful chemical investiga- 



* Dana, E. S., Ov- cil., p. 487. 



t Clarke, F. W., The Constitution Of the Silicates, Bull. 135, U. S. Geol. Survey, 

 1895, p. 75. 



t Van Hise, C. R., A Treatise on Metamorphism, Mon. 47, U. S. Geol. Survey, 

 1904, p. 315. 



§ Kemp, J. F., Traris. Amer. Inst. Min. Engrs., 1898, vol. 27, p. 200. 



U Hussak, E.^ Tschermak's Min. u. Petr. Miltheil., 1898-99, vol. xviii, pp. 339-341. 



° A Brazilian word meaning bean-shaped pebbles. 

 ® Hussak, E. and Reitinger, J., Zeitschr.f. Kryst. u. Min., vol. xxxvii, pp. 566-574. 



