VESUVIANITE AND ZIRCON. 315 



VESUVIANITE. 



Vesuvianite: 



HR // 6 Al 3 Si 5 0, 1 (Clarke). 



Tetragonal. 



Sp. gr. 3.35-3.45. 



Clarke states that the R in the typical mineral is replaced by calcium 

 and magnesium in the proportion of 5: 1, giving- HCa 5 MgAl 3 Sig0 2 i. 



occurrence. — Vesuvianite occurs in ancient ejections of Vesuvius. It is 

 most abundant in marbles. It is also found in various gneisses and schists, 

 especially those which are calcareous. It often forms in connection with 

 contact action. It is frequently associated with such other metamorphic 

 minerals as garnet, and also the micas and chlorites. 



Alterations. — From the literature it is impracticable to ascertain which 

 particular garnet, mica, or chlorite forms from a certain vesuvianite, and 

 the accompanving- minerals which must simultaneously form are unknown: 

 it therefore does not seem advisable to attempt to write equations represent- 

 ing the alterations, since they must be so largely speculative. 



ZIRCON GROUP. 



The only important rock-making mineral of the zircon group is zircon. 



Zircon: 



ZrSiO*. 

 Tetragonal. 

 Sp.gr. 4.66-4.70. 



occurrence. — Zircon is especially common in marble. It also occurs both 

 in massive igneous rocks, such as syenite and granite, and in the schists and 

 gneisses. 



Alterations. — According to Clarke the only alteration described is that of 

 hydration, producing hydrous zircon (malacon) (tetragonal; sp. gr. 3.905), 

 the reaction being: 



3ZrSi0 4 + H,C - H 2 Zr 3 Si 3 13 . 



The increase in volume in the change is 24.05 per cent. 



