ALTERATIONS OF RUTILE. 231 



In this connection the experiments of Hautefeuille" are very interesting'. 

 He produced rutile, brookite, and octahedrite from the same compounds, 

 but at different temperatures, rutile forming when red heat was used, 

 brookite when the temperature was between that required for the volatiliza- 

 tion of cadmium and zinc, and octahedrite when the temperature was a little 

 below that for the volatilization of cadmium. Rutile, the mineral with the 

 highest specific gravity, forms at the highest temperature, and high tempera- 

 ture is especially characteristic of the zone of anamorphism. 



Rutile may alter into ilmenite (rhombohedral; sp. gr. 4.75) and into 

 titanite (monoclinic; sp. gr. 3.48). In the change to ilmenite the reactions 

 may be: 



(1) Ti0 2 +Fe 3 4 =FeTi0 3 +Fe 2 3 +k, or 



(2) Ti0 2 +FeC0 3 =FeTi0 3 +C0 2 +k. 



In (1) the decrease in volume of the ilmenite and hematite as compared 

 with the rutile and magnetite is 1.88 per cent. In (2) the decrease in 

 volume is 34.77 per cent, provided the iron carbonate is present as solid 

 siderite and the C0 2 escapes. 



In the change to titanite the most probable reaction is: 



(3) Ti0 2 +CaC0 3 +Si0 2 =CaTiSi0 5 +C0 2 +k. 



The decrease in volume is 28.17 per cent, provided the compounds which 

 unite with the rutile are solids and the liberated C0 2 escapes. 



Those who have described the changes of rutile to ilmenite and titanite 

 have not indicated whether or not they occur as deep-seated alterations. It 

 may, however, be anticipated that such is the case, for they are changes 

 which involve liberation of heat and condensation of volume, and therefore 

 the kind which normally occur in the zone of anamorphism. 



DIASPORE GROUP. 

 DIASPORE AM) LIMOMTE. 



Diaspore: 



AIO(OH). 



Orthorhombic. 



Sp. gr. 3.3-3.B. 

 lAmonite: 



2Fe 2 3 .3H 2 0. 



Amorphous. 



Sp. gr. 3.6-4.00. 



«Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. a Dana; Wiley & Sons, 

 New York,' 6th ed., 1892, p. 239. 



