230 A TREATISE ON METAMORPHISM. 



tion with serpentine. In these positions it is very frequently a secondary 

 product of olivine. The reactions occurring in its production are given 

 under that mineral. 



Alterations. — The alteration of chromite into other minerals has not been 

 noted. 



RUTILE GROUP. 



RUTILE, OCTAHEDKITE, ASD BKOOKITE. 



Rutile: 



Ti0 2 . 



Tetragonal. 



Sp. gr. 4.18-4.25. 

 Octahedrite: 



Ti0 2 . 



Tetragonal. 



Sp. gr. 3.82-3.95. 

 Brookite: 



TiOV 



Orthorhornbic. 



Sp. gr. 3.87^.082. 



occurrence. — Rutile is a pyrogenic constituent in igneous rocks, and has a 

 widespread occurrence in the clastic and metamorphic rocks, both as an 

 allogenic and as an autogenic constituent, in the latter case generally being 

 derived from ilmenite. Rutile is also derived from brookite, ilmenite, octa- 

 hedrite, and titanite. 



Octahedrite in the metamorphic rocks is a secondary alteration of 

 other titanium-bearing minerals, especially of titanite. It is also derived 

 from ilmenite. 



Brookite occurs sparingly, both in altered igneous rocks and in sedi- 

 mentary rocks. In some cases it is secondary to titanite. 



Alterations. — Both octahedrite and brookite alter to rutile (tetragonal; sp. 

 gr. 4.18-4.25). In the case of octahedrite the decrease in volume is 7.83 per 

 cent; in the case of brookite the decrease is 5.69 per cent, The heat change 

 is undetermined, but probably the alterations occur with the liberation of 

 heat. If this be the case the alterations involve recrystallization, diminu- 

 tion of volume, and liberation of heat. In the case of octahedrite the sym- 

 metry remains the same ; in the case of brookite the symmetry is increased. 



It may be inferred that such changes as these occur in both zones, 

 being in all respects analogous to the changes which take place in dolomiti- 

 zation. (See pp. 238-240.) However, the geological occurrences of these 

 alterations have not been given with such definiteness as to enable one to 

 make definite statements as to the actual facts. 



