ALTERATIONS OF TITANITE. 355 



Alterations. — Titanite alters into rutile (tetragonal; sp. gr. 4.18-4.25), 

 octahedrke (tetragonal; sp. gr. 3.82-3.95), and perovskite (isometric or 

 psendoisometric ; sp. gr. 4.017-4.039). 



Rutile and octahedrite may be supposed to be produced by the follow- 

 ing reaction: 



(1) CaTiSi0 5 +C0 2 =Ti0 2 +CaC0 3 +SiO,+k. 



The expansion of volume is 39.22 per cent for rutile, provided all of the 

 compounds separate as solids, and 42.07 for octahedrite. 



Perovskite may be supposed to be produced by the simple breaking 

 up of titanite, according to the reaction : 



(2) CaTiSi0 5 =CaTiO s +SiO,, 



The expansion of volume is 0.14 per cent provided the silica also separates 

 as a solid. 



Information as to the natural conditions under which these changes 

 take place is not obtainable from the papers giving the above minerals as 

 secondary to titanite. From the character of the first change one would 

 expect, however, that it would occur in the zone of katamorphism, and 

 especially in the belt of weathering. Under such conditions there would be 

 a reason for the change, for there carbonation of the silicates, with libera- 

 tion of heat and with expansion of volume, is the rule. As so frequently 

 indicated before, the freed- silica may be taken into solution, and if this 

 occurs the volume is decreased. Under what conditions the second reaction 

 is likely to have taken place I can only conjecture from its nature. I 

 should expect it to occur in the zone of katamorphism. 



PEROVSKITE. 



occurrence. — Perovskite occurs as an original constituent in eruptive 

 rocks, and also in the metamorphic rocks, such as the schists and gneisses. 

 As a secondary mineral it has been observed as a product secondary to titan- 

 ite. It may be suspected that in the schists and gneisses it forms in the 

 zone of anamorphism by the union of rutile and octahedrite or brookite, 

 with calcium carbonate; but this is a pure conjecture, as the details of its 

 formation are not found in literature. 



Alterations. — The mineral does not readily alter into other compounds, 

 although it has been observed to alter into an undetermined substance, and 

 it is said to alter into ilmenite (hexagonal-rhombohedral; sp. gr. 4.75). 



