ALTERATION OF ILMENITE. 227 



liberates heat and diminishes the volume. The alteration of hematite to 

 pyrite and marcasite is best known where organic compounds are present 

 to reduce sulphuric acid to hydrosulphuric acid and to furnish carbonic acid 

 to form the carbonates. The reaction is especially characteristic of the belt 

 of cementation, and to this position the expansion of volume and the 

 liberation of heat correspond. 



ILMENITE. 



occurrence. — Ilmeiiite is an abundant pyrogenic constituent of the igneous 

 rocks. It is found both as an allogenic and as an autogenic constituent in 

 metamorphic rocks. As an autogenic constituent the compounds which 

 unite to produce it have not been worked out. As a metamorphic mineral 

 ilmeiiite is derived from perovskite and rutile. 



Alterations. — Ilmeiiite alters to titanite (monoclinic; sp. gr. 3.48), to rutile 

 (tetragonal; sp. gr. 4.18-4.25), and to octahedrite, or anatase (tetragonal; 

 sp. gr. 3.82-3.95). With these minerals magnetite (isometric; sp. gr. 5.174) 

 or hematite (rhombohedral ; sp. gr. 5.225) or limonite (amorphous; sp. 

 gr. 3.80) is simultaneously produced. One of the most frequent reactions 

 in the production of titanite is probably along the following lines: 



3FeTi0 3 +3CaC0 3 +3Si0 2 +0=3CaTiSi0 5 +Fe 3 4 +3C0 2 +k. 



The decrease in volume of the ilmeiiite, calcite, and quartz in passing into 

 titanite and magnetite, supposing the C0 2 to escape, is 22.35 per cent; but 

 the increase in volume of the titanite as compared with the ilmeuite alone 

 is 76.35 per cent. The alteration of ilmeiiite to rutile and octahedrite, with 

 combined magnetite, is as follows: 



3FeTi0 3 +0=3TiO,+Fe 3 4 +k. 



In case hematite is produced instead of mag'netite the reaction is: 



2FeTi0 3 +0=2TiO,+Fe 2 3 +k. 



Ill case limonite is produced, one and one-half molecules of water are added 

 to both sides of the equation. 



The increase in volume of the ilmeiiite in passing into rutile and mag- 

 netite is 6.02 per cent; into octahedrite and magnetite, 11.07 per cent. In 

 case hematite or limonite be produced, the increase in volume is corre- 

 spondinglv greater. 



It is certain that titanite forms from ilmeiiite in the lower zone. In this 

 zone, as explained on pp. 764-765, the CaC0 3 and Si0 2 can not be supposed 



