228 A TREATISE ON METAMORPHISM. 



to have been brought in from the outside, and therefore the change takes 

 place with decrease in volume. It is also certain that titanite forms exten- 

 sively in connection with chlorite, which commonly develops in the belt of 

 cementation. In this case the calcium carbonate and silica may be intro- 

 duced in solution from an outside source, under which circumstances the 

 volume is increased. 



The alterations of ilmenite to rutile and octahedrite, or any combina- 

 tion of them, certainly occur in the zone of katamorphism, and to this 

 position the heat and volume reactions correspond. However, I have not 

 found sufficient information on the subject to assert that these reactions do 

 not also occur in the zone of anamorphism. 



SPINEL GROUP. 



SPINEL. MAGNETITE, AND CHKOMITE. 



Spinel: 



MgAl 2 4 . 



(Hercynite, FeAl 2 4 . ) 



(Pleonaste, [MgFe] [ALFe] 2 4 .) 



(Picotite, [MgFe] [AlCr]A) 

 Isometric. 

 Sp. gr. 3.5-4.1. 



FeA. 

 Isometric. 

 Sp. gr. 5.168-5.180. 

 Chromite: 

 FeCr-A- 

 Isometric. 

 Sp. gr. 4.32-4.57. 



SPINEL. 



occurrence. — Spinel occurs as an original constituent in the igneous rocks, 

 but is much more abundantly present as a secondary constituent in the 

 metamorphic rocks, especially those which are rich in magnesium. In 

 many cases it is secondary to olivine and other minerals rich in magnesium. 

 The more important minerals from which spinel is derived are almandite, 

 biotite, chlorite, corundum, diaspore, garnet, gibbsite, olivine, and pyrope. 



Alterations. — According to Dana, spinel has been observed as altering to 

 talc (orthorhombic or monoclinic; sp. gr. 2.75), serpentine (monoclinic; 

 sp. gr. 2.575), and mica (monoclinic; sp. gr. 2.88-2.90). However, the 

 character of the alterations and the conditions under which they occur are 

 so little known that I shall not attempt to treat them from the physical- 

 chemical point of view. 



