344 A TREATISE ON METAMORPHISM. 



According to Clarke, penninite, one of the chlorites, is composed of 

 one molecule of biotite-chlorite and one molecule of phlogopite-chlorite; 

 and clinochlore is composed of two molecules of biotite-chlorite and one 

 molecule of phlogopite-chlorite. It is therefore easy to combine the equa- 

 tion given under biotite for the production of chlorite with the one under 

 phlogopite producing chlorite, and thus produce penninite (pseudorhombo- 

 hedral and monoclinic; sp. gr. 2.6-2.85) and clinochlore (monoclinic; sp. 

 gr. 2-2.5). However, as the alterations for the production of chlorite from 

 biotite and of chlorite from phlogopite, reactions of hydration, carbona- 

 tion, and liberation of heat occur in the zone of katamorphism, it may 

 be said that where penninite and clinochlore are produced from biotite and 

 phlogopite the physical-chemical reactions are of the same class as those 

 which have been g'iven for hydrobiotite and hydrophlogopite. 



CLINTONITE GROUP. 

 MARGAKITE, CH10KIT0ID, AXD OTTREUTE. 



The clintonite group includes the following rock-making minerals: 



Margarite: 



HjCaAl^SiAs- 



Monoclinic. 



Sp. gr. 2.99-3.08. 



Chloriloid: 



H 2 (MgFe)Al 2 SiO,. 



Monoclinic (G) or triclinic (D). 



Sp. gr. 3.52-3.57. 



Ottrelite: 



H 2 (FeMn)Al 2 Si 2 9 - 

 Monoclinic or triclinic. 

 Sp. gr. 3.3. 



occurrence. — The most common development of margarite is in connection 

 with corundum. In a number of cases it is recorded that the alumina of the 

 margarite is directly furnished by the corundum. Margarite is also found 

 as a metamorphic mineral in schists and gneisses, associated with the heavy 

 minerals staurolite, tourmaline, etc. As a metamorphic mineral, margarite 

 is also recorded as being derived from diaspore and gibbsite. 



