180 Decomposition of Iron Pyrites. 



ed surfaces of many common minerals, such as hornblende, 

 or chlorite, present similar oxidation -colors ; but even within a 

 crust of compact marcasite, the change is more ra^^id, and the 

 exact determination of the color on a perfectly fresh fracture 

 may require a somewhat careful inspection under the loup, on 

 account of the filmy greenish, yellowish or iridescent planes 

 which descend through the crust along the surfaces of its radiat- 

 ing plates. 



The lustre of marcasite appears to be, in all cases, at least as 

 high as that of pyrite, on a fresh fracture, however soon dulled 

 by the tarnish of incipient oxidation. 



A typical variety of stable character seems to have been the 

 favorite decorative material of the Incas or ancient Kings of 

 Peru, being used in rings and amulets, and the larger pieces 

 even polished as mirrors. Under the name, pierre des Incas, it 

 has been thus described :^ '*A kind of marcasite having a bril- 

 liant lustre, and a colour somewhat approaching to tin-white, when 

 first found, and bearing the same relation to European marcas- 

 ite, which is generally of a bronze color, that white gold does to 

 ordinary gold." 



Marcasite was, together with pyrire, largely used in the last 

 century for ornamental stones, of which it is stated: ''The lustre 

 of the polished surface was so brilliant that the stone, although 

 opaque, formed a rough substitute for diamond ; and this lustre 

 was not readily impaired by atmospheric influences. 



Much of the old marcasite jewelery is of so pale a color as 

 almost to resemble burnished steel ; such kinds generally belong 

 to the true modern marcasite, sometimes called 'white pyrites'." ^ 



''These are what are called health-stones (pierres de sante), 

 because it is supposed that they become tarnished when their 

 wearer becomes sick.''^ 



C. Pyrite. 

 My principal object has been the examination of pyrite, espec- 

 ially in all its crystallized forms, free from gangue-matter. The 

 variation of physical properties with the decreasing density, 



' Bristow, Glossary of Min., (1861), 228. 



2 Encyc. Brit., (1883) XV, 532. 



3 Diderot et D'Alerabert, Nouveau Diet., (1778), III, 801. 



