1S94.J -'-'* [Brown. 



formed is FeS, but probably by action of a ferric salt, or carbonic acid 

 (HjCOj), causing a loss la iron FeSj results, and this is almost always 

 pyrite. On the other hand, where ferrous sulphate has been reduced by 

 slow action of decomposing organic matter, the resulting sulphide seems 

 to be generally marcasite, which if not in crystals may be recognized by 

 its ready weathering to ferrous sulphate (FeSO^). This compound may, 

 however, in many cases, be a mixture of pyrite and marcasite, as much 

 pyrite seems to be.* These several ways in which pyrite may be formed 

 are of importauce as indicating the condition of the iron in the compound, 

 and will be referred to later on. 



Equally important as bearing on the constitution of these sulphides are 

 the observations that have been made as to their decomposition under at- 

 mospheric agencies. On exposing crystallized pyrite to atmospheric 

 weathering it is generally fouad that but little, if any, change takes place 

 even in a year's time, while crystallized marcasite, under the same condi- 

 tions, shows a rapid weathering. The main pruduct of the weathering 

 of pyrite in nature is the compound limonite, Fc403(OH)g, which occurs 

 in large quantities in nature, evidently derived from pyrite. Its pseudo- 

 morphs after pyrite are very common. On the other hand, when marca- 

 site weathers a veiy large percentage of ferrous sulphate (FeSO^) is 

 formed with a comparatively small percentage of limonite, unless the 

 marcasite decomposes underground and under pressure, when limonite is 

 largely produced.f Much of the excess of sulphur with marcasite is 

 changed to sulphuric acid, but with pyrite much remains behind as sul- 

 phur. Some comparison of the rate of oxidation of these two minerals 

 in the air is afforded by an examination of specimens in a collection. 

 Here if will be found that most of the pyrite is unchanged, but nearlv 

 every specimen of marcasite will show tarnish if no other sign of oxida- 

 tion, and often a considerable coating of oxide can be seen, or a white 

 elflorescence of FeSO^. 



Chemical investigations of these two minerals have been mainly in the 

 way of analysis, but some experiments have been made in the way of 

 studying their relative behavior towards certain reagents. Before de- 

 scribing my experiments, it will be necessary to briefly mention some of 

 these. 



Both minerals are decomposed by a solution of silver nitrate and cold 

 chloride, the decomposition taking place quite rapidly.:}: Jkly experiments 

 in this connection are mentioned later. 



A. A. Julien § has shown that different samples of pyrite show a differ- 

 ence in their reaction with bromine vapor. His experiments consisted in 

 exposing finely ground pyrite to the action of bromine vapor at the tem- 



* Compare A. A. Julien, "Decomposition of Pyrite," Ann. X. V. <lcacZ. Sc/., Vols, i.i 

 and iv. 

 t Blum, Pseudomorphosen, 1843, pp. 197-199. 

 JS. Meunier, Synth. Min., p. 309. 

 ?A. A. Julien, .1/1)1. y. Y. Acad. ScL, Vol. iv, pp. 151, 155. 



