12 COALFIELDS AND COLLIERIES OF AUSTRALIA. 



tapped from the seam beforehand, by means of bore-holes, and 

 may be conducted along pipes to the surface. 



Pyrites, known as "brasses," is of common occurrence in 

 coal seams. There are two forms of pyrites, having the same 

 formula, viz., iron pyrites, and marcasite, composed of two 

 parts of sulphur to one of iron; yet these tw r o minerals vary in 

 other respects, both physically and chemically. Marcasite is 

 more readily decomposed, and in doing so generates consider- 

 able heat within a shorter period than iron pyrites. As the 

 "brasses" of coal miners is often marcasite, it is highly prob- 

 able that the oxidation of this mineral may be an important 

 factor in some cases of spontaneous combustion. 



A. P Brownt makes some interesting suggestions to ac- 

 count for the difference between pyrite and marcasite. He 

 writes: "In nature, it would seem that in most cases the sul- 

 phide of iron first formed is Fe S (ferrous), but probably by 

 action of a ferric salt or carbonic acid (HoCO^) causing a lose 

 in iron, FeSo results, and this is almost always pyrite. On 

 the other hand, where ferrous sulphate has been reduced by 

 the slow action of decomposing organic matter, the resulting 

 sulphide seems to be generally marcasite, which, if not in crys- 

 tals, may be recognised by its ready weathering to ferrous sul- 

 phate (FeSO.;). This compound may, however, in many cases, 

 be a mixture of pyrite and marcasite, as much pyrite seems to 

 be." 



Pyrites contains a large amount of ferric iron, while in 

 marcasite the iron apparently exists in the ferrous condition. 

 Mr. Brown then goes on to explain that if we look upon the 

 iron in marcasite as a dyad (Fe^S^), the bonds of the sulphur 

 atoms are not all saturated with iron, so that a bond of each 

 has to combine with sulphur. Such combinations are weak, 

 and liable to oxidation more readily than when sulphur and 

 iron bonds are connected. He expresses the probable struc- 

 tural form of marcasite by the following graphic formula: 



Iron pyrites (Fe S*>), he looks upon as having a constitu- 

 tional formula (4 Fe iv $V,, Fe i{ S.i). 



B A 



tA. P. Brown, "A Comparative Study of the Chemical Be- 

 havior of Pvrite and Marcasite." Proc. Am. Phil. Soc. 

 .\ XXIII. 226." 



