174 



Allen, Crenshaw, Johnston, and Larsen — 



Fig. 2. 



Methods of distinguishing pyrite from marcasite. — To 

 distinguish between the disulphides pyrite and marcasite, two 

 methods were employed. 



1. Crystals were prepared large enough for goniometric meas- 

 urement. When hydrogen sulphide and sulphur act directly 

 on a solution of ferrous sulphate, the crystals are in general 

 minute. They increase in size with rise in 

 temperature and several other conditions. 

 One of the most important of these is slow 

 formation. This can be brought about by 

 a simple device (fig. 2). Into a glass tube 

 of about 18 mm inside diameter and 65 cm in 

 length, is poured through a funnel about 

 '7-10 g. of sodium thiosnlphate (Na o S 2 3 . 

 5H 2 0) dissolved in 30 cc water. The fer- 

 ric salt is contained in another smaller 

 tube which slips into the larger. The 

 small tube has a diameter of 15 mm out- 

 side and a length of 45 cm . It is supported 

 above the thiosnlphate solution by a piece 

 of glass tubing a little longer than the depth 

 of the liquid. When the outer tube is 

 sealed, the whole is heated in a bomb as 

 usual. At a temperature of 200°, hydrogen 

 sulphide is slowly generated from the thio- 

 sulphate solution, according to the follow- 

 ing equation: Na 2 S 2 3 + H 2 = J^a 2 S0 4 -f- 

 H 2 S. By the action of the gas on the ferric 

 salt solution, crystals of measurable size 

 have been repeatedly formed. This scheme, 

 however, could not be satisfactorily used in 

 studying the influence of various conditions 

 on the crystal form. Obviously, not all the 

 crystals of any product could be measured, 

 and it was, of course, a thankless if not 

 impossible task to identify all of them micro- 

 scopically. Therefore, a quantitative esti- 

 mation of the two minerals in mixtures 

 could not be made. Besides, the method of 

 synthesis is very slow. 



2. Stokes's oxidation method. Some years 

 ago, a chemical method for distinguishing 

 between pyrite and marcasite was worked 

 out by H. N. Stokes.* Since free use of 

 Fig. 2. Apparatus this method has been made, it will be nec- 



for the slow forma- , i • •,_ • t j_ «i n 



tion of crystals of iron essary to explain it in some detail. It 



disulphide. * H N stokeSj BulL jj. g Geol Survey, 186. 



