188 Allen, Crenshaw, Johnston, and Larsen — 



portion gave no sign of change, and at 300° a powdered sample 

 which was repeatedly heated for several days' time with 

 sodium sulphide and polysulphide solutions was equally 

 un affected. 



Density. — The density of the marcasite heated to 610° rose 

 from 4*887 to 4*911. The density of pure pyrite is 5*02 + 

 The change in color, and, more convincingly, the oxidation 

 number, show that the substance is pure pyrite after heating, 

 yet its density is too low. The explanation is probably to be 

 sought in the porosity of the product. 



Influence of pressure on the change marcasite -> pyrite. — Dr. 

 A. Ludwig, at our request, kindly undertook some experiments 

 on the influence of pressure in transforming marcasite to 

 pyrite. A few grams of marcasite were compressed for five 

 hours at a pressure of about 10,000 atmospheres. At the end 

 of the period, the oxidation by Stokes' method showed no 

 change in the substance. Later Johnston and Adams devised 

 an apparatus in which the marcasite could be heated by a 

 resistance coil while subjected to hydrostatic pressure of about 

 2000 atmospheres in petroleum oil. A number of experiments 

 were tried between 300° and 400°, but Stokes' reaction showed 

 no pyrite formation. The oxidation method was perhaps not 

 quite so certain here on account of the fact that the oil was 

 partially decomposed at the higher temperatures and the 

 product may have contained some reducing matter which could 

 not be removed by petroleum "ether." A mixture of pyrite 

 and marcasite containing any such reducing impurity, as we 

 have seen (p. 184), would give too low an oxidation number. 

 Thus it might happen that a little pyrite could be overlooked. 

 These experiments are of considerable interest because there 

 are very few data on the effect of pressure in irreversible 

 changes.* We do not know whether a difference in density in 

 the right direction would favor the change or not, since 

 Le Chatelier's law applies only to reversible changes. If the 

 speed of the change is influenced by pressure quite apart from 

 the volume relation, it may perhaps be retarded rather than 

 accelerated. Until apparatus is developed which will give 

 higher temperatures and at the same time high pressures, this 

 problem must wait. At present the assumption, f which has 

 been made in geology, that pressure favors all changes which 

 are accompanied by reduction in volume, irrespective of their 

 reversibility, is unwarranted. 



Monotropic relation of marcasite to'pyrite. — A crucible con- 

 taining 50 g. pyrite was rapidly heated (20° per minute) in H 2 S 



* Van't Hoff, Vorlesungen, 2nd. Ed., Braunschweig, 1901, vol. I, p. 236. 

 f See Van Hise, A Treatise on Metamorphisrn , Monograph No. 47, U. S. 

 Geol. Survey, 1904, pp. 215, 363. 



