OF PYRITES AND GYPSUM. 3I7 



normal marcasite about 4-8 ; and of normal pyrrhotite 4-6. But 

 abnormal forms are not infrequent, and in these impure varieties 

 the specific gravity may suffer disturbance of a more or less 

 erious character, according to the proportion of impurity present- 

 Prof. Julien's conclusions have been criticised by Mr. H. N.. 

 Stokes in his valuable paper published as a " Bulletin " of the 

 Geological Survey of the United States, No. 186. 



Of the native " sulphides" iron-pyrites, speaking broadly, is 

 one of the most unstable, but the different kinds of pyrites vary 

 greatly in their stability. Some of the pyrites from mineral 

 veins and from crystalline rocks is indeed durable enough, and 

 specimens of mundic, after exposure for years on the burrows or 

 waste-heaps of a mine, may still preserve a bright and brassy 

 appearance. Sharp cubic crystals in clay-slate also are often 

 resistent. But such cases are not very common. As a rule 

 marcasite seems much more prone to alteration than pyrite. 

 Thus the marcasite found in coal, lignite, clay-shale and chalk 

 readily suffers decomposition. The instability has been referred 

 by some authorities to the presence of foreign impurity, such as 

 arsenic or to the unstable proto-sulphide called troilite, whilst 

 others have connected it witli the state of aggregation of the 

 mineral. Prof, Julien's study of the subject led him to the con- 

 clusion that " difference of chemical composition has nothing to 

 do with the tendency to decomposition." It is more probably 

 connected with the molecular constitution of the mineral. 



Two distinct types of alteration may be recognised. In the 

 more familiar mode of alteration the pyrites suffers conversion 

 into brown oxide of iron. Many nodules and some crystals, on 

 exposure to moist air, become coated with this brown substance, 

 which is a ferric hydrate, or iron-hydroxide, known to 

 mineralogists as Limonite. The change proceeds slowly from 

 without inwards, and if continued long enough, the entire mass 

 of the mineral may become transformed. As the limonite has 

 practically the same volume as the pyrites from which it 

 originated, there is usually little or no change of form, so that a 

 perfect pseudomorph is obtained. Most mineral collections 

 contain beautiful pseudomorphs, wliich, though consisting more 

 or less completely of limonite, retain with perfect fidelity the 

 smooth faces and sharp edges, and even the characteristic striae 

 on the crystal-faces of the original pyrite. Such altered crystals 



