OF PYRITES AND GYPSUM. 319 



The tendency to limonitization, though common enough in 

 some kinds of pyrite, is even more marked in marcasite. Thus, 

 the well-known nodules from the chalk, often called " thunder- 

 bolts " and " fairy balls," are usually converted superficially into 

 limonite, so that tlie appearance of the globular masses is rather 

 suggestive of rusty cannon-balls. In some cases the ochreous 

 oxide forms only a thin brown rind, whilst in others it may 

 •occupy the greater part, or even the whole, of the nodule. If 

 there is any fissure, the meteoric agents may gain access to the 

 interior and attack the mineral in the immediate neighbourhoc)d 

 of the crack. The nodules tend to break readily in the direction 

 of the radiating fibres, and these fibres lose their brass-like 

 appearance when atmospheric influences penetrate along the 

 planes of weakness. 



Not only may pyrites suffer hepatic alteration, or limonitizaHon^ 

 but it may also undergo a saline decomposition, sometimes called 

 vitviolization. This type of change is sometimes very con- 

 spicuous in marcasite, and, as ever}' one knows, is a source of 

 great inconvenience to the collector of pyritized fossils. The 

 iron sulphide in the presence of moist air becomes oxidised 

 to f-rrous sulphate, which is the salt known, when formed 

 naturally, as melanteviie, and when formed artificially as " green 

 copperas " or " green vitriol." There is also frequently formed, 

 concurrently with the melanterite, free sulphuric acid ; and to 

 this product may be ascribed the corrosive action too often seen 

 in our cabinets and museums. The labels, the cardboard tra^s, 

 •even the wooden tablets and the drawers themselves, may be 

 attacked more or less seriously by the action of this acid 

 eliminated by the decomposition of the pyritous fossils. 



Although the name melanterite suggests a black mineral, the 

 ferrous sulphate is normally green. Being a very unstable body, 

 it tends to pass readily into the state of ferric sulphate, or per- 

 fiulphate of iron, a yellowish substance often seen as an 

 incrustation or efflorescence on rocks containing decomposing 

 pyrites. Moreover the ferrous sulphate also passes by oxidation 

 into the condition of ferric hydrate, as seen in common copperas 

 on exposure to air. 



It should be noted that Mr. H. N. Stokes shows that the old 

 notion that marcasite tends to saline change and pyrite to 

 limonitization is quite untenable. He has observed rapid 



