3l6 ON 'IKE NATURAL HI«;TORY 



It frequently happens that Pyrite and Marcasite, though not 

 isomorphous, occur in intimate association, and it is certainly 

 convenient in such cases to refer to the mixture as iron-pyrites. 

 "In clay-beds," says Julien, ''the pyritous nodules generally 

 consist of successive crusts or transition mixtures of pyrite and 

 marcasite." This remark is not without interest to us, inasmuch 

 as the iron-pyiites ol the London clay of Essex may often be of 

 such a mixed character. Julien described the pyritic mineral of 

 the well-known Nipadites, or fossil palm-nuts from the London 

 clay of Sheppey, as " marcasitic pyrite," and says that some of 

 the Sheppey fruits shew octahedra of true pyrite. 



When Marcasite is well-crystallized, its forms are so distinctive, 

 so very different from those of pyrite, as to be immediately 

 recognised. Very commonly the crystals of marcasite are 

 aggregations of wdiat are called twin-crystals. Some of these 

 groups present a shape which has led to the trivial name of 

 Cock's-covih pyrites, whilst others have a suggestive resemblance 

 to brass spear-heads, whence the term speav pyrites. The latter 

 is the common type in the Chalk-marl between Dover and 

 Folkestone, where according to Prof. Miers, 'they are locally 

 supposed to be the heads of Roman weapons. ^"^ 



Whilst the two minerals marcasite and pyrite are thus 

 readily enough distinguished when well crystallized, it is not 

 always easy, in the absence of crystalline characters, to say off- 

 hand whether a given specimen should be referred to pyrite, 

 or to marcasite. It is true that marcasite is paler in colour, so 

 that it is sometimes called white pyrites, but then by tarnish it is 

 apt to become yellow, and so tends to resemble true pyrite. 

 Strictly speaking it may be said that the three forms of iron- 

 pyrites have characteristic colours ; marcasite, or rhombic 

 pyrites, being tin-white ; pyrite, or cubic pyrites, being brass- 

 yellow ; and pyrrhotite, or rhombohedral pyrites, being bronze 

 coloured. But these colours refer to pure and fresh specimens, 

 and much of the marcasite seems to be impure ; that is to say, 

 it appears to be in intimate mechanical association with pyrite, and 

 thus acquires a yellowish tint. 



It has been said that the surest means of diagnosis is pro- 

 bably to be found in the specific gravity of the several minerals. 

 According to Julien the density of normal pyrite is 5*01 ; of 



10 Mineralogy : an Introduction to the Scientific Study of Minerals, 1902, p. 330. 



