34 



PSEUDOMORPHISM IN MINERALS. 



Again we have a blue mineral, also a hydrous carbonate of 

 copper, namely : Azurite, Ou3 C2 O7 + H2O, with a streak like 

 its colour — smalt blue. The crystals are monocline, in the form of 

 thin plates, or short prisms, not seldom however in long prisms, 

 mostly in tufts or groups, also in compact or earthy varieties. 

 The chemical composition, is 69 CuO, 26 00 + 5 H2O. 



The minerals already mentioned are enough for our present 

 purpose We may now consider the modes whereby a new com- 

 pound is produced in the shape of another from the decomposition of 

 which it has originated, that is of a pseudomorph, the accepted 

 meaning of pseudomorph being a mineral occurring in a form 

 which does not naturally belong to it, which form it has assumed 

 by the gradual substitution of the atoms of another mineral by its 

 own, Of this nature is the rhombic Atacamite or hydrous 

 oxy-chloride of copper when it appears in the tesseral shape of 

 cuprite, the red oxide of copper, in octahedrons and cubes. Here 

 we appear to have cuprite crystals, with a green colour and a 

 green streak ; the red oxide is in fact substituted by the chloride. 

 Not only do its tesseral crystals pass into atacamite, but they even 

 renew the fine lustre of that mineral. In short it is atacamite in 

 a false form. There can be no doubt that this change, or rather 

 partial change, has been effected by interpenetration of chlorine 

 in one state or another, among the molecules of the cuprite. On 

 a larger scale, veins of atacamite are frequently to be seen pene- 

 trating the mass, and occupying fissures, or at least lines of least 

 resistance, to reactionary force. Should the decomposing agent 

 reach an open space in the substance of the mass, and meet there 

 no crystalline form of the ore, it lines it with its own proper 

 crystals, but if it there finds crystals of the oxide it removes them 

 by slow degrees, replacing them by atacamite, and so producing 

 a pseudomorph. 



To have a clear idea of this class of pseudomorphism we must 

 suppose changes resulting from the reception of new elements 

 entering from within. 



Specimens are met with having one portion of their cuprite 

 crystals completely translated into atacamite, another in which 

 the operation is not yet perfected, and another in which it has not 

 yet commenced. 



In another pseudomorph, however, we have proof that the 

 direction of the agency of decomposition has been reversed. The 

 blue hydrous carbonate of copper, azurite, with its prismatic 

 crystals, beautiful blue tint, and transparency, are accompanied by 

 crystals, some of which show partly the green colour of malachite, 



