of Mineral Species. 81 



atoms of oxide of copper, two of water, and four of carbonic acid, 

 while malachite contains three atoms of e"ach. One atom of 

 carbonic acid is therefore exactly replaced by one of water. 



HAUY does not consider the crystals formed by aggregated 

 masses of the green filamentous malachite as epigenies of the 

 blue copper, as he unites the two species into one, and rejects 

 the slight difference in the results, of the chemical analysis as ir- 

 relevant. BEUDANT seems to be the first naturalist who viewed 

 this process of decomposition in a proper light *. 



Not only the blue copper, but also the imbedded octahedrons 

 and dodecahedrons of octahedral copper-ore, are found in that 

 locality in a state of incipient decomposition, resembling it in so 

 far as the form of the crystals is not altered. There is one cu- 

 rious difference, however, in the progress of this decomposition. 

 In the octahedral copper-ore, the surface first turns green by 

 the absorption of oxygen and water, since the protoxide is con- 

 verted into a hydrate of the peroxide, and then the decomposi- 

 tion penetrates deeper into the mass, whereby a more or less 

 considerable coating of compact malachite is formed ; whereas 

 the reverse takes place in blue copper, the surface of the crystals 

 being the last portion which is converted into malachite, since 

 the decomposition begins from the point of support. There are 

 crystals of an octahedral form, which consist, near the surface, of 

 fibrous malachite, of the same kind as that which often consti- 

 tutes the body of crystals, having the shape of blue copper ; they 

 generally contain a nucleus of octahedral copper-ore, not decom- 

 posed. A dodecahedral crystal of octahedral copper-ore, changed 

 into blue copper on the surface, is preserved in Mr ALLAN'S ca- 

 binet ; but such examples are rare. 



The cuivre hydro-siliceux of HAUY, comprehending chry- 

 socolla, is a species not yet well established, as the crystals 



* Traite de Mineralogie, p. 158. 

 VOL. XI. PART I. 



