552 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 63 



minerals commonly found on ancient buried bronze. After study, it 

 was determined that it is a hydrous double carbonate of copper and 

 sodium having the composition Na2Cu(C03)2*3H20 and that it is a 

 new mineral species not previously decribed. It has been given the 

 name "chalconatronite." This mineral was also observed on an Egyp- 

 tian bronze group, "Cat and Kittens," in the Gulbenkian Collection in 

 Lisbon, and on a Coptic censer in the Freer Gallery of Art. Chal- 

 conatronite seems to be a product peculiar to the arid soils of Egypt 

 where in certain districts alkali carbonates occur abundantly. 



COPPER CHLORIDES 



On ancient copper and bronze objects found after long contact with 

 the saline soils of desert regions or buried in the sea, the green cor- 

 rosion crusts are usually a mixture of the chlorides of copper. Sur- 

 face crusts containing chlorides do not form a desirable patina on 

 bronzes but are usually ugly and disfiguring. They give rise to a 

 troublesome kind of alteration product which has long caused concern 

 among collectors for the safety of bronze objects and have stimulated 

 more scientific inquiries in museum laboratories than any other class 

 of corrosion phenomena. A description of several of these copper 

 chloride minerals follows. 



Atacamite, Cu2(OH)3Cl, the most common copper chloride mineral, 

 gets it name from the desert of Atacama in northern Chile where it 

 occurs in secondary copper ore deposits. Since sodium chloride is 

 highly reactive toward copper and its alloys, ancient artifacts of 

 these metals exposed to it are often converted to fissured and nearly 

 formless masses of atacamite. The color of the mineral ranges from 

 emerald to blackish green. On the surface of many bronze objects 

 from Egypt and Mesopotamia atacamite occurs as a continuous, sugar- 

 like coating of dark green glistening crystals. 



Atacamite is a fairly common product. Rooksby and Chirnside 

 (1934) , who were among the first to apply X-ray diffraction methods of 

 analysis to metal corrosion products, observed that the green corrosion 

 product on modem electrical copper wire exposed to sea water and also 

 to artificial salt solutions is atacamite. 



Often crystalline atacamite is associated with a paler green powdery 

 product which gives the X-ray diffraction pattern of paratacamite, 

 a mineral identical in chemical composition, but having hexagonal crys- 

 tal form, whereas atacamite is orthorhombic. Such related minerals 

 are called dimorphs. One form appears to occur about as abundantly 

 as the other. Although both atacamite and paratacamite are observed 

 most commonly on bronzes from arid areas, they may also be found on 

 bronzes from regions of normal rainfall. Otto (1959) cites the occur- 

 rence of both minerals on bronze objects from Germany and other 

 parts of Europe, and he adds that paratacamite occurs much more f re- 



