184 Mr. A. Gages on Vivianite. 



carbonate of ammonia, when a milky-white precipitate, appa- 

 rently of phosphate of the protoxide of iron, is thrown down. This 

 white precipitate, kept from contact with air, sometimes assumes 

 a bluish tint on exposure to the light. If, instead of using weak 

 cold hydrochloric acid ih the preceding experiments, boiling acid 

 be employed, the solution assumes a reddish tint, and the preci- 

 pitate thrown down by the sesquicarbonate of ammonia is of a 

 reddish colour ; the absoi-ption of oxygen by the precipitate also 

 takes place more rapidly in this case. 



If we employ caustic ammonia in the first instance instead of 

 sesquicarbonate, the precipitate, instead of being milky white, 

 has a reddish tint. The colour of the crystallized Vivianite also 

 deepens, as well as the streaks, which, being bluish white, soon 

 change to an indigo colour on exposure to the air ; and even 

 colourless crystals, as those from the greensand of Delaware, 

 only become blue on exposure. The clay upon which the blue 

 iron earth is found is generally whitish, showing that the iron 

 present is protoxide; and it has been observed that the blue 

 bloom is often formed only after exposure to the air. Berzelius 

 considers that the white compound which becomes blue has the 

 composition of 3 FeO, PO^, or rather a hydrate of it. It is no 

 doubt the same compound which is first formed in the elegant 

 process by which Becquerel has succeeded in producing crystal- 

 lized Vivianite. He forms two cells, separated by a diaphragm 

 of moist clay ; in one he puts a solution of sulphate of copjjei-, 

 and in the other one of phosphate of soda ; he then dips a copper 

 rod into the solution of the copper salt, and an iron rod into the 

 solution of the phosphate, and brings the two rods into connexion. 

 After a time white crystalline nodules are deposited on the iron, 

 which quickly become blue on exposure to the air. 



It is not, however, by oxidation alone that Vivianite can be 

 produced ; the circumstances under which many specimens are 

 found, show that its formation is due perhaps oftener to deoxida- 

 tion than to oxidation : thus in Cornwall it always occurs asso- 

 ciated with pyrites, or magnetic iron pyrites. In the first, its 

 production may easily be accounted for by supposing the forma- 

 tion of the sulphate of iron and a double decomposition between 

 the sulphate so formed and apatite. Perhaps the Vivianite 

 often found filling the cavities of amygdaloidal trap may have 

 been likewise formed in a similar manner. I have on one occa- 

 sion observed it coating, like blue paint, zeolitic minerals found 

 in such cavities, clearly proving that its formation was posterior 

 to that mineral. 



The most favourable circumstances, however, under which 

 Vivianite appears to be formed by deoxidation, is during the 

 decay of the organic matter of the bones. During the process 



