86 WATSON AND GOOCH: VIVIANITE from FLORIDA 



very rapid change in color of the streak of vivianite or of its 

 powder on fine grinding, from white or faint bluish white to deep 

 or indigo blue, is a well known property of the mineral. 



Further chemical study of the mineral was undertaken to deter- 

 mine, if possible, whether the change in color was due to oxidation 

 or to inversion. * For this purpose a lot of uniformly colored pale- 

 green crystals was selected and divided into two portions. The 

 first portion was accurately weighed, dissolved' without grinding, 

 and ferrous iron determined. The second portion was reduced 

 to powder by fine grinding and exposed to sunlight for five hours, 

 after which a weighed sample of the exposed powder was dis- 

 solved and the ferrous iron determined. The results follow: 



FeO. 



UNG ROUND 

 VIVIANITE 



per cent 



42.88 



FINELY GROUND 

 VIVIANITE 



per cent 



38.43 



These figures clearly show that fine grinding of the mineral 

 resulted in the oxidation of 4.45 per cent FeO to Fe203, equiva- 

 lent to 4.94 per cent FcoOs. A second lot of pale-green crystals 

 of the Florida vivianite was submitted to Prof. F. P. Dunning- 

 ton, of the University of Virginia, who found on analysis 40.58 

 per cent FeO and no Fe203 on dissolving the crystals without 

 grinding. 



The results show beyond reasonable doubt that the presence 

 of Fe203 in blue vivianite, which is probably the common variety 

 of the mineral, is due to oxidation and not to inversion. The 

 rapidity with which oxidation takes place on fine grinding, 

 manifested in very rapid change of color .(blue), is noteworthy 

 and of special interest. 



COMPOSITION OF THE MATRIX 



The matrix is a dark brownish yellow ocherous earth, locally 

 hardened and red in color, containing besides vivianite irregu- 

 larly scattered phosphate pebbles of chalky white color with 

 surfaces partly smooth but usually more or less pitted, and scant 

 rounded grains of pellucid quartz. It readily dissolves in hot 

 dilute hydrochloric acid. 



