550 



Annals New York Academy of Sciences 



In FIGURE 4 is illustrated a common geological phenomenon, i.e., the inter- 

 relations between iron oxides and iron sulfides in water at different Eh and 

 pH values. The stability relationships on the diagram were calculated by 

 Garrels^ for 25° C. temperature and for 1 atmos. pressure at a total dissolved 

 sulfur activity of 10~^ Under such circumstances, magnetite is stable under 

 mildly reducing conditions and at a pH higher than 7. The SO^T ion is stable 



UJ 



+1.0 

 +0.8 



+0.6 

 +0.4 

 +0.2 



0.0 

 -0.2 



-0.4 

 -0.6 



-0.8 

 -1.0 



OXIDIZING 

 VANADIUM 



^^DE POSITS 



% 



V. 



^a 



% 



%, 



MINE '3" 



WATERS, 



'A/ /^ OXIDIZING \ ' 



'/3 DEPOSITS 



RAIN, "^C 



V. 



> 



% 



STREAM '^/V- 



WATER, ^ /■/ 



NORMAL V/C^ 

 OCEAN ^ 

 WATER 



-^/^ 



H 



Os, 



%. 



V. 



^^. 



^e 



% 



% 



\ 



BOG 



\ 



^T/J/^^WATERS 



GROUND 

 '6'/ WATER 



r^^ WATER- '^/•^ 

 <0, \ LOGGED ^O f^ 





%,"\"^SALINE "%/), 

 ^^/>.^ ^ WATERS //^ 





% 



N» 



_L 



_L 



4 6 8 10 12 14 



pH 



Figure 5. Approximate position of some natural environments as characterized by Eh. 

 and pH. After Garrels.^^ 



in the magnetite field, but the stability field of elementary sulfur extends 

 toward a more oxidizing environment and an acidic pH. The missing mineral 

 phases in a given suite are also indicative of environment. Note that py- 

 rite forms at the same pH as magnetite but under a more reducing environment 

 than magnetite; whereas, hematite forms under a more oxidizing environment. 

 Neither pyrite nor hematite has crystallized out in Orgueil, although their 

 elementary components are present. Layer lattice silicates are known to occur 

 under conditions similar to the magnetite environment in figure 4. 



