Nagy et al. : Environment of Orgueil Meteorite Parent Body 549 



on the thermodynamics of dilute aqueous solutions that /ih-pH relationships 

 may be used to detine mineral stability. £h-pH diagrams had been used to 

 deduce environmental conditions from low temperature mineral paragenesis 

 and from sedimentation data; Carrels'^ presented a comprehensive treatise on 

 the geological aspects of £h-pH relationships. 

 Because of a lack of information, an evaluation of the parent environment in 



(UPPER LIMITED OF WATER 

 .STABILITY) 



^^0^ 



Fe203 

 (S04--) 



0.0 



-0.2 

 -0.4 

 -0.6 



-0.8- 



-1.0 

 



(LOWER LIMIT OF 

 WATER STABILITY) 



8 



10 



12 14 



pH 



Figure 4. Stability relationships of iron sulfides and oxides in water at 25° C. and 1 atmos 

 total pressure and total dissolved sulfur activity of 10~^ After Garrels.^^ 



terms of £h-pH relations must be basically speculative in nature. Speculation 

 is possible, however, if it can be assumed, that: (1) the temperature range was 

 approximately 0° to 200° C; (2) organic matter and other minor components 

 do not substantially affect known £h-pH relationships; and (3) minerals of the 

 Orgueil suite are gentically related. It is known^* from the stability of iron 

 oxides in water that pressure has only a limited affect on /?h-pH relationships. 

 A change of the temperature from 25° to 100° C. causes a shift in the stability 

 fields of solids relative to the £h-pH axes but it does not affect the shape and 

 size of the fields. 



