548 Annals New York Academy of Sciences 



A Discussion of Conditions on the Parent Body 



Experimental data establish that the Orgueil meteorite consists chiefly of the 

 following substances (Hsted in an approximate order of decreasing abundance). 

 (1) Hydrous layer lattice sihcate mineral(s), (probably chlorite or, less likely, 

 montmorillonite); (2) magnetite; (3) magnesium sulfate; (4) organic matter; 

 and (5) elementary sulfur. 



The terrestrial occurrence of the minerals (1, 2, 3, 5) must be briefly con- 

 sidered before one attempts to evaluate the environment of the Orgueil parent 

 body. As an initial consideration, one may note that hydrous, layer lattice 

 silicate minerals can form only in the presence of water (licjuid or vapor). 

 Clearly, the parent body must have contained water. 



The chlorite minerals occur in crystalline, metamorphic schists (which had 

 high temperature histories), in altered, basic igneous rocks as well as in soils 

 and sediments (low temperature history). Serpentine and talc have either 

 hydrothermal origins or they are alteration products of igneous rocks. The 

 montmorillonite minerals are known to occur both in soils and sediments and 

 in rocks altered by hot hydrothermal solutions. Layer sihcate minerals occur 

 under a rather wide range of temperatures. 



Magnetite is present in many igneous rocks (which crystallized from molten 

 silicates), and in sediments. Epsomite is known to crystallize from (low 

 temperature) mineral water; it is often found in limestone caves. Sulphur 

 may be the result of either volcanic activity, of the decomposition of H2S in 

 thermal springs or of bacterial action in rocks and Recent sediments. 



On the other hand, phase equilibria studies,^^"'*' have demonstrated that 

 certain, characteristic high temperature minerals, absent in Orgueil, beg'n to 

 form above 450 to 500° C. temperature. This then may be safely assigned as 

 the upper limit of the Orgueil temperature history. As to the lower limit of 

 the parent body, one must resort to speculation. It is difficult to visualize 

 how a great mass of crystalline siUcates could have formed through solid state 

 reactions, at temperatures below the freezing point of water. 



Other considerations may narrow down the temperature range. The fact 

 that some sulfur and hydrocarbons can be liberated from the stone at tempera- 

 tures as low as 150 to 200° C, at slightly reduced pressure, suggests that the 

 upper limit of the temperature range could not have been much higher than 

 200° C* Furthermore, the composition of the organic matter seems to have 

 been altered when the meteorite was heated with water in sealed glass tubes at 

 temperatures substantially higher than 200° C. DuFresne and Anders"*- noted 

 recently that some strained glass fragments found in the Mighei carbonaceous 

 chondrite indicated that the meteorite could not have been subjected to a 

 temperature of 180° C. for a period longer than a few weeks. The authors 

 claimed that Mighei temperatures could not have exceeded 300° C. It was 

 also suggested that the magnesium sulfate veins in Orgueil were produced by 

 liquid water. 



There are 2 other useful indications of environment : the oxidation-reduction 

 potential (Eh) and the pH. It is known from Pourbaix's*^ fundamental work 



* Gas chromatographic and mass spectrometric analyses indicate that hydrocarbons as 

 small as C9 are present in Orgueil. The boiling point of H-nonane is 150° C. at atmospheric 

 pressure. 



