72 Rankin and Wright — Ternary System CaO-Al 2 0-SiO^ 



conditions of natural formation are very different from those 

 under which this system was studied in the laboratory. The 

 main differences are two : (1) That under geologic conditions 

 the reactions take place under high pressure (which moreover 

 is probably not constant or uniform during the process) ; (2) 

 the complexity of the mineral solution, particularly the pres- 

 ence of volatile ingredients, especially water, at unknown con- 

 centration (pressure) ; both of these factors would exert an 

 influence of unknown magnitude upon the configuration of the 

 various fields of stability, and might well change their number 

 and character.* For this reason we find in nature minerals made 

 up substantially of CaO, A1 2 3 , Si0 2 (e. g. kyanite) which have 

 not been produced in the laboratory ; on the other hand, com- 

 pounds of these oxides which have not been observed hitherto 

 by penologists may be made synthetically (e.g. CaO.Al 2 3 ); 

 but from these circumstances we should conclude — presuming 

 that none of this compound does in fact occur in the earth — 

 merely that the compound may have formed and subsequently 

 disappeared, forming compounds more stable under the particu- 

 lar conditions. In any case the first step in a complete investi- 

 tion — which would include the effects of pressure, of presence 

 of water, etc., — is a thorough study of the relations of the pure 

 components and of the compounds formed under the least 

 complicated experimental conditions. 



Some of the possible applications to geologic problems will 

 be treated in a later paper; for the present, we desire merely 

 to emphasize the fact that these very important differences 

 affect the validity of all deductions made from the behavior 

 and mutual relations of the pure, dry oxides at high tempera- 

 tures and at atmospheric pressure. 



The synthetic minerals composed of CaO,Al 2 3 ,Si0 2 which 

 we have prepared are : quartz, tridymite and cristobalite (Si0 9 ) : 

 corundum (A1 2 3 ) ; wollastonite (CaO.Si0 2 ) ; sillimanite (A1 2 3 . 

 Si0 2 ); anorthite (CaO.Al 2 3 .2Si0 2 ) ; all of which have the same 

 compositions and practically the same optical characteristics as 

 the natural minerals. The compound 2CaO.Al 2 3 .Si0 2 has the 

 same optical characteristics but not the same composition as 

 has been assigned to the natural mineral gehlenite, but the 

 analyses of the natural mineral vary within such wide limits 

 that it seems reasonable to suppose that the true composition of 

 natural gehlenite is 2Ca0 2 Al 2 3 .Si0 2 . The compounds which 

 are synthetic minerals are underlined in fig. 19 ; from which 

 one sees that there are five triangles in each of which the 

 only compounds present are synthetic minerals, each triangle 



* For a discussion of some of these points see Johnston and Niggli, J. 

 Geology, xxi, 589-99, 1913. 



