340 Foote and Bradley — Solid Solution in Minerals. 



deposited simultaneously. This is the case, for instance, when 

 solid solutions crystallize as eutectics. If dolomite and calcite 

 could be obtained which had crystallized simultaneously from 

 solution, this material would therefore be ideal for determining 

 the mixing limits of both minerals. A very careful search 

 through the extensive Brush collection has shown no specimen 

 where this condition of affairs was realized, nor have we been 

 able to obtain such a specimen from other sources. This was 

 not unexpected, for it can be shown that at a given tempera- 

 ture, both minerals could be deposited only when the ratio of 

 lime to magnesia in the solution has one fixed value. On the 

 other hand, dolomite or calcite alone could be formed from an 

 infinite number of solutions, with varying proportions of lime 

 and magnesia, Almost the only chance of obtaining such 

 specimens appears to be from the concentration of a large quan- 

 tity of solution containing salts of both metals, in which case, 

 one mineral would first crystallize to be followed ultimately 

 by the crystallization of both. 



There appears to be another way, however, in which crystals 

 of one of the minerals may be obtained saturated with the 

 other. If, for instance, a solution capable of depositing dolo- 

 mite comes in thorough contact with calcite, the solution should 

 become saturated with the latter and the dolomite resulting 

 should contain the maximum amount of calcium carbonate. 

 A specimen of dolomite, therefore, deposited on calcite, or of 

 calcite on dolomite, may usually be expected to contain the 

 maximum amount of the other salt in solid solution. On the 

 other hand, the calcite or dolomite originally present would 

 not, of necessity, be changed in composition to the limiting 

 value. As will be seen below, one specimen of calcite depos- 

 ited on a dolomite was apparently not saturated with the latter. 

 In this case, the calcite crystal stood out from the mass of 

 dolomite and the solution' from which it formed was probably 

 not saturated with the latter. In general, where the secondary 

 mineral crystallizes in intimate contact with the primary one, 

 the composition should approach closely to the limiting value. 



A number of specimens have been obtained showing the 

 associations mentioned above, where either dolomite or calcite 

 has been deposited on the other, and in each case the secondary 

 mineral has been analyzed. In some cases, the material 

 required for analysis was so closely associated with the primary 

 mineral that separation by means of heavy solution was neces- 

 sary. In the case of one dolomite, No. Ill, the material was 

 put through the heavy solution twice to remove particles of 

 calcite carried down in the first treatment. Where the specific 

 gravity of the samples is not given in the table, the material 

 was in such good crystals that it could be separated in pure 



