PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 595 
At rt atm. and 21°95 there is equilibrium between the four 
phases (MgCl, - 6H.O), (CaCl, - 6H,O), (CaCl, 2MgCl, 12H,0),7 
and solution, the composition of which is 100H,O, 4.92MgClL, 
g-27CaCl,. The reaction may therefore be written as follows: 
1 (MgCl, -6H.O)+1.188 (CaCl, + 6H.O) =o. 252 (CaCl, - 2MgCl, - 12H.O)+ 
Solid Solid Solid 
A Cc B 
0.101 (100H,0, 4.92MgClL, 9.27CaCl.) 
Solution 
L 
Now if the temperature is changed, the pressure remaining constant, 
one of the above 4 phases must disappear; and it has been found 
that the right-hand side is favored by increase of temperature. 
Therefore at temperatures below 21°95 and 1 atm. there can 
coexist: either 
(MgCl, - 6H.O), (CaCl, -6H,O), solution (1) 
or 
(MgCl, -6H.O), (CaCl: 6H.O), (CaCl. - 2MgCl,: 12H,0) (11) 
while at higher temperatures the possible stable configurations are 
either 
(CaCl, - 2MgCl, - 12H,0), solution, (CaCl, - 6H,O) (IIT) 
or 
(CaCl, - 2MgCl,- 12H,O), solution, (MgCl. -6H.O) (IV) 
The solution has a definite composition corresponding to each 
temperature. For instance, at 16°7 in contact with (MgCl, - 6H,O) 
and (CaCl, . 6H.O) its composition is 100H,0, 6.69CaCL, 5.94 
MgCl; while at 28°2, in contact with (CaCl, - 2MgCl, - 12H,O) 
and (MgCl, - 6H,0), it is 100H,O, 8.84CaCL, 5.37MgCl. 
From this it is evident that not all the systems whose compo- 
sition can be expressed in terms of MgCl,-6H,O, CaCl,-6H,O and 
H,O can give the 3-phase combinations I, II below 21°95, or III, 
IV at temperatures above 21°95. This may be illustrated by the 
isotherm for 23° (and 1 atm.), which has the general form given in 
Fig. 2. PQis the solubility curve of A, OR that of B (tachhydrite), 
and RS that of C. The point Q gives the composition of the solu- 
tion in equilibrium at 23° with A and B; R, of that in equilibrium 
* The so-called tachhydrite. 
