MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 335 
curve, located quite close to the eutectic between the microcline 
components and quartz. 
As microcline (from dikes of granite-pegmatite) always contains 
considerable Ab;an, most often 25-30 Abian: 75-70 Or, it almost 
certainly has a somewhat lower melting-point than pure Or, and we 
must therefore assume that the eutectic Qu:Or contains a little 
more Qu than the eutectic Qu: microcline. 
As the binary eutectic we shall assume 28 Qu:72 Or. As pure 
Ab has nearly the same melting-point as pure Or, nearly the same 
molecular weight (Or=279.4, Ab=263.3), and possibly also 
almost exactly the same melting-heat, it must be supposed that 
the eutectic Qu:Ab holds about the same percentage of Qu as 
the eutectic Qu:Or. As an approximation we may consequently 
assume 28 Qu:72 Ab as the binary eutectic. 
QU:AN AND QU:AB, QU:OR 
For the pressure of one atmosphere we have the synthetic 
determination: Eou-an=48 Qu’: 52 An, at 1353°. Further we refer 
to the determinations just mentioned (for a very high pressure): 
EQu-Or= ca. 28 Qu: 72 Or 
EQu-ab= ca. 28 Cu:72 Ab. 
Even if an error of a few per cent may be found in the latter 
statements, it is evident in any case that the binary eutectic 
~Qu:Or or Qu:Ab contains much less quartz than the binary 
eutectic Qu:An. This is in accordance with Ab (as well as Or) 
having an essentially lower melting-point, in round numbers 450°, 
than An. 
The course of the melting-curve (see Fig. 3) on the Qu side 
in the neighborhood of Qu will be about the same, whether the 
second compound is An or Ab (or Or). If we extend the curve, 
experimentally determined for Egu-an on the quartz side, and 
draw the curve on the feldspar side for Equa, (respectively 
Ho.-o;) about parallel with Eou-an, a binary eutectic Eouap 
(respectively Egu-or) will appear with composition about 25-30 
t Qu here signifies christobalite from the melting-point to 1470° and tridymite 
from 1470° to 1353°. 
