286 HAROLD L. ALLING 
composition richer in the component that possesses the higher 
freezing-point than the margins. Such zonal crystals are common 
among the plagioclase feldspars. Not all zonal textures, however, 
are to be explained in this way, as will be explained later in more 
detail. All available information leads to the conclusion that this 
type of diagram is the one to which the plagioclase feldspars (the 
soda-lime series) are to be assigned. In the field of mineralog- 
raphy our knowledge of other and similar systems is very incom- 
plete. In addition to the plagioclase feldspars, the garnets, the 
scapolites, the micas, the alums, and certain ranges of the pyroxene 
and amphibole families may be mentioned. But in the field of 
metallography, binary systems of solid solutions are better under- 
stood. The following systems may be noted: Ag-Au, Ag-Pd, 
Au-Pd, Bi-Sb, Co-Fe, Co-Ni, Cu-Pd, Cu-Pt, Fe-Mn, In-Pb.* 
There is no fundamental difference between isomorphous minerals 
and the alloys belonging to this group. 
Now let us examine in more detail the other extreme (0), 
that of a eutectiferous system, the two components of which are 
entirely insoluble in each other when in the solid state. In 
Figure 17 the composition of the melt chosen is the same as that 
used before, namely 60 per cent of N and 4o per cent of M. 
On cooling the melt the freezing commences at A; the resulting 
crystal having a composition of A, or pure NV, the liquid, a composi- 
tion of A. This can be expressed by saying that the composition 
of the crystal “varies” or “‘slides” down the line A,B,C,D, while 
that of the remaining liquid “slides” down the liquidus from A 
through B and C toward E where it freezes, but on passing to the 
solid phase the solution £ is rendered a mechanical mixture as the 
two components separate, theoretically at least, into pure NV and 
pure M. This mechanical mixture is termed the eutectic mixture. 
This tells us how such a system will look under the microscope, 
either in thin sections or as etched polished slabs. It will consist 
of crystals of pure N surrounded by minute crystals of pure M@ 
and pure N. The groundmass of the small crystals will be the 
eutectic mixture. It is possible to reverse the scheme and deter- 
mine the composition of the original melt by determining the 
1C. H. Desch, Meiallography (1913), p. 401. 
