SOLIDIFICATION OF ALLOYS AND MAGMAS 579 
solid solutions of CaSiO, and diopside will separate out between o 
and 2 per cent. and 46 and 47 per cent. of MgSiO,. From 2 per cent. 
to 46 per cent., the structure will be a composite eutectic of CaSiO, + 
diopside, each saturated with a small amount of the other, and an 
excess substance of CaSiO, or diopside, each saturated with the other, 
depending upon whether the composition is below or above the 
eutectic composition of 28 per cent. MgSiO,. 
Likewise for the right-hand half of the diagram, isomorphous 
mixtures of diopside and MgSiO, are indicated between 47 and 67 
per cent. and 98 and roo per cent. of MgSiO,. Between 67 and 96 
per cent. a eutectic will form of MgSiO, and diopside each saturated 
with the other, and an excess substance diopside below the eutectic 
composition of 68 per cent. of MgSiO,, and MgSiO, above this ratio. 
This diagram also furnishes an example of a condition frequently 
met with in alloys, that of allotropy, or transitions below the solidifica- 
tion range. Thus above 1,190° the a CaSiO,, or pseudo-wollastonite, 
is the stable form, but is unknown in nature; below this temperature 
the S form, the mineral wollastonite, is stable. With MgSiO, the @ 
form is magnesian pyroxene occurring in meteorites and intergrowths 
with enstatite. At 1,365° this is transformed into the orthorhombic 
a form distinct from enstatite and unknown in nature. 
Microscopic examination confirmed these deductions except that 
the eutectic texture was rarely met with, probably due to the very 
slow cooling and consequent segregation of the constituents. 
EUTECTICS AMONG ROCKS 
Work of this general nature, but not so reliable and accurate, has 
been done by J. H. L. Vogt on the silicates, and he mentions a number 
of eutectic mixtures, as follows: 
68 per cent. diopside with 32 per cent. olivine 
74 per cent. melilite ‘“ 26 per cent. olivine 
65 per cent. melilite 35 per cent. anorthite 
40 per cent. diopside 60 per cent. akermanite 
74.25 per cent. anorthite 25-75 per cent. quartz 
75 per cent. albite 25 per cent. quartz 
(73 
THREE COMPONENT MIXTURES 
So far the discussion has been confined to the freezing of two 
component mixtures. However, more generally, particularly in 
