OCCURRENCE OF MICA IN CENTRAL AUSTRALIA. 
45 
crystallise. The pegmatites have formed at temperatures between 
],000° C. and 500° C., the last solutions of potassie mother-liquor pro- 
bably crystallising as mica and quartz at the last-mentioned lower 
temperature. 
In crystallising, the order of crystallisation of pegmatites depends 
on the following combination of factors: — (a) specific gravity, (&) for- 
mula or molecular volume, (c) oxide volume, (d) pressure and tempera- 
ture and (e) stress. 
( a ) and (b). Goodchild’s researches into the physical chemistry of 
rock-forming minerals proved that minerals with the highest specific 
gravity and lowest formula volume crystallised out first. This accounts 
for the early crystallisation of minerals like rutile, magnetite, tourma- 
line and other minerals occurring as inclusions in pegmatite minerals, 
though most of these commenced to form early enough to be enveloped 
by and retained in the parent granite. 
(c) As regards the felspars, Goodchild found that the oxide volume 
was the most important factor controlling the order of their formation. 
Lime, with an oxide volume of 16-4 to 23-4, enters into combination before 
soda, with an oxide volume of 22 to 22*7. Therefore most of the lime 
is retained in the parent granite. But, as potash has the greatest oxide 
volume of the common alkalies, it remains in solution longest and is the 
last to enter into combination. 
The mother-liquor from which the pegmatites are formed consists 
mainly of soda, potash, silica, and the various mineralisers already men- 
tioned which hold in solution alumina, iron, and traces of other heavy 
metals. As the oxide volume of quartz (silica) is only 22*7 to 27, it is to 
be expected from Goodchild’s work that it would start to enter into 
combination about the same time as the lime and soda and before the 
potash, at least in static solutions. In the Hart’s Range pegmatites 
oligoelase and microeline, the soda felspars, almost always formed before 
the potash felspars. 
(d) Pressure and temperature become important factors in the 
sequence of crystallisation. As stated before, carbon dioxide is, with 
alkali, responsible for holding large quantities of silica in solution. 
While the dyke is stoping its way upwards in plastic metamorphic rock 
of the deepest zone, it is under very high temperature and pressure, and 
the mineralisers are confined in it. As soon as it reaches cooler and 
fractured rocks — like those of Grubenman’s Middle Zone — the carbonic 
acid is enabled to escape together with much of the water and haloid gas. 
The lowering of pressure, and concomitant lowering of temperature, and 
the loss of the solvent gases, cause an immediate precipitation of silica, 
which then closes the faults or cracks. The cooling of the magma now 
brings about further precipitation of quartz and felspar as a eutectic 
and later the formation of felspar. At this stage excess of silica has 
been reduced. The closing of fissures causes a renewed rise of tem- 
perature and pressure, and the hydrous potash-rich residual magma 
crystallises as quartz, felspar and mica, but the excess potash and the 
remaining mineralisers hold in solution silica and alumina, but not in 
the proportion needed to form orthoclase. Then it is that this solution 
with falling temperatures precipitates mica and even corrodes felspar 
and sometimes the wall rocks, to abstract the alumina necessary to 
crystallise out as mica and quartz. 
