OCCURRENCE OF MICA IN CENTRAL AUSTRALIA. 
47 
This theory of the origin of mica is the only one which will stand 
critical examination in the light of facts observed in the mines. The 
order of crystallisation in the mica-rich pegmatites is therefore : — 
1. Massive quartz. 
2. Eutectic quartz and felspar. 
3. Felspar, 
4. Muscovite with or without residual quartz. 
Crass Seel ion of k'ismef Dyke showing Mica 
developed on wat/s and in bunches along jomfs . 
Tig. 4. 
( b ) Effect of Country Rock. 
The nature of the dyke retinue of a parent intrusion of large size is 
largely dependent on the character of the rocks invaded. If the latter 
contain great beds, or bodies, of limestone, which is calcined, silicated 
and partly assimilated by the granitic mass, differentiation is promoted 
into dioritic and acid, calcic and alkaline granites, giving off dykes of 
different kinds. But the most important effect, from the point of view 
of mica-rich pegmatites, is that the alteration of limestones in the contact 
charges the acid portions of the granite differentiates with water and 
carbonic acid. These constituents favour the production of pneuma- 
tolytic dykes. 
The pegmatite dykes themselves are also affected by the rocks which 
they encounter in working their way up either through a fissure or by 
the stoping process. Where they pass through limestones the consolida- 
tion and crystallisation of potash is delayed and the production of mica 
favoured above the calcic formation. 
In the Spotted Tiger Mine, the most productive mica mine in the 
Territory, the pegmatite seems to be richer in book-mica where it lies 
between biotite gneiss walls than in the acid gneiss and quartzite. (See 
figure 5). This has also been a matter of general observation with most 
of the mines. Almost all the best mines are situated on pegmatites 
intruding basic gneiss and amphibolite. This is probably due to two 
causes — firstly, the open texture of the basic gneisses and amphibolites, 
which permits pent up carbonic acid from the intrusion to invade the 
formation; and, secondly, the greater susceptibility of basic minerals to 
alteration by carbon dioxide and moisture — they become carbonated and 
chloritised. 
Dykes in very acid gneisses rarely contain payable mica, except when 
the gneisses are interbedded with limestone or amphibolite and these 
rocks occur nearby. 
