OCCURRENCE OF MICA IN CENTRAL AUSTRALIA. 
39 
Mica therefore possesses a unique combination of properties, 
unequalled by any other natural or manufactured substance. As the 
electrical industry is constantly expanding with the march of civilisation, 
the demand for mica is constantly increasing. In ancient and medieval 
times muscovite, or white mica, was in limited demand — on account of 
the size of sheets obtainable — for making windows and skylights. With 
the expansion of the metallurgical industry mica came into use for 
making windows in furnaces to enable the operator to see the progress 
of smelting, but even a generation ago the demand for muscovite was 
only moderate. To-day production cannot supply the demand. Con- 
sequently thinly split scrap mica, powdered mica, and fine mica from 
the concentration of mica in river sands (recovered by dredging or 
sluicing) are to-day employed in the manufacture of micanite, which is 
composed of fine mica plates, or powder, cemented with certain shellacs 
and gums, and compressed into sheets. Micanite can be used as a 
dielectric where temperatures are not excessively high. 
Ground mica is also used as backing for rolled asphalt roofing, to 
prevent sticking ; also to give lustre to paints and wallpaper, to give a 
finish to stucco and concrete, and as a dusting material to prevent rubber 
goods from adhering to the moulds in which they are cast, and for many 
other purposes. 
A decade ago the market for mica was precarious, and in the hands 
of speculators. The miner who faced hardship and risk did not get a 
fair deal. These conditions are not likely to recur. 
IV. MODE OF OCCURRENCE AND ORIGIN OF THE 
COMMERCIAL MICAS IN CENTRAL AUSTRALIA. 
Phlogopite and muscovite are very different in their mode of 
occurrence and mineral associations. 
A. Phlogopite is a much rarer mineral than muscovite. It is in 
great demand for the manufacture of aeroplane spark-plugs and com- 
mutators. 
While muscovite is a widespread mineral in acid igneous and meta- 
morphic rocks, phlogopite is associated with basic igneous and 
metamorphic rocks, and occurs in them only sparingly and sporadically. 
There is only one phlogopite mine in Central Australia. It is 
situated on the southern slope of the Strangway Range, a spur of the 
Hart’s Range. (See plate I.) It was opened up by the Government in 
1943, essentially because supplies of phlogopite from Madagascar and 
Canada were cut off through the war with Japan. The mineral was badly 
needed for Australian aeroplane manufacture. 
The occurrence of phlogopite in this locality had been discovered 
some years earlier by a prospector named Johannsen. The find attracted 
some interest, and several government geologists had cursorily reported 
on it before me, but they did not have the advantage of examining the 
deposit after it had been properly opened up. 
The phlogopite occurs in a dunite, a rock composed wholly of olivine. 
The. geologists referred to above, Owen and Sullivan, regarded this 
dunite as an alteration product of dolomite, which according to them, 
had been intruded by a basic dyke (amphibolite). They attributed the 
