Vol. LVII., No. 4. 
37 
THE ORIGIN, DISTRIBUTION AND MODE OF 
OCCURRENCE OF MICA IN CENTRAL 
AUSTRALIA 
By H. I. Jensen, D.Sc. 
(Three Maps and Five Text-figures.) 
{Read before the Royal Society of Queensland, 21th August, 1945; 
issued separately 20 th January, 1947.) 
I. DEFINITION OF MICA. 
Mica is mineralogically a general term applied to any one of a group 
of minerals which are of a very glistening lustre, and highly cleavable 
parallel with the basal plane. In commerce and industry the term is 
applied principally to one variety of mica, namely white mica or mus- 
covite, though the darker brownish phlogopite is also in great industrial 
demand. 
II. CHEMICAL NATURE OF THE MICAS. 
The true micas form a peculiar group of primary hydrous silicates, 
in which an alkali oxide enters into combination with an oxide of 
aluminium, iron, or magnesium, and with silica, and a little water of 
constitution. The alkali may be potassium, lithium or sodium. The 
sodium-mica, paragonite, occurs in rocks as fine pearly scales, sometimes 
forming a compact mass of mica rock, but it is of no commercial impor- 
tance as it does not form large plates. The lithium micas, lepidolite and 
zinnwaldite, are present as scales, or compact masses, in some gneisses 
and pegmatitic granites, but are of no commercial value, except for the 
extraction of lithium, for which purpose the occurrence would have to 
be exceptionally large since these micas contain only about 4 per cent, 
lithium. The commercially important micas are the potash micas, 
muscovite and phlogopite. Black mica, the iron-potash mica, mineralogi- 
cally called biotite, is of no commercial importance, although it frequently 
occurs in pegmatite dykes as very large crystals, up to 12 inches or more 
in diameter, and cleaves well ; it lacks the transparency and the dielectric 
properties which are measures of the value of mica. The micas in demand 
are muscovite, the potash-aluminium mica, and phlogopite, the magne- 
sium-potash mica. 
III. PROPERTIES AND USES OF THE COMMERCIAL MICAS. 
The properties which give mica its present day wide commercial 
use and high value are its perfect basal cleavage into sheets often as 
thin as a thousandth of an inch, transparency or translucency, flexibility, 
elasticity, lustrous sheen of cleavage faces, low thermal conductivity, 
infusibility, and, for electrical applications, its dielectric property of 
high resistance to the passage of electricity. This last property, in con- 
junction with its low thermal conductivity and infusibility, accounts for 
the high and ever-increasing demand for mica, which is absolutely 
essential to the electrical industries in the manufacture of condensers, 
telephones, dynamos, commutators, sparking plugs and various types of 
heating equipment, in which the wires, or heating elements, are coiled 
round the mica. 
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