OCCURRENCE OF MICA IN CENTRAL AUSTRALIA. 
41 
The phlogopite (-bearing) dyke, on underground examination, is seen 
to have a large number of sub-parallel partings, or major joints, dipping 
gently to the south-south-east. In the weathered zone solutions pene- 
trating along these joints have decomposed the olivine rock to form a 
pug consisting of magnesite, with nodules of magnesium silicate 
(saponite) . Along these major joints, and in neighbouring branch joints, 
the best books of phlogopite w T ere developed. Very abundant books of 
phlogopite also occurred in a pipe-like olivine formation, which went 
straight down only a couple of yards from the main shaft. It was missed 
in sinking the shaft. (See figure 3.) 
The parallel partings, joints, or floors in the dyke, which have per- 
mitted decomposition into secondary magnesian minerals, were probably 
also passages for solutions in the remote geological period when the 
pegmatites were injected. It would therefore appear from the study of 
this mine that phlogopite was not an original component of the dyke rock, 
but was formed by the action of solutions containing potash, emanating 
from granitic instrusions in the vicinity. Its occurrence in pipe-like 
bodies, as well as in proximity to the floors and joints mentioned, points 
to its being a pneumatolytic mineral. The occurrences of phlogopite 
rocks, consisting almost Avholly of phlogopite, are sporadic, and mostly 
near one or other of the walls. They are probably portions very severely 
affected by hot solutions rich in potash. In these places the phlogopite 
has not been able to grow freely into large books, but occurs as inter- 
grown crystals up to 1 or 2 inches in diameter. 
A little fine-grained phlogopite also occurs in similar association 
in a large basic dyke known as the Black Pinnacle in the Hart’s Range, 
and in metamorphosed limestone near Jenkin’s apatite deposit, on the 
Hart ’s Range road, but no large books seem to exist in either locality. 
The phlogopite mine (Johannsen’s) was developed at great expense, 
but was abandoned in 1944, as soon as overseas supplies of the mineral 
could again be obtained. Owing to the sporadic occurrence of good 
books of the mineral, even in the best portions of the mine, the expense 
of mining it was not warranted. It is of course quite possible that 
further development of the mine may lead to the discovery of richer 
A/ IV 
S£ 
Fig. 3. 
