MINER ALOGICAL NOTES. — COOKSEY. 
Ill 
MINERALOGICAL NOTES. 
By T. Cooksey, Ph.D., B.Sc. 
(Mineralogist to Australian Museum). 
1. — Precious Opal from White Cliffs, N.S.W. 
On examining a cut and polished specimen of a fossil-bearing 
ferruginous sandstone boulder from White Cliffs, N.S, Wales, I was 
struck by the appearance of the precious opal which had replaced 
the carbonate of lime of the shells. The rock is permeated with 
the opal, and particularly when polished has a very beautiful appear- 
ance. The minute surfaces in the interior of the opal which 
produce the play of color, when viewed with a lens, appear to be 
quite flat and terminated by perfectly straight edges often parallel. 
On a closer examination under the microscope with reflected 
light, the appearance in many places was strikingly similar to 
that of a section of crystalline marble viewed with crossed nicols. 
The light and dark banded appearance due to twinning in the 
marble was perfectly imitated in the opal and on rotating the 
S23ecimen on the stage the bands became alternately coloured. 
As the play of colour in the opal is produced by minute cracks in 
its substance, the planes of colour seen by reflected light are there- 
fore produced by cracks which apparently occupy the same position 
as the cleavages of the calcite displaced by the opal ; occasionally 
the traces of cleavages could be seen distinctly on one of the 
bright surfaces and the angles formed by their intersection were 
approximately those found in calcite. Other portions again 
showed a somewhat fibrous structure. In many places on focussing 
into the substance of the opal these cleavage planes could be 
distinctly seen, and the rhombic forms produced by them were so 
exactly similar to those obtained by cleavage in calcite that a 
doubt as to their origin seemed out of the question. 
From the above observations it is evident that the carbonate of 
lime of which the shells were originally composed had first been 
converted into crystalline calcite (by which all shell structure had 
necessarily been lost), and then the calcite replaced by opal. The 
latter had also reproduced the cleavages of the former, and it is these 
that cause the play of colour which gives to the opal its precious 
character. Cracks or fractures of a conchoidal form are also 
present and also produce colour by reflected light but the brilliancy 
of the specimen for the most part results from the presence of 
these characteristic cleavages. 
2. — Basic Sulphate of Iron from Mount Morgan. 
A specimen supposed to have been a fossil bone was sent to this 
Museum for examination and identification by Mr. R. L. Jack, 
Government Geologist for Queensland, he having received it from 
Mount Morgan. 
