MINERALOGICAL N0TE3. — COOKSEY. Ill 



MINERALOGICAL NOTES. 

 By T. CooKSEY, Ph.D., B.Sc. 



(Mineralogist to Australian Museum). 



1. — Precious Opal prom 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 

 specimen 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. 



