ON KAOLINITE FROM THE HAWKESBURY SANDSTONE. 173 



detected in all [improperly prepared] kaolin,' on which Johnson 

 and Blake, followed by Dana, founded their orthorhorabic mineral 

 Kaolinite, do not appear to have been separated and analysed." The 

 italics are mine. 



I have thus taken this opportunity of investigating and analys- 

 ing what I consider to be Kaolinite, and which is seen to be under 

 the microscope, entirely composed of six sided crystals and plates 

 which have cleaved off. Unfortunately the quantity at my dis- 

 posal was small, but I have still sufficient for a good number of 

 microscopic slides. The mineral I found in a little pocket in the 

 sandstone at Marrickville, near Sydney, with a small quantity of 

 coal, which coal was blocking up the entrance. Being in the 

 solid rock with nothing like a crevice near, it was well protected 

 from all impurities. When first found it much resembled an 

 accumulation of minute scales of mica ; it had a pearly lustre, an 

 unctuous feel, a mealy appearance, and was of a very light cream- 

 colour — almost white. A trace of iron was removed by dilute 

 hydrochloric acid, but the colour of the thus treated substance 

 was not altered in any perceptible degree. 



Under the microscope it appears to be composed of six sided 

 crystals, the basal cleavage of which is so perfect that the whole 

 crystal appears to separate into plates. Where the plates are 

 isolated they are perfectly transparent, and where two or three 

 are placed one above the other, the edges of the lower are distinctly 

 seen through the upper plates. The general appearance is repre- 

 sented in Plate xxiii., fig. 1. 



The angles of the crystals are, as pointed out by Descloizeaux, 

 120°, and if the faces are extended till they meet, the resulting 

 angle is 60°; several plates that I measured gave these as the 

 correct figures, or very near it. 



Under the blowpipe it exfoliates and increases considerably in 

 size. It commences to expand at once, glows with a brilliant 

 light, and becomes intensely white. With cobalt-nitrate solution 

 gives a fine blue colour. 



