240 ACCOUNT OF THE NATIVE HYDRATE OE MAGNESIA, 



In this state of our information respecting native magnesia, 

 Dr Hibbert, who has distinguished himself by his excellent 

 mineralogical survey of Shetland, and augmented our national 

 resources by the discovery of Chromate of Iron in large quan- 

 tities, put into my hands a mineral from Shetland, which had 

 been considered by mineralogists as White Talc, but which, he 

 was persuaded, differed materially in the nature of its ingre- 

 dients from that substance. In consequence of being familiar 

 with the Hoboken Magnesia, I considered the Shetland speci- 

 men as the same mineral ; and I put this opinion beyond a 

 doubt, by establishing the identity of their optical properties, 

 and also by a chemical examination of the two substances. 



Mineralogical Character. — The structure of Native Hydrate 

 of Magnesia is distinctly lamellar. The laminae sometimes 

 diverge from a central line, and frequently occur in groups, 

 with the laminae of one group inclined to those of another, 

 like the masses of mica in granite. 



The colour of the laminae is white, and a slight tinge of 

 green is sometimes observed, when we look upon their edges. 

 They are perfectly transparent when separate ; but I have no- 

 ticed in specimens exposed to the weather, a dull and white 

 opacity, which had been induced by the separation of the mi- 

 neral into a greater number of minute laminae. This white 

 part has the same relation to the transparent part as Albin has 

 to Apophyllite *, and, as happens with this mineral, the disinte- 

 gration follows the crystalline structure of the body. One 

 specimen of this kind exhibited a six-sided prism, the interior 

 of which was undecomposed, while all the external part had a 

 white opacity. 



The Native Hydrate of Magnesia scratches Talc, from 

 which it may be easily distinguished, as the former marks 

 white paper with a silvery powder, whereas the latter gives 



only 



* See Edin. Phil. Journal, vol. i. p. 5. 



