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XXXIX. On the Optical Phenomena and Crystallization of Tour- 

 maline, Titanium, and Quartz, within Mica, Amethyst, and 

 Topaz. By Sir David Brewster, K.H., D.C.L., F.R.S., 

 and V.P.R.S. Edin.* 



[With a Plate.] 



T^HE existence of certain minerals imbedded in others, — the 

 optical phenomena which they exhibit, — their form and 

 mode of distribution, and the mechanical influence which has 

 been exerted during their formation on the mineral that contains 

 them, are among the most curious and instructive facts in phy- 

 sical science. 



The dissemination of perfectly-formed crystals of titanium, 

 both in the form of titanite and anatase, in Brazilian crystals of 

 quartz, is a fact so well known that I shall take no further notice 

 of it, but shall proceed to give an account of a series of facts of 

 a much more general and interesting character, which I have 

 had occasion to observe during an extensive examination of 

 minerals, undertaken with a different object. 



1 . On the Distribution of Tourmaline in Mica. 



When fluids and condensed gases are imprisoned in the cavi- 

 ties of topaz and other hard minerals, they retain their place till 

 some powerful agent releases them from confinement, or till heat 

 gives them such an expansive force as to burst the mineral. In 

 mica, however, where the laminae of which it is composed are 

 held together by a very feeble cohesive force, the fluids in their 

 cavities, and the extraneous materials which were present at 

 their formation, have experienced no difficulty in quitting their 

 place and spreading themselves between the plates of the mineral. 



Tourmaline and quartz, though thus distributed between the 

 laminae of mica subsequent to its crystallization, have yet found 

 a place in it contemporaneously with the crystallization of the 

 mica itself. In this case they are large crystals, equivalent in 

 thickness to many laminae, and may be taken out and subjected 

 to examination. Some of the crystals of tourmaline are so large, 

 indeed, that I have used them with their own natural faces as 

 analysing prisms ; and the quartz crystals, which are amorphous 

 and very irregularly formed, occupy a still greater space. In 

 both cases, however, the tourmaline and the quartz when taken 

 out leave large openings in the laminae, and have greatly dis- 

 turbed the structure of the mica around them. 



The crystals of tourmaline thus formed in the mica have 

 almost always the faces of the flattened hexagonal prism parallel 



* From the Transactions of the Royal Society of Edinburgh, vol. xx. 

 part 4. 



