
XXXVIL.—On the Optical Phenomena and Crystallisation of Tourmaline, Titanium, 
and Quartz, within Mica, Amethyst, and Topaz. By Sir Davin Brewster, 
K.H., D.C.L., F.R.S., and V.P.R.S. Edin. (With a Plate.) 
(Read 4th January 1853.) 
The existence of certain minerals imbedded in others,—the optical phenomena 
which they exhibit,—their form and mode of distribution, and the mechanical in- 
fluence which has been exerted during their formation on the mineral that con- 
tains them, are among the most curious and instructive facts in physical 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 farther 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 fiuids and condensed gases are imprisoned in the cavities 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 laminze of which it is com- 
_ posed are held together by a very feeble cohesive force, the fluids in their cavi- 
ties, and the extraneous materials which were present at their formation, have 
experienced no difficulty in quitting their place, and spreading themselves be- 
tween the plates of the mineral. 
Tourmaline and quartz, though thus distributed between the laminze of mica 
subsequent to its crystallisation, have yet found a place in it contemporaneously 
with the crystallisation of the mica itself. In this case they are large crystals, 
_ equivalent in thickness to many laminze, 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 lamine, and have greatly disturbed 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 to the laminze of the mica. I have 
found, however, a few cases in which the flat summit of the hexagonal prism is 
parallel to the laminee. The crystallisations of quartz have also the axis of the 
prism, or its hexagonal faces parallel to the lamine. 
VOL. XX. PART IV. 71 
