268 Prof. J. P. Cooke on the Vermiculites. 



We have in our possession a plate of Elba tourmaline cut per- 

 pendicular to the axis, in which the polarizing microscope shows 

 on different zones a separation of the hyperbolas, which amounts 

 in some positions to 8 degrees ; and in moving the plate across 

 the field the optical divergence varies precisely as on plates of 

 phlogopite and vermiculite. There is certainly no external evi- 

 dence of lamination on tourmaline crystals ; for the mineral is 

 remarkably compact, and the crystals have not even a basal 

 cleavage : but it will be remembered how readily some of the 

 varieties pass by alteration into micas of the magnesian type ; 

 and this change to a foliated structure, in which the lamination 

 is parallel to the base of the original hexagonal crystal, may be 

 facilitated by a grouping of the molecules of the tourmaline in 

 the manner represented by fig. 12. 



We have also a plate of amethystine quartz, in which a beam 

 of parallel polarized rays of light exhibits a twinning almost as 

 symmetrical as that shown in fig. 10, the three zones being most 

 beautifully mapped out by the alternating bands of right- and 

 left-handed quartz, which are such a familiar phenomenon of 

 these crystals. But, besides this, in each of these zones, near the 

 border of the plate, can be distinguished a biaxial structure with 

 an optical divergence of several degrees ; and on one other plate 

 of amethyst we have had an opportunity of examining, we have 

 also seen under the polarizing microscope the biaxial curves at 

 one or more points. 



These facts most distinctly suggest the theory that the optical 

 phenomena of quartz are produced by a molecular structure 

 similar to that by which we have obtained identical phenomena 

 in our artificial plates of mica, and that the two orders of crystals 

 are aggregates of compound molecules, whose parts are twinned 

 together in the one case in right-handed, and in the other in 

 left-handed spirals, and, lastly, that the simple molecule, if de- 

 veloped normally, would produce a biaxial structure*. This 

 theory is most markedly in harmony with the chemical relations 

 of silica. The compound SiO 2 is the only one of the tetrad 



* Since the above was in type we have received the Amer. Journ. Sci. 

 (IV.) February 1874, containing a description of the rhombic silica which 

 Professor Maskelyne, of the British Museum, has discovered in the meteo- 

 rite of Breitenbach. This new species of silica, which Professor Maske- 

 lyne calls Asmanite, has the form of a right rhombic prism, with an angle 

 of 120° 20', and the crystals are optically biaxial; but while the specific 

 gravity of quartz is 26, that of Asmanite is said to be 2 - 245. It is perhaps 

 to be expected that such a molecular macling as we have described would 

 determine an increase of density, since thereby three molecules coalesce to 

 form one; or it is possible that the remarks made infra in regard to 

 calcite apply also to quartz ; but still the marked difference remains to be 

 explained. 



