Prof. J, P. Cooke on the Vermiculites. 269 



oxides which crystallizes in the hexagonal system ; and ever 

 since, by the study of the organic compounds of silicon, the 

 quadrivalent character of the element has been made evident, 

 this fact has been a striking anomaly in our chemical classifica- 

 tion. Assume, however, that the molecule SiO 2 would develop 

 normally into a rhombic structure, and that the hexagonal form 

 of quartz is solely a result of molecular twinning, and the ano- 

 maly disappears. The molecule SiO 2 may be approximately of 

 the same form as the molecule TiO 2 in Brookite ; but having 

 the exact dimensions and polar conditions which favour the 

 mode of molecular twinning, described above and represented by 

 fig. 12, it may always develop into hexagonal shapes. 



Are, then, all hexagonal forms thus closely related to the 

 rhombic systems of crystals ? And do all molecules of the di- 

 mensions and polar conditions illustrated by the figures of this 

 article (that is, those which correspond to the rhomb of 60° and 

 120°) usually develop into hexagonal forms ? May not the 

 whole difference between an hexagonal and a rhombic form arise 

 from a slight difference of dimensions, which determines a mo- 

 lecular macling in the one case, and a normal development of 

 the single molecules in the other ? These questions point out 

 most interesting lines of investigation, and will recall to the mi- 

 neralogist a number of facts bearing upon the subject. Allow 

 me to refer to two of the most striking and most obvious. 



On the crystals of chrysoberyl the rhombic angle is 119° 46'; 

 and every mineralogist is familiar with the hexagonal macling, 

 similar to fig. 7, which is so very characteristic of this species*. 

 Corundum differs chemically from chrysoberyl in that a portion 

 of the alumina in the former is replaced by glucina in the latter. 

 Corundum has a perfect hexagonal form ; and, fundamentally* 

 may not the only crystallographic difference be that, in conse- 

 quence of the replacement, a rhomb of 120° changes to a rhomb 

 of 119° 46' ? Now we have a plate of macled chrysoberyl, show- 

 ing the normal wide divergence of the optical axes at certain 

 points on its borders, and a nearly uniaxial structure at the 

 centre, where there is an obvious interpenetration between the in- 

 dividuals of the made, and where the superposition of the several 

 laminae is most beautifully shown by a polarized beam of parallel 

 rays. We have also a section of a corundum crystal, presenting 

 phenomena similar to those seen with the plate of tourmaline, 

 described above. Further, we have observed like phenomena on 

 a section of phenacite; and although the last mineral contains 

 silica, yet if the molecules of SiO 2 are crystallographically equi- 

 valent to those of Al 2 O 3 , it may be that the molecular structure 



* See also Dana's * System of Mineralogy,' fifth edition, figs. 154, 155, 

 p. 156. 



