PhacoUte. 61 



Chabasito is, 3 Al Si^ + (Cal, N, K), Si- + 6 Aq. Rammelsberg is 

 inclined to regard Phacolite as a mixture of Acadiolito and Scolecite 

 (lime mesotype), the latter containing an additional atom of water.* 

 By uniting the atoms of both, ho thus states the chemical formula 

 for Phacolite; 2 R Si + Al^ Si^ + 10 H. As the analyses stand 

 (compare Berzelius's and Thomson's with the two just referred to), 

 Phacolite differs from Chabasito in containing three per cent, less of 

 silicic acid, and three atoms less of water. Now it is obvious, that 

 these differences are insufficient to authorise a separation of the two 

 minerals, unless there be a want of agreement in crystallographical 

 and other characters, greater than that as yet pointed out. An 

 equally valid reason could be urged for the separation of Acadiolito 

 from Chabasite, on the ground of a difference in their composition, 

 had not the examinations of Professor G. Rose proved an exact 

 agreement in the angles of their primary crystals. So also of Le- 

 vyne and Gmelinite, which are now admitted to be only varieties of 

 Chabasite, their occurring forms all being secondaries to the same 

 primary rhombohedron. The evidence of the identity of any two 

 minerals is best shewn by the incipient or intermediate passages of 

 one into the other, in the same specimen. I am not aware that, in 

 the case of the Irish or Bohemian Phacolite, such evidence has been 

 adduced ; no tendency of the sort is shewn in the specimens I have 

 examined from those countries. Now, one of my specimens from 

 New York has the distinct form of Chabasite (the perfect rhombohe- 

 dron), and of Phacolite (perfect double six-sided pyramids). The 

 first form, however, is rare; the incipient replacements are also 

 shewn; but these crystals have not the full perfection of waxy 

 lustre reflected by the ultimate form of Phacolite, — a singular effect, 

 attributable, probably-^ to the nature of the solvent in which the 

 molecules were suspended. 



Approach of twin-crystals to the Phacolite form. — These, as they 

 are sometimes presented, would, unless carefully examined, be mis- 

 taken for the true form of Phacolite. The most perfect specimens 

 I have seen, are from Nova Scotia. They consist of two rhombo- 

 hedrons united in the usual manner, each crystal turned half round, 

 but having their superior edges and lateral angles deeply replaced. 

 The approach to the form of Phacolite is thus produced ; the edges 

 and angles not standing out in relief, as they ordinarily do in these 

 twin forms. The striae, parallel with the edges of the two rhombo- 

 hedrons, so intersect as to shew the compound nature of the crystals. 

 Dr C. T. Jackson has a fine specimen of this variety from the Two 

 Islands, in Nova Scotia, of a wine-yellow colour ; I have another, 

 pure white, froni the same place. 



* See First Suppl. to his Handworterhuch, p. 112. It was on these grounds 

 that HoflFman proposed to separate acadiolite, as well as the Gustafsberg va- 

 riety, from chabasite — Poggendorft Annalen, xxv., 495. 



