142 Prof. Chapman's Miner alogical Notes. 



(4.) Phenacite and Beryl. — Phenacite was at first mistaken 

 for quartz, hence the derivation of its name ; it is, however, very 

 closely related to that suhstance. Both quartz and phenacite 

 are hemihexagonal : in the former, the relative length of the ver- 

 tical axis =1*095 j in the latter, 05471 ; so that the common 

 phenacite rhombohedron = |R compared to the quartz form as 

 unity. In beryl, again, from the two triaxial pyramids occasion- 

 ally present in that mineral, we obtain for the relative length of 

 the vertical axis in the protaxial form, the values 0-9968 and 

 0"4980. The first is in close accordance with that of the prot- 

 axial form of quartz. 



Phenacite and beryl may therefore be considered members of 

 the quartz group, in which, if glucina be looked upon as a ses- 

 quioxide, there can be no difficulty in placing them. On the 

 other hand, if the formula of glucina be written BeO, the iso- 

 morphism of 3BeO with Si O 3 , and the sesquioxide isomorphs of 

 the latter, must be allowed. Of the other glucina compounds, 

 the chrysoberyl may represent a trimetric, and the euclase a 

 monochnic quartz. With the former are associated staurolite, 

 andalusite, topaz, &c. Helvine, as shown in note 2, belongs to 

 the garnet type. 



(5.) Sphene and Epidote. — If sphene constitute not a type of 

 its own, the only group to which it can be referred is that of the 

 epidote series. In epidote, the general formula — uniting the 

 Si 3 and R 2 O 3 — becomes r 3 R 5 . In sphene, the number of 

 atoms =3CaO, 3Ti0 2 , 2Si0 3 ; and by uniting the acids, we ob- 

 tain equally with epidote the formula r 3 R 5 . 



That silica and the sesquioxides are at times isomorphous with 

 titanic and stannic acid, we have evidence in the isomorphism, 

 on the one hand, of idocrase, hausmannite and anatase ; and, on 

 the other hand, of zircon, rutile and cassiterite. All of these 

 forms belong to the dimetric system, but their axial relations 

 separate them into two distinct groups. 



(6.) Chlorite spar and Chloritoid. — These minerals, which 

 closely resemble each other, may be looked upon as allied to 

 epidote. Chlorite spar is, in fact, an iron epidote. Erdmann's 

 analysis gives in atoms 3FeO, 3A1 2 3 , 2Si0 3 , = r 3 J R 5 . 



The chloritoid, according to Bonsdorff's analysis, contains 

 3RO, 2A1 2 3 , 2Si O 3 , 3H 2 0. If we admit that three atoms of 

 water may replace one atom of silica, or of alumina, these num- 

 bers produce, as before, the formula i^R 5 . It may be remarked 

 in support of the above suggestion, that Rammelsberg considers 

 one of the iolite metamorphs — the praseolite — to be an iolite in 

 which one atom of silica has given place to three atoms of water. 



