Prof. Chapman's Mineralogical Notes. 143 



(7.) Wichtyne. Minerals of the Epidote Type. — Laurent's 

 wichtyne, the wichtisite of Hausinann, appears also to belong to 

 the epidote type — if it be not actually an altered variety of epi- 

 dote. It yields, however, by Laurent's analysis, 3RO, R 2 O 3 , 

 4Si O 3 , whilst the epidote contains 3RO, 2R 2 O 3 , 3Si O 3 . 



The epidote type may thus consist of the following minerals : — 

 allanite or orthite (including bagrationite, &c), gadolinite, epi- 

 dote, sphene, wichtyne, chloritoid and chlorite spar. The allan- 

 ite and epidote are strictly isomorphous, but their chemical 

 formulae are by no means alike. The former contains, in atoms, 

 3RO, AT 2 O 3 , 2Si O 3 ; the latter, 3RO, 2R 2 O 3 , 3Si 3 . Isomor- 

 phism, therefore, is no proof of kindred composition, or some 

 extended hypothesis must be adopted to meet the above case. If 

 we assume that S?- = 1R — id est, that one atom of silica, or of a 

 sesquioxide, = three atoms of RO — the difficulty vanishes, and 

 the two formula? enter of course under one common term. This 

 hypothesis is necessarily at present a purely gratuitous one, ad- 

 mitting, in fact, of the widest licence, and consequently of the 

 widest abuse ; but unless some hypothesis of the kind be, at 

 least provisionally, adopted, we cannot retain our existing formulae 

 and effect at the same time a satisfactory distribution of minerals. 

 Every fresh observation shows the insufficiency, for instance — 

 even if convenience plead for its retention — of the division of the 

 silicates according to the oxygen relations of their so-called bases. 

 Few mineralogists will now disallow the propriety of placing truly 

 vicarious or isomorphous compounds underthe saniecornniontype; 

 but the difficulty lies in the legitimate employment ofheterome- 

 rous isomorphism as a classification-element. That heteromero- 

 isomorphous compounds should in some cases be placed together 

 and in others be kept distinct, is, however, sufficiently evident ; 

 the grounds of union or separation constitute, therefore, the 

 question at issue. Besides crystallization characters, three other 

 elements should here be looked to ; — first, the general chemical 

 nature of the substance ; secondly, its other physical characters ; 

 and thirdly, its circumstances of occurrence. On these data I 

 would place phenacite and beryl with quartz, but not quartz with 

 cliabasite; acinite and augite, again, together, but not augite 

 with borax. Numerous other examples will readily occur to 

 those conversant with the subject. 



(8.) Chrome Tourmalines. — Many of the Siberian tourmalines 

 contain a small amount of chromium, probably as Cr 2 O 3 . These 

 ■pecimens are generally in acicular groups, and of an extremely 

 fine green colour. At first sight they might be mistaken for 

 actynolite; and, indeed, a specimen which I examined had a 

 label attached to it bearing that name. 11 = 70; sp. gr. =3"181. 

 Fusible. 



