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XXI. Mineralogical Notes. By Edward J. Chapman, Pro- 

 fessor of Mineralogy in University College, London*. 



UNDER the above title, it is the intention of the writer to 

 offer from time to time a series of remarks and investiga- 

 tions on subjects relating to mineralogy. 



(1.) Crednerite. — If we allow the isomorphism, atom for atom, 

 of silica and alumina, and through alumina, of silica and the 

 sesquioxides generally f, Rammelsberg's crednerite — the man- 

 gankupfererz of Credner — may be admitted into the augite group. 

 The cleavage form of crednerite is certainly monoclinic ; and the 

 angles, so far at least as they can be estimated in the specimens 

 hitherto obtained, assimilate to those of the augite prism. The 

 minerals of the augite type have the general formula 3RO, 

 2Si O 3 , which, with the substitution of sesquioxide of manganese 

 for silica, is exactly that of crednerite as deduced by Rainmels- 

 berg, viz. 3(CuO, BaO), 2Mn 2 O 3 . A proof of the isomorphism 

 of Si O 3 and Mn 2 O 3 is afforded by the manganese garnets and 

 spinels, and more particularly as belonging to a system of vari- 

 able angles, by Vesuvian and Hausmannite. 



(2.) Helvine. — The helvine in the classification of Mohs bears 

 the name of tetrahedral garnet ; and with the spinel and garnet 

 group it must in fact be placed, unless it stand alone, for to no 

 other type amongst the silicates and their isomorphs can it be 

 referred. On the supposition that Si O 3 and Mn 2 O 3 are iso- 

 morphous, and that sulphur and oxygen are equally so, the 

 atomic constitution of the helvine falls into the common garnet 

 formula, r + R. The composition of helvine, for instance, as 

 usuallyrepresented, = (Be 2 3 ,Fe 2 3 ), SiO 3 + 2MnO, Si0 3 +MnS. 

 This may be reduced into three atoms of (MnO, MnS), and three 

 atoms of (R 2 O 3 , Si O 3 ) ; or into equal atoms of base and acid, 

 r + R. 



(3.) False cleavage in Garnet. — A small garnet in the author's 

 possession, from the Zillerthal, exhibits a peculiar and interesting 

 example of false cleavage in relation to the crystalline structure 

 of metamorphic rocks. This garnet is a combination of a tra- 

 pezohedron, ^O, with the rhombic dodecahedron ; and entirely 

 through its mass run lines of false cleavage, parallel to the clea- 

 vage planes of the mica-slate in which it is imbedded. 



* Communicated by the Author. 



t See a paper by tin- Author on the isomorphous relations of silica and 

 alumina, iu the report of the British Association for 1850. 



