MINERALOGY. 



75 



this group is (3 M"0, M 2 '"0 3 , 3 Si0 2 ), where M" represents an 

 atom of Calcium, Magnesium, Iron, or Manganese, and M'" an 

 atom of Aluminium, Iron, or Chromium ; the principal sub- 

 divisions are : — 



Grossularite . 3 CaO, A1 2 0 3 , 3 Si0 2 

 Pyrope . . 3 MgO, A1 2 0 3 , 3 Si0 2 

 .3 FeO, A1 2 0 3 , 3 Si0 2 

 . 3 MnO, A1,0 8 , 3 Si0 2 

 . 3 CaO, Fe 2 0 3 , 3 Si0 2 

 . 3 CaO, Cr 9 (X 3 SiO, 



Almandine 

 Spessartite 

 Andradite 

 Uwarowite 



and thus present great variations of chemical composition, 

 though the type remains constant. Garnets, however, are 

 rarely found to illustrate any separate subdivision, the usual 

 composition being that of a mixture of two or more of them 

 together in indefinite proportions. 



Again, while minerals differing very much in chemical com- 

 position may present almost exactly the same crystalline form r 

 on the other hand, many minerals having the same percentage 

 chemical composition present a quite different type of sym- 

 metry in the development of their crystals. Thus, to take a* 

 well-known instance, the carbonate of lime, having a compo- 

 sition represented by the formula CaC0 3 , occurs in two quite 

 distinct crystalline forms, sometimes appearing as calcite in 

 crystals belonging to a system with trigonal symmetry, some- 

 times as aragonite in crystals developed according to an ortho- 

 symmetrical type. 



A mineral collection, then, to be complete, must present 

 all the different varieties of chemical composition, and at the 

 same time illustrate the often very extensive varieties of 

 crystalline form assumed by the minerals of a species or 

 group. But besides these chemical and morphological features 

 other important characters have to be illustrated, among 

 which are the various modes of occurrence of a particular 

 mineral, including its associations with other minerals ; and in 

 a great national collection that is to illustrate the mineralogy 

 of the world, it is important that there be specimens from 

 all localities where the mineral occurs under special and note- 

 worthy circumstances ; and it must be a special object that 

 examples of each mineral species should show its most com- 



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