222 



THE GEOLOGIST. 



Hematite (¥e) crystallize in nearly identical forms : the same is the 

 case with Rock-salt (Na CI) and Fluor (Ca Fl) ; and the Carbonates of 

 Lime, Calcite (Ca C), of Magnesia, Magnesite (Mg C), and of Protoxide 

 of Iron, Chalahite (Fe C), have also similar crystalline relations. What 

 is the case with these, and numerous other minerals occurring 

 naturally, also holds good with many other bodies, which are only 

 produced artificially in the laboratory. 



As isomorphous bodies are found to have the same form when they 

 crystallize in an un combined state, so it is likewise found, when they 

 enter into combination as the components of substances of a more 

 complicated chemical nature, that the isomorphic property is still 

 continued, and that they are capable of mutually replacing each other 

 without causing any essential change in the crystalline relations of the 

 compound substance. For example, Al and Fe are isomorphous, as 

 are likewise Ca and Fe. In the mineral Garnet, one variety is thus 

 composed : Si 40 per cent., Al 25 per cent., and Fe 33 per cent. In 

 another variety the Al and Fe are wholly replaced by their respective 

 isomorphs Fe and Ca, the composition of the mineral being Si 40 per 

 cent., Fe 29 per cent., and Ca 30 per cent. : and all this substitution 

 without any change of crystalline form. 



Although isomorphic replacements do not alter the essential form 

 of a mineral (which is its crystalline form), they often give rise to 

 considerable changes in its other physical characters. This is pecu- 

 liarly the case when the oxides of the heavy metals replace the earths 

 and alkalies, giving rise to changes particularly in colour and specific 

 gravity. The many varieties which characterise some mineral species 

 are generally due to this. See the cases of Epidote, Garnet, Augite, 

 Hornblende, &c. 



Among the large number of isomorphous bodies enumerated by 

 mineralogists and chemists, those that concern us in rock-minerals are 

 comparatively few. Those most worthy of attention are : — 

 Elements Fl, CI. 



Compounds of 1 atom of base united c a. — Ca, Mg, Fe, Mn, K, Na, Li. 



with 1 atom of oxygen .... I h. — Ca (as Atragonite), Ba. 

 Compounds of 1 atom of base united ] ... 



with 3 atoms of oxygen . . . , / 



