Till. RELATION or MINI .IJAl.s To Till! KLKMK.N 



:i ine.-isurement of the angles. The angles are then a function of 

 the chemical composition. 



Mrsititr is ;i aatunilly occurring carbonate, forming crystals 

 in which iron :md magnesium carbonates have crystallized in pro- 

 portions of two of magnesium to one of iron, and they should each 

 h;ive tin- >.nm proportional influence on the crystalline angles: 



r A r 



145 12' 

 73 



Mesitite, 2 MgCO 3 , FeCO 8 2 MgCO 3 = 



FeCO, = 



218 12' H- 3 = 72 46' ; 

 the measured angle is 72 46'. 



The increase or decrease of the topic parameters with the addi- 

 ion of isomorphous substances in the molecule may not be the 

 same in all directions. These parameters of mixed crystalline 

 substances will increase more rapidly in one direction with the 

 increase of the percentage of certain elements than in others. This 

 influence will have more effect upon molecules which are compara- 

 tively simple in their structure than on those which are complex ; 

 and compounds which are isomorphous in complex mineral mole- 

 cules may not be able to replace each other in such simple mole- 

 cules as the chlorides. Thus potassium and sodium are isomor- 

 phous in many silicates, yet their simple chlorides crystallize in 

 different types of symmetry. 



Owing to the unequal increase in the topic parameters, complex 

 isomorphous groups, as the pyroxenes and amphiboles, may pass 

 through three entire cystallographical systems. 



Generally the physical properties of mixed crystals, or those 

 formed by isomorphous groups replacing each other, will stand as a 

 mean between the properties of their constituents ; and, in the strict 

 sense of the term, compounds are isomorphous when the physical 

 properties of their mixed crystals are continuous functions of their 

 chemical composition. 



Elements which stand in the same column in Mendeleef's table 

 }f the elements, or those of the same group, are usually isomor- 

 phous, especially in complex mineral molecules, and those which 

 fall directly under each other in the odd and even groups are iso- 

 morphous to a great extent in simple molecules. 



In group I, the alkali or univalent metals, it will be noted that 

 lithium is written on the left, while sodium is written on the right 

 and not directly under lithium. Two columns of elements are 

 t hu> formed ; any metal, as potassium, is more closely related to the 



