230 MINERALOGY 



metals of the same column, as lithium and rubidium, than it is to 

 the metals of the other column, as sodium. The elements lithium, 

 potassium, rubidium, and caesium are isomorphous in such simple 

 salts as the chlorides, nitrates, iodides., or sulphates ; but sodium is 

 isomorphous with these only in more complex compounds, as the 

 feldspars, pyroxenes, or amphiboles, and complex silicates gener- 

 ally. 



Copper, silver, and gold are isomorphous in their sulphides and 

 as elements. 



II. In the bivalent metals there are two distinct groups; the 

 first, calcium, strontium, barium, and lead, are isomorphous in their 

 carbonates, sulphates, silicates, and practically in all minerals. 



The second bivalent group is composed of calcium, magnesium, 

 manganese, ferrous iron, nickel, cobalt, zinc, and cadmium. Their 

 oxides are isomorphous in the spinel group, carbonates, arsenates, 

 tuffgstates, silicates. Like sodium in the univalent groups, cal- 

 cium is a connecting element in the bivalent group ; it is a member 

 of both, forming a carbonate which is rhombohedral, crystallizing 

 with the second group, and a second carbonate which is ortho- 

 rhombic, aragonite, crystallizing with the first group. The two 

 groups can replace each other to some extent in the complex 

 silicates, as the pyroxenes and amphiboles. 



III. The trivalent elements, with the exception of aluminium 

 and ferric iron, are not common ; these two replace each other in 

 such simple molecules as the, spinels ; Cr 2 Os is also included here. 

 They are found replacing each other throughout the silicates, 

 where Mn 2 Os and Ti 2 Os may be added, as in the garnets. 



IV. In the fourth group, TiO 2 , SnO 2 , ZrO 2 , SiO 2 , and ThO 2 are 

 found replacing each other in the rutile-cassiterite group of the 

 tetragonal system. In silicates Zr0 2 , TiO 2 , and SiO 2 are found the 

 more often replacing each other. 



V. In the group of pentoxides, phosphorus, arsenic, and vana- 

 dium replace each other,, as in the apatite group, and added to 

 these are antimony and bismuth, which are all isomorphous in their 

 sulphides and thiosulphates. 



VI. In the sixth group, sulphur, selenium, and tellurium are iso- 

 morphous in the bivalent state only. In the sexvalent state the 

 sulphates, molybdates, chromates, and tungstates are very closely 

 related. 



VII. In the fluorine group, fluorine itself stands apart from the 

 other members of the group, the three most important of which 



