232 MINERALOGY 



placing all groups of isomorphic elements within brackets, and the 

 most important element in each group first. It is always under- 

 stood that elements thus written in a mineral formula may replace 

 each other, and they are always written in the order of their im- 

 portance in that mineral molecule. If it is wished to still further 

 simplify the above formula, it is at once seen that both titanium 

 and manganese are present in only very small amounts and may be 

 neglected in the formula. 



Classification of minerals. Any natural classification of min- 

 erals must consider both the chemical and physical properties of 

 the mineral, but as the physical properties depend to a large extent 

 upon the elements present, the chemical relations of the elements in 

 the mineral molecule are given more weight in the scheme of classi- 

 fication than the presence of any one element. Minerals are therefore 

 classified as chemical compounds, following the order of the natural 

 classification of the elements, but the acid radicle will determine 

 the group ; for example all sulphates are placed in the same large 

 group. The subheads are determined by the character of the salt ; 

 whether it is anhydrous, basic, acid, or hydrated, as well as by a 

 consideration of its crystalline type. By this method of classifica- 

 tion those minerals which form natural isomorphous groups are kept 

 together ; for example, barium, strontium, calcium, and lead sul- 

 phates are placed in the same group of anhydrous sulphates, as 

 they should be, since they crystallize in the same type and are 

 isomorphous. If they were classified from their basic or metallic 

 elements, they would be widely separated in four different groups, 

 even though they are all found in nature under the same condi- 

 tions and often several are mixed in the same crystals. 



The system of classification here adopted is that of Dana, which 

 may not be in all respects the most desirable, yet most of the min- 

 eral collections in the American museums follow this system in their 

 arrangement. Not taking into account the hydrocarbons, there 

 are eight large groups in this scheme, as follows : 



I. Native elements. Includes the elements which occur in na- 

 ture in an uncombined state, as gold, copper, silver, sulphur, dia- 

 mond, etc. 



II. Sulphides, including the arsenides, antimonides, and other 

 similar compounds. Mostly salts of hydrogen sulphide, H 2 S, 

 as galena, PbS ; also including the corresponding compounds of 

 tellurium and selenium. 



III. Sulpho-compounds, as the sulpharsenites, sulphanti- 



