GUIDE TO THE MINERALOGIC COLLECTIONS 45 



mineral species are compounds of two or more elements united 



according- to the laws of chemical combination. 



A few predominant chemical compounds make up the greater 

 part of the earth's crust. Of these, silica (Si0 2 ), a combination 

 of silicon and oxygen, is the most important. This forms quartz 

 and its numerous varieties, amethyst, agate, flint, etc.; and, com- 

 bined with other elements, often with an extremely complicated 

 chemical composition, silica makes the great group of silicates, 

 which includes the larger number of the common rock forming 

 minerals. Oxygen combined singly with an element forms 

 another great group, the oxids to which many ores, such as 

 those of iron, belong. Combined with aluminium oxygen forms 

 alumina (Al 2 O a ), a common mineral; and this combined with 

 silica is the base of our clays and an important rock constituent. 

 Oxygen with carbon and some other elements forms the carbon- 

 ates to which limestone belongs; with sulfur and some other ele- 

 ments it forms the sulfates (gypsum, etc.); and with phosphorus 

 and another element the phosphates. Sulfur, without oxygen, 

 combined with an element forms a sulfid, fluorin a fluorid, 

 chlorin a chlorid, etc. 1 



The most satisfactory classification of mineral species is that 

 based on chemical composition. Under sections having a similar 

 chemical composition, species are divided into groups which 

 usually embrace minerals closely allied crystallographically. 

 Throughout the succeeding section the chemical composition of 

 each species is given in words and symbols, which, while appeal- 

 ing specially to the chemist, can be readily understood by those 

 who bear in mind that in each mineral those elements are found 

 whose abbreviations appear in the symbol. Numbers below the 

 sign indicate the relative number of atoms of each element. 

 Example, realgar is a sulfid of arsenic, and the signs of sulfur 

 1S1 and arsenic (As) appear in its symbol (AsS); or, there is one 

 atom of sulfur and one of arsenic united, but arsenic is rela- 

 tively heavier than sulfur (see table of elements in appendix) 

 therefore the composition by weight is in percentages: sulfur, 

 29.9; arsenic, 70.1; 100. 



Isomorphism, dimorphism, etc. 



It has been found in a number of cases that mineral species 

 so related by chemical composition us to form purl <>f nne of the 



T.'irr. Kaliili S. Economic -eolo-y uf the U. S. 1894. 



