222 MINERALOGY 



Aluminium 7.26 Hydrogen .94 Barium .05 



Iron 5.08 Titanium .41 Sulphur .04 



Calcium 3.51 Carbon .22 Nitrogen .02 



Magnesium 2.50 Chlorine .15 Strontium .02 



Chromium, fluorine, lithium, and uranium are all less than .02 and 

 probably greater than 0.01 ; bromine and all other elements are 

 each, in turn, less than .01. 



In a classification of the minerals their most important charac- 

 teristics must be considered, and the starting point of a natural 

 classification is without doubt one in which the chemical composi- 

 tion 4 and physical properties are the most important consideration, 

 though the latter to a large extent are derived from the elements 

 a mineral contains. Those minerals which are composed largely 

 of the same elements should stand in any scheme of classification 

 near together, especially since mineral species are mostly deter- 

 mined by the chemical tests for the elements which they contain. 

 The classification of minerals* will follow closely the natural classi- 

 fication of the elements themselves. It has been shown, especially 

 by Mendeleef, that the properties of the elements are determined 

 by their atomic weights, and from a consideration of this fact a 

 natural classification of the elements has been adopted which places 

 the elements in the order of their atomic weights. 



In a list of the elements placed in the order of their atomic 

 weights, starting with lithium, the first element which possesses a 

 well-developed chemical character, the atomic weight increases un- 

 til sodium is reached, with an atomic weight of 23. Sodium is an 

 element very similar to lithium in its chemical properties and very 

 different from fluorine, with an atomic weight of 19, directly pre- 

 ceding it. Sodium is then written in the column with lithium and 

 a second horizontal line or group is started. 



In each case, elements having similar or related chemical prop- 

 erties will fall in the same column, and these properties will be 

 repeated periodically. This whole scheme is known as the periodic 

 classification of the elements. 



Elements falling under each other in the same column of the table 

 of elements are of the same valence and are capable of replacing 

 each other in mineral molecules to a large extent, though this prop- 

 erty may not extend from top to bottom of the column in all 

 groups. In the first column or group, lithium, potassium, rubid- 

 ium, and caesium fall directly under each other. These are all alkali 



