364 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



It will be seen from these analyses that tourmaline is a silicate of very complicated 

 composition ; in fact, no other precious stone is so complex in character ; in this respect, 

 therefore, there is a marked contrast between tourmaline and diamond, the composition of 

 the latter being the simplest possible. No tourmaline has all the fourteen elements indicated, 

 and those which do occur are always present in variable amounts ; this tends to make the 

 constitution still more complex. Besides silica, there is always present boron, aluminium, 

 magnesium, sodium, potassium, and water, the last of which is expelled only at a red-heat. 

 Fluorine is very rarely absent, but lithium and manganese enter into the composition of only 

 a few tourmalines ; iron, as ferrous oxide, is an important, but variable, constituent, 

 which may bp absent, present in small amount or in considerable amount, as much as 20 per 

 cent. Tourmalines containing a large amount of iron are always dark in colour and 

 imperfectly transparent, frequently quite black and opaque, and are therefore unfit for use 

 as gems. 



The number of the constituents of tourmaline, together with the difficulty of accurately 

 determining the proportions in which they are present, and the fact that these proportions 

 vary in every specimen that is analysed, make it impossible to arrive at a chemical formula 

 which satisfies every condition and is unexceptionable in every way. Every formula hitherto 

 proposed is based to a certain extent on supposition, and no single one has been generally 

 accepted as final. The first two analyses quoted above have been recently made (1899) by 

 Professor S. L. Penfield and Mr. H. W. Foote, with the object of establishing a formula for 

 the mineral. Every care and precaution was adopted, and their results have given rise to 

 much discussion. The general formula they propose is H9Al3(B.OH)2Si^Oig, the nine 

 atoms of hydrogen being supposed to be replaceable by variable amounts of aluminium, 

 alkalies, magnesium, and iron. The special formulae constructed to suit particular cases are 

 necessarily very complex. According to another view, all varieties of tourmaline are, like 

 the garnets, isomorphous mixtures of a small number of fundamental molecules of perfectly 

 fixed and definite composition, the differences in the chemical composition of the different 

 varieties of the mineral being due to the relatively varying amounts in which these funda- 

 mental molecules are present, as are also the differences in physical characters. Even if this 

 theory be correct, the determination of the exact constitution of the fundamental molecules 

 is still one of the problems of mineralogical chemistry. 



In contrast to the chemical composition, the crystalline forms of all varieties of tourmaline 

 are in close agreement. The crystals belong to the hexagonal system with hemimorphic- 

 rhombohedral symmetry. A prism of greater or less length is nearly always developed, and is 

 terminated by rhombohedra, scalenohedra, or by the basal plane, singly or in combination. 

 Corresponding faces of these forms are inclined to each other and to the prism faces at angles 

 which vary in different crystals irrespective of the chemical composition, only, however, to a 

 small extent, at most only about a degree of arc. Such close crystallographic agreement 

 in substances of different chemical composition is explained on the principle of isomorphism, 

 the different varieties of tourmaline being regarded as an isomorphous series of minerals. 



There is a peculiar and characteristic feature connected with crystals of tourmaline 

 which may be seen in the accompanying figures (Figs. 70a to e), the arrangement of the faces 

 at one end of the prismatic crystals differs from that of the faces at the other end ; 

 Such crystals are said to be hemimorphic. The hemimorphism of tourmaline is rarely 

 very distinct, for the crystals are, as a rule, attached at one end to the matrix and 

 terminal faces developed only at the free end. Doubly terminated crystals of tourmaline 

 may be readily distinguished by this hemimorphic development from any other minex'al 

 which they may resemble in appearance. In Fig. 70, this hemimorphic development is 



