346 



SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



which are known, are distinguished according to the particular oxides present, 

 those of less importance, these are : 



Omitting 



1. Calcium-aluminium garnet 



2. Iron-aluminium garnet . 



3. Magnesium-aluminium garnet 



4. Calcium-iron garnet 



5. Calcium-chromium garnet 



33iOs. 

 3FeO.Al203.3Si02. 

 3MgO.Al203.3SiO2. 

 3CaO.Fe303.3Si02'. 

 3CaO.Cr,0,.3SiO„. 



The results of chemical analyses show that each of these garnets may occur in nature in 

 a moderately pure condition. In most garnets, however, the part of the monoxide or of 

 the sesquioxide is not played exclusively by calcium oxide, ferrous oxide, &c., or by 

 aluminium oxide, ferric oxide, &c., but by mixtures of two or more oxides. Thus, there are 

 garnets which contain, beside silica and alumina, two monoxides — lime and feiTous oxide — of 

 which now the first, and now the second, predominates. Thus in one case the mineral 

 approaches in character to a calcium-aluminium garnet, and in the other to an iron- 

 aluminium garnet, and it may be considered to be a mixture in varying proportions of the 

 two molecules SCaO.AljOg.SSiO^ and SFeO.AljOg.SSiOg. The members of the garnet group 

 are thus isomorphous mixtures of certain fundamental compounds, five of the most important 

 of which have been given above ; a few others will also be mentioned later when each kind 

 of garnet receives special consideration. The garnet group is divided into a number of 

 species known by particular names, the division being based upon differences in chemical 

 composition. The following table of the analyses of a few gem-varieties of garnet gives an 

 idea of the diversity in their chemical composition, a composition which can, however, be- 

 reduced to one common type by applying the principle of isomorphous mixtures : 



The different varieties of garnet, though so diverse in chemical composition, nearly 

 all occur in well-developed crystals of the same kind. The crystals are met with embedded 

 in rock, with faces fully developed on all sides, like the crystal represented in Plate XIV., 

 Fig. 3, from which the surrounding matrix has been partly removed ; also attached to the 

 walls of drusy cavities or crevices in rocks, such a druse being shown in Fig. 7 of the same 

 plate; The crystalline forms are those of the cubic system ; the most important are 

 represented in Figs. QQa to d. The rhombic dodecahedron (Fig. 69a) is so common and 

 characteristic of garnet that it is sometimes known as the garnetohedron. The twenty-four 

 edges of this form are often more or less widely truncated by planes, which, as shown in 



