92 



MINERALS AND GEOLOGY 



arrangement of this kind would lead to a separation of varieti 

 of one and the same mineral. In the garnets, for example, cer- 

 tain varieties contain magnesia, and others lime or oxide of iron, 

 <fec., in place of magnesia, these bases being capable of mutual 

 substitution without the general or essential character of the sub- 

 stance being altered by the change a peculiarity known as isomor- 

 phism. The silicates possess representatives of all the crystal sys- 

 tems. In their hardness they vary from 1.0 (in tale) to 8.0 (in 

 topaz). Their aspect is most commonly vitreous, resin o- vitreous, 

 stony, or pearly, but the micas and some few other silicates 

 (bronzite, &c.) exhibit a pseudo-metallic lustre (see Part I). The 

 colour frequently varies greatly in examples of the same species, 

 as it is due chiefly to minute and accidental proportions of 

 foreign matters, or to variations in the isomorphous bodies which 

 form the base. Thus, where protoxide of iron or ferro-ferric oxide is 

 largely present, the mineral will generally possess a dark-green or 

 black colour, but where these bases are replaced by lime or magnesia 

 in greater or less proportion, the same mineral may be quite pale or 

 light in colour, or even colourless. The different garnets, pyroxenes, 

 amphiboles, tourmalines, &c., are familiar examples of this fact. The 

 streak, however, is always white (or nearly so) under normal con- 

 ditions, but it may exhibit a slight or indefinite tinge of grey, green, 

 or brown, in a very dark or ferruginous variety, especially if the sub- 

 stance be slightly altered or decomposed. Many silicates unless pre- 

 viously ignited or fused with potash or alkaline carbonates, resist 

 altogether the action of acids. Others becomj partially attacked or 

 decomposed (some by boiling hydrochloric acid, and others by sul- 

 phuric acid), the silica separating in a granalar, slimy or gelatinous 

 condition (See under "Action of Acids," in Part I). Some silicates, 

 which do not gelatinize in their ordinary state, exhibit this peculi- 

 arity if previously fused or strongly ignited. Certain silicates are 

 quite influsible in the blow-pipe flame. Others, if held, in the form 

 of a thin or pointed splinter, in the platinum forceps (Part I), be- 

 come rounded and vitrified at the point or edges ; and others, again, 

 melt into a perfect globule. In. some cases, the substance exfoliates, 

 or swells up and forms an intumescent branching mass, on the first 

 application of the flame ; and in many instances the fusion of a sili- 

 cate is accompanied by continued bubbling. Silicates which contain 

 a large proportion of silica form a clear transparent glass with carb. 



