88 



MINERALOGY. 



c fed, spathose-iron, or Ohalybite, enters as an ingredient in a high 



I percentage. 



3a&es Among the hydrated carbonates, and carbonates combined with 



II & 22. hydrates, or with compounds belonging to the previous divisions, 



attention may be called to the green and blue copper ores, Malachite 

 and Chessylite, of which latter a very fine series of crystals is exhibited. 



Case 22d contains also fine specimens of Cromfordite, a combination 

 of the chloride and carbonate of lead; and of Parisite, an analogous 

 compound from the Emerald Mines of Santa Fe de Bogota, containing 

 the fluoride combined with the carbonates of calcium and the rare 

 metals of the cerium group. 



The Silicates, occupying no less than twelve Cases, form the next silicates 

 class in this section. The minerals comprised in this large, varied, 

 and important class are arranged in series distinguished by the type of 

 oxide that characterises the bases in the silicate. Thus the silicates cor- 

 responding to monoxide-bases (ferrous oxide, magnesia, &c.) are arrayed 

 in one series; those the bases of which are sesqui-oxides are in another; 

 and such as contain bases of both kinds fall into a third. The respec- 

 tive hydrates are comprised under the series to which the minerals 

 of corresponding anhydrous types belong. 



The first of these series is composed of such silicates as are formed 

 by the combination of silica with monoxides only, or in which sesqui- 

 oxides are met with only as accidental or intrusive ingredients. The 

 anhydrous section of this series contains, among others, the following 

 minerals. Phenakite, the di-glucinum silicate (Be 2 Si0 4 ), and Wille- 

 mite, a zinc-silicate (Zn 2 Si0 4 ) corresponding and isomorphous with 

 it, represent a rhombohedral series of dibasic silicates. The speci- 

 mens of Phenakite from the emerald mines of the Urals are extremely 

 fine. In this group is placed another rhombohedral mineral, Dioptase, 

 a hydro-cupric silicate having the analogous formula (H 2 CuSi0 4 ). 



Of the same chemical type are the minerals comprised in the Olivine 

 group, which are orthorhombic in their forms, and include Tephroite, 

 di-manganous silicate; Fay alite, di-ferrous silicate ; with Olivine and 

 Hyalosiderite, which are the magnesium and magnesio -ferrous silicates 

 of the series. The Chrysolite is the name of the pale yellow gem 

 into which the larger and clearer specimens of Olivine are occasion- 

 ally cut ; while the Peridot is a pistachio-green variety, of which fine 

 crystals and cut specimens are exhibited in Case 22f. Gadolinite, 

 a di-yttrious silicate (containing also cerium, &c), is represented by 

 fine crystallised specimens; and Humite, a mineral containing Fluor- 

 ine, and likewise belonging to the more basic silicates, is arranged 

 here. 



; ases Among the monosilicates are arranged the large series of impor- 



3 & 24. tant minerals which form the two parallel groups of the Augites 

 and the Hornblendes. In juxtaposition with these is seen Wollas- 

 tonite, the calcium monosilicate, and the anorthic minerals Rhodonite 

 and Babingtonite, homotypic in composition, but crystallographically 

 differing from the other members of the series. The Augitic and 

 Hornblendic groups present two distinct crystallographic types. In 



