MINERALOGY. 



89 



Enstatite, the magnesian, as in Bronzite a maguesio-ferrous mono- 

 silicate crystallises in the orthorhombic system, though with certain of 

 the angles of an Augite ; while in Diopside, and the other Augites, 

 clinorhombic in crystallisation, part of the Magnesium is displaced by 

 Calcium, and also by Iron, Manganese, or Zinc. So Anthophyllite, a 

 magnesio-ferrous monosilicate, corresponding with Bronzite, presents 

 orthorhombic forms with angles belonging to the type of the Horn- 

 blendes, as exemplified in Tremolite and the other members of the 

 group, which, however, crystallise in the oblique system. Certain 

 varieties of Jade or Nephrite are assigned to these groups, as are also 

 different kinds of Asbestos. 



The hydrated section of this series contains the Serpentines and the Cases 

 talcose minerals. It comprises, also, Chrysocolla, a cupric silicate 25 & 2 $< 

 hydrate ; Hemimorphite, the zinc silicate hydrate ; and Apophyllite, 

 a hydrated calcio-potassium silicate, extraordinarily fine specimens of 

 which are seen in Cases 23 and 24, and in the glazed ends of Case 25. 



The second series in the class of the silicates consists of those of 

 the sesquioxides. Foremost among them is the Topaz, an aluminium 

 silicate, in which part of the silicate is replaced by an analogous 

 fluosilicate. Many specimens of this mineral, collected by Col. de 

 Kokscharow, are here shown. Those from the Urulga river in Siberia 

 are singularly fine ; they are of a delicate sherry-colour, but are pre- 

 served in the dark, as light speedily bleaches them. 



The third series of the silicates is constituted of those in which the 

 monoxides and sesquioxides are associated in the same mineral. 



The various groups known by the general names of the Garnets, 

 Scapolites, Idocrase, Epidote, the Felspars, the Micas, and Dichroite 

 find their places in this series, into the hydrated section of which fall 

 the beautiful and extensive varieties of Zeolites and Chlorites. 



Among these the Garnets form a group of minerals belonging to the 



cubic system in which the chemical type (3M // 0,M 2 "'0 3 ,3Si0 2 ) re- 

 mains constant, while the isomorphous elements under that type replace 

 each other inunlimitedvariety. Among the familiar forms of thismineral 

 group, the violet-tinted Almandine, and the rich red Syriam Garnet are 

 ferrous-aluminic varieties ; the yellow and hyacinthine Garnets, known as 

 Cinnamon-stone and Essonite contain calcium and aluminium : the 

 calcium is replaced by magnesium in the deep blood-red Bohemian 

 Garnet and Pyrope, varieties which when cut en cabochon are the 

 Carbuncle of jewellery. Idocrase, a mineral with a smaller range of 

 chemical variation than the Garnet, is represented by a series of 

 crystals (of pyramidal forms) of remarkable variety and perfection. 

 Epidote is also well represented by specimens from Ala and from Cagps 

 the Obersulzbachthal. To the Epidote group also belong the minerals 27 & 93 

 Allanite, Zoisite, and Jadeite. To these succeed the various mine- 

 rals, Phlogopite, Biotite, Muscovite, Lepidolite, &c, included in a 

 group under the name of Mica. The group of Felspars follows, among 

 which will be found Labradorite, with its beautiful play of colours ; 

 the Moonstone, a partially decomposed Orthoclase; a fine specimen of 



