86 



MINERALOGY. 



valuable states of this mineral, of which Emery is an impure form. 

 Cases Identical in chemical and crystallographic type with Corundum, though 

 11 & 12. ver y different in aspect both in its crystalline and massive varieties, 

 is the valuable iron ore, Haematite, the ferric oxide. A tarnish on 

 J some of its crystals, especially on those from Elba, produces an irides- 



cent effect of great beauty. With Haematite is placed Ilmenite, or 

 Titanic-iron, one of the ambiguous species of this class. Intimately 

 blended with the former mineral in all proportions and crystallising 

 in its forms, it yet presents the formula of titanate of Iron, a formula, 

 however, which, as containing two equivalents of metal united to three 

 of oxygen, is in fact homotypic with a sesquioxide. 



The hydrates of this class include the important iron ore Limonite 

 (Brown-hsematite), and Gcethite, which is monohydrated ferric oxide, 

 the latter represented by fine specimens from the Eestormel mine, 

 Cornwall. Next follow Manganite and Diaspore, respectively the 

 monohydrated manganese and aluminium oxides, isomorphous with 

 Gcethite. 



The class of dioxides is illustrated by a series of crystals and (d)Diox |i 

 other forms, — especially rich in the Cornish varieties — of Cassiterite 

 or Tin-stone (stannic acid), the ore of tin: and next follows 

 Cases the Zircon, consisting of the associated zirconic and silicic diox- 

 13 & 14. ides (zirconic and silicic acids). Its crystals, like those of Cassi- 

 terite, with which it is nearly isomorphous, are pyramidal. Its 

 pellucid varieties are gems. The 'dull green is the Jargoon, while 

 peculiar (" hyacinthine") red lints characterize the gem known as the 

 Hyacinth or Jacynth, of which fine cut specimens are in Case 13b. 

 The yellow and blue tints are rare, but the more pellucid and colour- 

 less zircon, from its exceptionally high refractive power, approaches 

 even the diamond in brilliancy. 



In the same continuous series is Rutile, the titanium dioxide 

 (titanic acid), isomorphous with Zircon, and approximately so with 

 Cassiterite. Anatase is the same substance, also in pyramidal forms, 

 but with different parameters ; while in yet a third series of forms this 

 trimorphous titanic dioxide is to be seen as the orthorhombic mineral 

 Brookite, of which the specimens from the Snowdon district are re- 

 markable. 



The Rhombohedral system is represented in the class of the 

 dioxides by Quartz and its varieties. 



This important mineral is silica, the oxide of silicon (silicon being 

 an element of the carbon group). This oxide occurs in a state 

 physically distinct from Quartz, in the Opal, which is amorphous: 

 specimens of it will be found at the end of the crystalline series of 

 the dioxides. Among the purer varieties of these are the Mexican Fire 

 Opal, and the beautiful and almost exclusively Hungarian gem, the 

 Noble Opal, conspicuous for its fascinating play of colours. Specimens 

 of Tridymite will be seen in Case 14b; it is a crystallised form of silicic 

 acid, with the specific gravity of opal. Its crystalline forms, however, 

 are distinct from those of Quartz, which is the more common and 

 more dense variety of Silica. The latter is seen in its purest form as 



