FLOOE.] 
MINERALS. 
47 
the ferric oxide. A tarnish on some of its crystals, especially on those 
from Elba, produces an iridescent effect of great beauty. With Haema- 
tite is placed Ilmenite, or Titanic-iron, one of the ambiguous species 
of this class. Intimately blended with the former mineral in all 
proportions and crystallizing in its forms, it yet presents the formula 
of titanate of Iron, a formula, however, which, as containing two equi- 
valents of metal united to three of oxygen, is in fact homotypic with 
an epideutoxide (sesquioxide). The hydrates of this class include 
Limonite (Brown-hsematite), and Gcethite, which is monohvdrattd Cases 18 
ferric oxide. The Cornish specimens of this mineral, from the Res- (ii-)andl9. 
tormel mine, are very fine. In juxtaposition with Gcethite are Man- 
ganite and Diaspore, respectively the monohvdrated manganic and 
aluminic oxides, isomorphous with Gcethite. 
The class of dioxides is illustrated by a series of crystals and Case 20. 
other forms, — especially rich in the Cornish varieties — of Cassitente 
or Tin-stone (Stannic acid), the ore of tin. 
In the same Case is placed the Zircon, consisting of the associated 
dioxides of Zirconium and of silicon (zirconic and silicic acids). Its 
crystals, like those of Cassiterite, with which it is nearly isomorphous, 
are pyramidal. Its pellucid varieties are gems. The dull green is 
the Jargoon, while peculiar (" hyacinthine ") red tints characterize the 
gem known as the Hyacinth or Jacynth. The yellow and blue tints are 
rare, but the more pellucid and colourless zircon, from its exceptionally 
high refractive power, approaches even the diamond in brilliancy. 
In the same continuous series is Eutile, the dioxide of Titanium Case 21. 
(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 prismatic mineral 
Brookite, of which the specimens from the Snowdon district are very 
remarkable. 
In this Case is also exhibited the dioxide (the -'peroxide") of 
manganese, Pyrolusite, the mineral employed for the production of 
oxygen gas, and for the evolution from the chlorides of their chlo- 
rine, so largely employed in the arts. 
The Bhombohedral system is represented in the class of the 
binoxides by Quartz and its varieties. 
This important mineral is silica, the binoxide of Silicon (a chemical Cases 
element of the carbon group). This oxide occurs in a state chemi- 24 (ii.) and 
cally and physically distinct from Quartz, in the Opal, which is ' 1 
amorphous, and will be found at the end of the crystalline series 
of the binoxides. Among its purer varieties are the Mexican Fire 
Opal, and the beautiful and almost exclusively Hungarian gem, the 
Noble Opal, conspicuous for its fascinating play of colours. The other 
allotrcpic state of silica is seen in its purest form in Quartz crystal in Case 21 
Case 21. Its tinted specimens may vie in point of colour with jewels (u.) 
of denser substance and higher refrangibilitv. Among these are the 
lilac-hued specimens of the Amethyst, of which as well as of the Case 22. 
yellow Brazilian kind, there are specimens showing the ** rippled " 
