The titaniferous oxide of iron, or iron sand, is also attractable by the magnet. Its colour is a deep black, with some metallic lustre; it is perfectly opaque: its fracture is conchoidal; it is hard and difficult to grind under the pestle into a dull black powder, which stains the fingers when it is very fine; it melts at a high heat into a black enamel without lustre. All volcanic rocks contain a greater or less quantity of titanic iron-ore, disseminated through them, which may be recognised by its brilliant metallic lustre, and its perfect conchoidal fracture.
7. Fer oligiste, iron-glance, specular iron and red iron-ore.—This ore has the colour of polished steel; and the light transmitted through the thin edges of its crystals appears of a beautiful red. Its powder is always of a well marked brown-red hue, passing into cherry-red, which distinguishes it from the black-oxide ore. Its fracture is rough, or vitreous in certain varieties; it breaks easily; but it is hard enough to scratch glass. It usually contains from 60 to 70 of metallic iron in 100 parts; the equivalent proportion of oxygen in the pure red oxide of iron being 30 parts combined with 70 of metal. It is a mistake to suppose any specular iron ore capable of yielding 85 per cent. of iron, for 100 parts of even protoxide of iron contain only 77·77 parts of metal.
The compact variety comprises the crystals of the island of Elba, and of Framont in the Vosges, which have a rough-grained fracture. It exists in very great masses, constituting even entire mountains; in the cavities and fissures of these masses, the beautiful crystals so much prized by collectors of minerals, occur.
The island of Elba is equally celebrated for its inexhaustible abundance of rich specular iron-ore, and for the immemorial antiquity of its mining operations. [Fig. 581.] is a vertical section passing through the three workings, called Pietamonte (D), Sanguinaccio (E), Antenna (F), through an antient excavation a, through the coast o, and the mole p, ending at the canal of Piombino. The total height of the metalliferous mountain above the level of the sea, is no more than 180 metres, or 600 feet.
The rock which constitutes the body of this little mountain d l, is called bianchetta by the workmen. It is a white slaty talc, slightly ochreous, or yellowish, consisting chiefly of silica and alumina, with some magnesia.
The ore of Antenna (F) is a very hard compact fer oligiste, of a brilliant metallic aspect. The workable bed has a height of 66 feet, and consists of metalliferous blocks mixed confusedly with sterile masses of the rock; the whole covered with a rocky detritus, under a brownish mould. From its metallic appearance and toughness, this bed is called vena ferrata, the iron vein. In Pietamonte the workable bed is composed entirely of micaceous specular iron ore (fer oligiste), with its fissures filled with yellow ochre. This bed rests upon the rock called bianchetta; the brilliant aspect of ore in this place has gained for it the name of vena lucciola.
The metalliferous hill d l, extends to the north-east, about a mile beyond the workings D E F. The ore contains about 65 per cent. of iron, and is smelted in Catalan forges.
The following description of the figure will make the structure of this extraordinary mine well understood. a, is a great excavation, the result of antient workings.