Haematites

The haematites consist essentially of ferric oxide, Fe₂O₃, and may be divided into two groups according as they are anhydrous or contain combined water. The former are usually red in colour, whence the word "haematite," from the Greek haima, blood. The hydrated haematites vary in colour from red to dark brown, according to the amount of water they contain. Perfectly pure anhydrous ferric oxide contains 70 per cent, of iron, but the red haematites used in this country for commercial purposes average more nearly 60 per cent, of metal. The crystalline variety is known as specular iron ore, iron glance or looking- glass ore, and is obtained mainly from metamorphic rocks, but also occurs as a product of volcanic heat - as, for example, in Italy, some very beautiful crystals being found in the neighbourhood of Etna and Vesuvius. The ore is found in France, Switzerland, and elsewhere in Europe, fine specimens having been obtained from St Gothard. The island of Elba has long been famous for its beautiful crystals, even Ovid (43 в.с. to a.d. 18) drawing attention to them. Other sources are India, Canada, Mexico, and the United States of America.

The crystals belong to the hexagonal system; hardness 5.5 to 6.5; density 4.5 to 5.3.

Micaceous iron ore is the name given to crystalline ferric oxide exhibiting a foliated structure of dark grey scales that glisten like mica. No mica, however, is present in the ore. It is found in the Lake Superior district, U.S.A., but the finest crystals come from Elba. Prior to the European war of 1914 micaceous ore was raised in Devonshire and sent to Germany under the name of shining ore. Hardness 6; density 5,2.

An octahedral variety is known as martite. It is black in colour, sometimes appearing bronzed, but it has a reddish brown streak, which serves to distinguish it from magnetite, as also the fact that it is not magnetic, or at best only very feebly so. Hardness 6 to 7; density 4,35 to 5.33. Martite is probably a pseudomorph of magnetite - that is, it may have been derived from that mineral by oxidation, without appreciable alteration of the crystalline form. This change may be effected in the laboratory by prolonged heating of magnetite in the blowpipe.

A hard, compact, nodular variety of haematite is found in Cumberland in the carboniferous limestone series of Cleator Moor, in Furness in North Lancashire, and to a less extent in Devonshire, and from its shape is termed kidney ore. The ore is very low in phosphorus, and is particularly valuable for making Bessemer pig iron - that is, pig iron suitable for the manufacture of steel by the acid Bessemer process. The Furness ore appears to have been worked in prehistoric times.

The colour of the ore varies from red to iron-black, and exhibits quite a metallic appearance. When scratched or powdered the colour becomes bright red. The Brixham ore is used in the manufacture of paint.

Softer, earthy forms of red haematite are also found in Cumberland, the colours of which vary from bright to dull red. They are known as ruddle or sometimes as puddle ore, as they are used for lining the hearths of puddling furnaces. Spain and America likewise yield large quantities of earthy red haematite.

Immense deposits of a variety known as red fossil ore occur in the United States, containing from 30 to 50 per cent, of iron and from 0.4 to 0.7 of phosphorus. Red ochre is another form of anhydrous ferric oxide, but the term is also used for a variety of turgite.

Haematite ores, rich in iron, are found in China.

The problem of the origin of the haematites has been the subject of much discussion. The view is largely held that, in the instance of Cumberland and Lancashire, the haematite has gradually replaced the calcium carbonate, molecule by molecule. This is supported by the fact that the ore and rock appear to have "grown together," as the miners express it, the ore gradually passing into the limestone and possessing precisely similar stratifications and dip. Again, casts of mollusca and other fossils characteristic of carboniferous limestone have been found in the haematite, as well as crystals of haematite pseudomorphic with calcite. It does not follow, however, that all haematites are necessarily formed in this way; local conditions must always be taken into consideration. Admitting for the sake of argument the foregoing or metasomatic origin of the north-country haematites, the next point of interest is to determine how this molecular substitution could have taken place. The formation of anhydrous ferric oxide would appear to postulate an application of heat in some way or other, and volcanic activity seems to be the most easy way by which such heat might be forthcoming. It seems possible, therefore, that the carboniferous strata were overlain by ferruginous Permian and Triassic deposits. Waters, charged with ferrous iron in solution by passage through these deposits, percolated through the limestone beneath, dissolving out the calcium carbonate and leaving ferrous carbonate. This slowly oxidised, and under the influence of heat yielded anhydrous ferric oxide.

Red haematite has been found at Torquay, apparently pseudomorphic with pyrites, which is remarkable, for, although pyrites is frequently oxidised to limonite, it is indeed rarely that it is converted into anhydrous haematite. It is suggested that pyrites was converted into ferric chloride by the action of sea water, and that this reacted with limestone to yield anhydrous ferric oxide 4 in some such manner as that indicated above.

Chromitite occurs as dull, magnetic crystals, resembling magnetite, and has a mean composition represented by the formula Fe₂O₃.Cr₂O₃.

As already stated, hydrated haematites usually differ from the anhydrous in colour. As the percentage of water increases the colour changes from bright red to dark brown. Turgite, or hydro-haematite, 2Fe₂O₃.H₂O, is a common ore which contains about 5 per cent, of water. It occurs as an earthy deposit frequently known as red haematite and as red ochre; it is also found in a compact, fibrous condition, and in botryoidal and stalactitic forms, similar to limonite, for which ore it has frequently been mistaken. It may usually be distinguished by its redder colour, its red streak, and its greater hardness. Hardness 5 to 6; density 3.56 to 4.7. When heated in a tube it splinters in a characteristic manner, which serves to distinguish it from limonite and other ores of analogous composition.

One of the most important of the brown haematites is limonite, 2Fe₂O₃.3H₂O, which contains some 15 per cent, of water. It is found in mammillary and stalactitic forms of various shades of brown. It also occurs in concretionary form. The name "limonite" is derived from the Greek leimon, a meadow, and was first used to designate the bog ores.

Limonite is found in relatively small quantity in the Forest of Dean, where iron ores were worked in Roman times. It is called brush ore, locally, when it occurs in stalactitic, reniform or compact masses, containing some 80 per cent, of ferric oxide or 56 per cent, of metallic iron; and smith ore when incoherent and containing only 54 to 58 per cent, of ferric oxide (38 to 41 per cent, of iron).

Fine varieties of earthy limonite, more or less contaminated with clay, etc., are known as ochres, siennas, and umbers, the colour of the last-named being darker, probably on account of the presence of manganese. Ochres and umbers are found in several parts of England, notably in Derbyshire, Oxfordshire, Devon, and Cornwall. The name umber is derived from Umbria, a town now known as Spoleto, in Italy, where umber was first obtained. Sienna likewise received its name from the Italian town of the same name. Turkey umbers are acknowledged to be the finest. They really come from Cyprus, but received their name through having been imported from Constantinople at a time when their true origin was unknown. Siennas are found in the Harz, in Tuscany, and in America.

Large deposits of brown haematite occur in Northamptonshire, the beds extending into Lincolnshire and Oxfordshire. The ore is abundant in the U.S.A.

A fibrous variety of limonite is known as wood iron ore.

Goethite, Fe₂O₃.H₂O, is named after the famous German poet, Goethe, who was also a mineralogist of considerable repute. It occurs as black or brown rhombic crystals, the parameters of which are

a: b: c = 0.66: 1: 1.089.

The crystals are usually blackish brown in colour, but appear red by transmitted light. They yield a red streak; hardness 5 to 5.5; density 4 0 to 4.4.

Goethite contains some 63 per cent, of iron. It has been found in Somerset, Cornwall, and Gloucester, some very fine crystals coming from Lostwithiel in Cornwall On the Continent it has been found in numerous districts such as Saxony, Nassau, etc., whilst quantities are present in U.S.A. Other names for the ore are pyrrho-siderite, and rubinglimmer. Acicular varieties are known as needle iron stone and onegite, the latter occurring in quartz in a similar manner to the penetrating needles of rutile so frequently met with. Aventurine oligoclase or sunstone owes its beautiful internal reflective powers to the presence of crystals of haematite or Goethite. A velvety variety of Goethite is known as sammet-blende, from the German Sammet, velvet. A variety of Goethite occurs as fine red scales which may be recognised under the microscope by their dichroism. This serves also to distinguish them from haematite. Lepidocrite is a form of Goethite presenting a fibro-scaly structure.

Hydrogoethite, 3Fe₂O₃.4H₂O, has been found in Russia in thin red veins in limonite. Under the microscope it is seen to consist of transparent plates or needles, with a distinct cleavage.

Limnite, Fe₂O₃.3H₂O, and xantho-siderite, Fe₂O₃.2H₂O, closely resemble limonite in appearance, but are usually slightly lighter in colour and contain more water. The former name is derived from the Greek limne, a marsh, and the ores are frequently termed bog or lake ore. They are found in different localities, such as Ireland, Sweden, and America. They consist of deposits of hydrated ferric oxide, probably oxidised from ferrous salts and thrown out of solution by lowly organisms. Large deposits of chromiferous brown haematite, containing about 2 per cent, of chromium, occur in Greece and in Cuba. Xantho-siderite also occurs in crystalline form as golden needles.

Laterite is a similar mineral, which is found in India, and contains varying quantities of iron and aluminium hydroxides.

Minette is a brown haematite found in Luxemburg, Lorraine, and Rhineland. It consists of oolitic grains bound together by a cement of chalk, clay, or silica, and may contain anything from 0.5 to 2 0 per cent, of phosphorus. Minette is therefore largely used in basic steel manufacture. Brown haematites also occur in Russia, Poland, and China. Spain exports large quantities of haematite, which vary in colour from red in the almost anhydrous " rubio " ores to brown.

Esmeraldaite, Fe₂O₃.4H₂O, occurs as pod-shaped inclusions in limonite in Esmeralda Country, Nevada. It is glassy, brittle, and possesses a yellowish-brown streak.

Many minerals are known in which ferric oxide is associated with other metallic oxides. Thus Arizonite or ferric titanite, Fe₂O₃.3TiO₂, is a dark, steel-grey mineral found in Arizona.

Plumbo-ferrite, (Pb, Fe, Cu)O.Fe₂O₃, occurs in Sweden and elsewhere as hexagonal plates, in appearance like molybdenite, but yielding a pale red streak.

Cuprous ferrite, CuFeO₂ or Cu₂O.Fe₂O₃, occurs as the mineral delafossite in Arizona. It yields tetragonal crystals,

a: c = 1: 1.94;

of hardness 5.5; black, with black streak, and non-magnetic.