286 IRON classify a particular ore otherwise than in broad general terms. According to the nature and amount of the admixed substances, the value of the ore varies largely. The pre sence of certain impurities, notably of phosphorus, in more than minute quantity prevents the use of certain ores for particular purposes, and thus reduces their value ; the admixture of particular kinds of gangue in other cases renders the ores unsuitable for working in the same kind of way that would otherwise be advantageous ; in smelting such ores by means of a blast furnace different kinds and amounts of flux for the earthy impurities are requisite in different cases, thus affecting the cost of production, so that in fine the value of an ore is by no means necessarily proportionate to the amount of actual iron present therein. The following table gives a rough idea of the general composition and characters of the leading classes of iron ores, such as are in actual use as sources of metal : Oxide of Iron Class. Ferrous Carbonate Class. Htematite. Brown Iron Ore. Magnetic Ore. Spathic Ore. Clay Ironstone. Ferric oxide 60 to 95 50 to 90 30 to 70 Oto 5 to 10 Ferrous ox ide i J usually ab sent or U,, 20 ,, CO 30 45 r nearly so. ) Manganese oxide - 2 to 2 ,, 1 1 25 2 Magnesia.... 1 2 2 10 1 10 Alumina 5 1 10 10 5 1 10 Lime ,.. ,, 3 ,, 5 ,, 5 25 1 ,, 10 Silica 1 25 1 30 ,, 25 5 2 25 Carbon di oxide } 2 5 5 35 40 20 ,, 35 ) ( usually ab ) Phosphoric anhydride V 3 3 2 < sent, or only present in V- 3 ) / traces. ) Sulphur ,, 1 1 2 ditto. ., 2 Water 5 5 20 5 to 5 o ;, 4 Anhydrous Hydrated Ferric and Crystalline Ferrous car ferric ox ferric oxide, ferrous ox ferrous car bonate dis ide, with a with more ides, more bonate, with seminated greater or or less ad- or less ap more or less through a less admix mixed proaching magnesium clayey mass, ture of sili- earthy and to the ratio and manga the former Main com c i o u s or clayey mat Fe 2 3 , FeO, nese carbo predominat position quartzose ter. and more nates, and a ing in the matter, and or less ad little gan richer ores sometimes a mixed with gue. of the kind little clay or clayey mat usually other alu ter and gan worked. minous gan gue. gue. Red haematite ores (including specular ore o- fer oligiste and mica- ceous ore) vary considerably in their external appearance; the variety known as " kidney ore " is well exemplified by the Cumberland de posits, and constitutes dark brownish-red botryoidal and reniform concretions, occasionally with a considerable amount of smoothness and lustre externally, and of crystalline frequently radiating struc ture ; softer varieties are known as "red ochre" and "puddlers ore," owing to their use for "fettling" puddling furnaces and as pigments, and are of unctuous consistency almost earthy in character. " Specular ore " is a hard well-crystallized form, deriv ing its name from the brightness of the surface of its crystals, which appear dark grey or black by reflected light ; this variety is well exemplified by the Elba ore ; its specific gravity is near 5 0, the crystalline system being the hexagonal. " Titaniferous iron ore " or "ilmenite " resembles specular ore in appearance and crystalline form ; it is more strictly a variety of magnetic ore, however, inasmuch as it usually contains a considerable amount of ferrous oxide ; the ferrous titanate present may on the other hand be re garded as FeTi0 3 , or Fe 2 3 , in which half of the iron is replaced by titanium ; whilst some of the ferrous iron is frequently replaced by magnesium. "Micaceous iron ore " is a crystalline scaly substance which, when of sufficient brilliancy, forms a good pigment for iron work, known as minium de fer. " As a rule haematites are consider ably free from phosphorus and sulphur ; various haematitic deposits in Spain, however, have been found by the writer and others to rontain large amounts of phosphorus, sometimes to the extent of several parts per cent, of that element in relation to the iron ; whilst occasionally pyrites veins are found in haematite beds. The chief hseniatitic ores worked are those from Cumberland and North Lan cashire (Ulverston, Furness, "Whitchaven, &c.); from Sweden and Norway (Dalkarlsberg, Uto, &c.); from Liege, Saxony, the Harz, Silesia, and Austria; from Elba and Brazil (specular ore); and from Missouri (Iron Mountain, Pilot Knob), Lake Superior, Ohio, Ten nessee, and Alabama ; many other deposits, however, exist, this class of ore being very widely spread ; thus it is found in some quantity in Cornwall (Restormel), Brixham, Ayrshire, Glamorgan shire, North Wales, the Isle of Man, the Erzgebirge, Russia, Spain, &c. Haematitic ores are usually found in the older geological formations, especially the Huronian, Cambrian, Silurian, Devonian, and Carboniferous rocks ; in many cases they are distinctly of sedi mentary character, i.e., they have obviously been deposited by aqueous agency. Some deposits have probably been originally thrown down either as ferric oxide detritus from the abrasion of rocks, &c., containing ferruginous matter, or as hydrated oxide from the oxidation of water containing ferrous carbonate in solution, the ochreous deposits thus formed having been rendered more or less completely anhydrous and indurated by the long-continued effect of pressure and the conduction of the internal heat of the earth to them. The Cumberland haematite largely occurs in pockets in Carboniferous Limestone, and has doubtless been produced by the latter kind of agency, the cavities of the limestone rock becoming gradually filled up by the deposition of iron oxide. Red sandstones, on the other hand, represent deposits of ferric oxide thrown down simultaneously with much sand ; whilst the earthy varieties of haematite have probably been less indurated by heat and pressure, and were doubtless formed by deposition from water containing clayey matters in suspension to a greater or lesser extent. The Alabama deposits exhibit distinct stratification, forming a bed be tween the Coal Measures and the Devonian Limestone upwards of 100 feet in thickness, and several square miles in extent. In Corn wall, North Wales, and especially in the Lake Superior and Missouri districts, the hsematitic deposits form large veins and lodes. The specular ores of Elba, Sweden, Missouri, and elsewhere usually occur as massive deposits ; portions of the latter occasionally show the passage of spathose ore into specular ore (Snelus), suggesting the effect of heat accompanied by oxidizing action. Occasionally brown haematite is found passing into red, indicating gradual dehydration more complete in one portion of the deposit than in another. The following analyses illustrate the composition of some haematites : Character of j ( Ore and - Locality.... ( Ulverstonc Red Haematite. African Mokta Ore. Elba Specular Ore. Lake Superior Rod Specu lar Ore. Pictou County, Nova Scotia. Pilot Knob, Missouri. R. Smith. Siemens. Laboratory of Phoenix Works. Geological Survey Reports. Thorpe. A. Blair. Ferric oxide Ferrous ,, Manganese oxide 90-94 6-35 trace 0-9:> trace trace 0-24 C-C8 0-78 79-74 6-43 2-92 0-52 0-25 4 V 75 5-11 87-84 6-07 3-47 0-22 0-34 0-02 5 : 97 1-90 0-17 90-52 trace 1-39 0-70 0-42 0-26 6-89 0-77 0-05 56-06 9-20 1-88 1-05 absent 2s -6S 2-54 absent 84-33 0-15 2 : 19 0-21 0-14 0-04 13 : 27 Magnesia Phosphoric ) anhydride ) " Sulphuric do Water, carbonic") acid, volatile j- matters, &c. ) Total metallic iron 99-88 09 72 100-00 100-00 100-00 100-33 C3-66 CO-8 Cl-81 62-92 43-40 59-15 Brown hasmatite (including gothitc, limonitc, log iron ore, lake ore, &c.) varies even more in appearance and character than red haematite, and is found of the most varied degrees of purity. Many deposits have been apparently formed by the alteration of argillaceous ferrous carbonate ; others form superficial sandy beds produced by the deposition of ochreous matters from solution either by purely chemical action, such as the oxidation of dissolved ferrous carbonate, or by the action of organized beings, especially Diatoniacese. As a rule much earthy matters are contained in this class of ores, together with considerable amounts of sulphur and phosphorus ; certain deposits found in Spain and Africa (Bilbao and Marbella ores, &c. ) are, however, often considerably free from these objectionable ingredients. Usually brown haematites are dis tinctly of sedimentary character, forming beds ; but they often occur also as veins, especially in the older formations, doubtless deposited (often along with other minerals, e.g., copper ores) from water flow ing through the cracks and crevices of the rocks. Sometimes the brown colour is much lightened, the tint being almost red and some times even yellow. In texture these ores usually differ considerably from the more compact kinds of red haematite, being cindery, earthy, or sandy in character, and only comparatively rarely massive, save whe i they have been subjected to indurating and compressing influ ences since their deposition, in which case they Lave usually lost water and become partially converted into something more like red haematite. Sometimes a definitely crystallized hydrate, Fe. 2 3 ,H 2 (gothite), is found ; scaly minerals of the same composition have also been described under the names of lepidocrocite, &c. The larger deposits of brown haematite arc found in the Secondary and
Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/302
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