Ironstone
Ironstones are a group of iron-rich sedimentary rocks. They are potential iron ores and have a characteristic rust-coloured appearance on weathering. Their origins and the nature of the iron-containing minerals present vary so widely that some people call them `iron formations' rather than ironstones. Precambrian banded iron formations occur in old cratonic (shield) areas. They usually form thick sequences of iron minerals interbedded with cherts. Such ironstones are of great economic importance. Deposits from the Cambrian period onwards are usually thin and oolitic. The only iron deposits forming today are the bog iron ores of mid- to high-latitude lakes and swamps. Ironstones often contain fossils.
| Colour | brown, red, green or yellow |
| Mineralogy | iron minerals present to give at least 15 per cent iron in the rock are commonly limonite, goethite, hematite, magnetite, pyrite, siderite, chamosite and glauconite; often cemented by calcite or dolomite |
| Texture | variable grain size and variable texture; sometimes oolitic |
| Structure | may be banded otherwise sedimentary structures common; fossils may be present |
| Occurrence | often interbedded with cherts, limestones and
sandstones; ironstones themselves may also be classified as other sedimentary rocks (e.g. sandstones, limestones, shales) |
| Examples | large Precambrian iron-formations are in the Lake Superior region (North America) and the Hamersley Group of NW Australia; later ironstones are the Clinton Ironstone, Central Appalachian Basin (USA) the Rhiwbina Iron Ore (S Wales) and the Northampton, Cleveland and Frodingham Ironstones (England); modern freshwater bog ores are in Sweden and Finland |
(Field Guide to the Rocks and Minerals of the World: David Cook and Wendy Kirk)
Weardale Iron Ore
In 1433 the bishop's iron mines in Evenwood, Raly, Hertkeld and several other places in the Weardale were farmed out to Sir William Eure at 112l. 13s. 4d. per annum. 33 The largeness of this sum implies that the production of iron was considerable.
Bog Iron
"Bog iron ore is widespread in the northern and western parts
of the British Isles. As its name implies it is not a rock but a
deposit formed under arctic or sub-arctic conditions. It is very
difficult to describe but in many ways it resembles peat and, in
areas where it contains manganese, has a definite indigo colour.
In Scandinavia it is often deposited on the bottom of lakes and
is recovered by cutting holes through the ice and dredging with
sieve-like spades. In Britain it can be found just under the
turf on moorland, and in Weardale it occurs in thicknesses of
3-4 inches at depths of about 8 inches. It is where iron-bearing
surface waters meet organic material that conditions suitable
for the precipitation of iron oxides are created. Deposition is
often continuous and once the ore has been removed, more will
form at the rate of several inches in a period of 30 years."
1 R.F. Tylecote 'Metallurgy in Archaeology'
p179 published by Edward Arnold 1962
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"The Romans, whose nation nineteen centuries ago was mistress to the world, during their residence in the north of England, were acquainted with our iron mines, and familiar with the mode of its manufacture. Remains of their forges or blomaries have been found in some parts of the county, and heaps of slag are sometimes found near the remains of their camps, testifying that ancient people availed themselves of its use, as witness the remains of the station at Lanchester, where immense pieces have been found together with portions of coal, showing that they knew also of that mineral and its use. Iron ore was formally worked and smelted in several parts of the county, as appears by the heaps of iron scoriae found on many of the commons or fells and elsewhere. Several of these large heaps of iron slag are found near Milkwellburn on the Derwent, at North Bedburn, and in Weardale. Crowley, Millington and Co's ironworks at Winlaton were founded in 1691, and were the principal works, where anchors, anvils, chains, cables, and other iron goods were manufactured in the north of England for many years. Gateshead ironworks were established about 1745 ; smaller ones at Lumley about 1780 ; and on the Teams at Urpeth, about 1800, by William Hawks, founder of the firm of Hawks, Crawshay, and Co. Consett ironworks were established in 1840, and Birtley and Witton Park at later dates. The amount of iron goods required in the northern counties of England was more than the works in the district could supply, and up to about 1845 the consumers of pig and malleable iron were obliged to obtain much of their suppliers from the ironworks in Scotland, Staffordshire, and South Wales. Tow Law ironworks being founded at such a period were bound to go on and prosper, and that they did so is shown by the fact that the black fell-top, with its single farmstead nestling on its side in 1841, was transformed in 1851 to an increasing, populous, and thriving manufacturing and mining village with a population of nearly 2,000 persons, increasing four years later to nearly 3,000.
The iron ore is worked in the Weardale mines much the same as the lead ore, and is found in the same veins, though where the lead is more abundant the iron becomes scarce, and vice versa. The iron ore does not confine itself to veins like lead, but flats out, as it termed, into horizontal beds, on one or sometimes both sides, varying very much in with and depth, reaching several hundred yards in the former, and from five to twenty yards or more in the latter. These bands are so near the surface in some localities as to admit of their being worked in the same manner as stone quarries ; whilst in other places they are worked in the same manner as the lead mines. All the lead found in the iron mines is worked by the Weardale Iron Company and turned over to T.W. B. Beaumont, Esq., the lessee of the lead royalties in the manor of Weardale, upon agreeable terms.
From the rich and almost pure ore of the Ryder ironstone Mr. Attwood commenced to manufacture iron of a splendid quality, smelted with coke, manufactured at the Company's own coke ovens at Black Prince, or that made at the Blackfields ovens.
The Weardale Iron Company, soon after its formation, worked a kind of ironstone which is often found in the coal measures, but in uncertain quantities, at Tow Law. One of these ironstone pits was in a field behind the Company's offices, to the south of the cemetery ; the large black heap lying near the railway being the refuse obtained from the mine. The other ironstone pit was in the dale between Tow Law and the farm of West Shield, through which the streamlet called Mosey, more properly the Moss burn, flows onward to Panburn, the large heap lying there being the waste obtained in working the mine. A well of clear pure spring water at the lower end of this heap is know as the " Ironstone Well." The kind of ironstone worked from these two mines, or pits as they were locally called, and which is associated with the coal measures, is often called clay band ironstone, from its being found in these horizontal beds interstrarifield with the plate beds, or clay bands of the carboniferous rocks. It is sometimes called the German band ironstone, from the ore found in such beds having been used by the colony of German swordmakers of Shotley Bridge, in the manufacture of swords and other articles in the seventeenth century. The small nodules of ironstone found in these bands are also known by the name of "catheads," and some may always be found on searching either of the above "seggar" heaps. The stone obtained from these pits not unfrequently yielded iron of a superior quality, but the small amount left after calcimining did nor render the working payable, owing to the limited supply."
(Tow Law Deerness and District History Society)
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Iron ore mining in Weardale was recorded as far back as the 12th century when the Bishop of Durham leased an iron ore mine at Rookhope for the making of ploughs.
In 1408 Bishop Langley gave consent to a John Dalton to experiment with the smelting of iron ore at Bedburn. The ground was cleared and wooden huts built beside a forge also made out of timber, roofed with turf. Two furnaces were built the Blomehahatl and the Stringlath. These early furnaces were probably no more than bowls with bellows to increase the draught, the fuel was charcoal and the need for this made inroads into the forests and in the 17th century concern was expressed about the number of trees that were being felled.
Other records from the 14th century show forges being let in Stanhope and Wolsingham. A Thomas Smith rented five forges from Bishop Hatfield in 1349. From then on more forges were licensed from successive Bishops covering the Dale from Redgatehead on the outskirts of Wolsingham to the area around Rookhope in the west.
It was here in Rookhope in 1430 that Robert Kirkhouse was granted from Bishop Langley a lease to mine ironstone, the ground for a forge and the wood for charcoal for the purpose of smelting iron ore that, “grows between Stanlawe Burn, Crawcrook and the woods of the Abbot of Newminster except oke, ash, hollywood, appiltre and also all wode fit for felyed before the coleyers make cole”
The smelting of iron ore continued using these primitive furnaces until the 18th century when the problem of fuel was overcome when a Shropshire ironmaster successfully used coke to smelt his iron ore.
(Ironstone Mining in Weardale (Part 1) by Barry Kindleysides)
Black-band ironstone
The term 'black-band ironstone' appears to apply to two different iron bearing rocks. The earlier in formation, also known as banded ironstone formations, is found in Precambrian sedimentary rocks and consists of iron oxides. The later is found in carboniferous rocks and owes its colour to the inclusion of a significant amount of "carbonaceous matter". It is the latter type which is found in Weardale (in addition to clay band ironstone)
"A dark variety of clay ironstone containing sufficient carbonaceous matter (10% to 20%) to make it self-calcining (without the addition of extra fuel)."
Jordanhill, Glasgow - At a depth of about 200 feet was the first of several beds of ironstone, a material had been identified while digging for coal seams. It was known as blackband ironstone and was of a high quality such that just over two tons of raw material could be smelted to give a ton of pure iron. The seams were generally quite thin, between 9 and 18 inches, so large quantities of surrounding material had to be brought to the surface in order to actually dig it. This resulted in huge bings of material such as blaes and clays and these bings led to the local industry of brickmaking.
Banded iron formations (also known as banded ironstone formations or BIFs) are a distinctive type of rock often found in primordial (Precambrian) sedimentary rocks. The structures consist of repeated thin layers of iron oxides, either magnetite (Fe3O4) or hematite (Fe2O3), alternating with bands of iron-poor shale and chert. Some of the oldest known rock formations, formed over , include banded iron layers,[1] and the banded layers are a common feature in sediments for much of the Earth's early history.
.jpg) |
This image shows a 2.1 billion years old rock
containing black-banded ironstone, which has a weight of
about 8.5 tons. The approximately two meter high, three
meter wide, and one meter thick block of stone was found
in North America and belongs to the National Museum of
Mineralogy and Geology in Dresden, Germany. (Click on
picture for larger image 1.66MB) From Wikipedia |
On the Origin of Clay-Ironstone
of the Geological Survey of England
A. 1. CLAY-IRONSTONE* is an impure carbonate of iron, containing generally from 30 to 33 per cent. of metallic iron mingled with varying proportions of clay, oxide of manganese, lime, and magnesia.
2. When the principal foreign matter in this mineral is of a
bituminous or combustible nature the variety thus formed
is called "Black band"
.
This combustible matter often amounts to 25 or 30 per
cent.
3. Sometimes lime so far predominates in Clay-ironstone as to give it the general appearance of a compact limestone, when the presence of iron may be detected by the great weight of the mass, and by the decomposition of the exterior, which turns a dark brown to the depth of a quarter of an inch or more on oxidation.
4. Through the intermediate forms it passes into a pure limestone. Such limestone bands are characterized by being densely compact, very hard, dark-coloured, and by being devoid of organic remains.
B. 1. The foregoing varieties occur throughout the vast thickness of delta-deposits whose remains now constitute the Carboniferous formations. They lie in beds varying in thickness from a fraction of an inch to about 2 feet.
2. When the top of a bed is exposed it is often found to be traversed by a series of cracks, which present the general appearance of a net of very unequal and irregular mesh. These cracks are filled with sediment like that in which the bed of Clay-ironstone lies.
3. Very frequently the beds of all varieties .....
Coal Measure ironstones
These are the only stratified primary rocks of any economic importance. In most coalfields, clayband and blackband ironstones were formerly of great importance, though only small tonnages are now mined. In 1857, 80 per cent, of Britain's ores were obtained from the argillaceous ironstones of the carboniferous rocks, but in 1890 the figure was just under 17 per cent. These ironstones occur as nodules in the beds of shale and fireclay or sometimes as thin veins of ironstone a few inches thick interspersed within the shale. They vary in colour from light grey to black.
Mine and Quarry Engineering June 1944