Sunday, October 25, 2009

How the Iron Ore Formed

Under the right circumstances both iron and magnesium ions are capable of acting as weak acids under the influence of alkali or alkaline mineralization as exemplified by the underlying Stockbridge marble formation, or as at the time of deposition a back arc basin coral reef. The re4actions that occurred between the CaCO3 and the proposed iron and magnesium oxides found in the overlying Walloomsac formation providing the iron to form the hydrous iron oxide (limonite) at the contact between the Stockbridge and Walloomsac formations. The Walloomsac also provided the necessary magnesium ions for converting the limestone to dolostone a mixture of calcium and magnesium carbonate. Undoubtedly there were chemical reactions occurring at the beginning of the depositation of the Walloomsac on top of the Stockbridge formation.

The real transference of ions did not really become strong until the Taconic orogeny during the late Ordovician that buried both formations underneath a nappe of older Cambrian rocks that were associated with the island arc that had formed earlier off the south coast of the North American craton. The position of older rocks on top of younger rocks was a puzzle to geologists for decades that was known as the ‘Taconic Problem.” This puzzle was finally unraveled in the 1970s with the discovery of plate tectonics.

Tuesday, October 20, 2009

The Walloomsac Formation - The Bearer of Iron

The Walloomsac Formation was formed from volcanic ash falling on top of a coral ref in the back arc basin of the volcanic arc that formed off the south coast of the North American craton in the mid-Ordovician. Since that time the North American Craton has rotated approximately ninty degrees counterclockwise making the ancient south coast today's east coast. The situation at the time was similar to today's Japan and the Sea of Japan between Mainland Asia and Japan.

The ash fall was intermittent depending on volcanic activity in the adjacent island arc. The prevailing wind would have blown the volcanic ash into the back arc basin. We must assume that the volcanics were mafic to andesitic in nature having ejecta that was enriched in iron and magnesium.

This was originally laid down as interbedded layers of ejecta and coral reefs that grew on top of each layer of ejecta after it was erupted. At the beginning of the eruptive cycle in the island arc the ejecta was more mafic then it became later as it approached andesite in composition. It was these early eruptives that supplied the iron and magnesium that later were converted into dolostone and iron ore.

Friday, September 25, 2009

The Discovery of Ore Hill

The Discovery of the Ore Hill Mine:

One summer afternoon Ezekiel Ashley and John Pell climbed a small hill on a 100 acre tract of land they were surveying in the Town of Salisbury Connecticut. At the top of the hill they discovered soil that looked exactly like iron rust. That was exactly what it was, hydrous iron oxide, more commonly known as “limonite” an ore of iron. In 1887 the decedents of Ezekiel Ashley were still proprietors of the mine on Ore Hill.

At first the local blacksmiths came to the mine and hauled the ore off in their saddlebags where they refined it into wrought iron at their forge. This is a way of smelting iron ore directly into iron by causing the ore to be reduced to iron by removing the oxygen in the ore by combining it with charcoal. This process makes a spongy mass of iron and slag that afterwards is hammered on an anvil. This process called louping causes the impurities to be driven from the iron. The result is a bar of wrought iron ready to be used for other articles made of iron.

The first furnace capable of making cast iron was erected seven miles away in Ancram, NY by Phillip Livingstone. This furnace was to take an active part in the Revolution as it was this furnace that forged one of the chains that George Washington caused to block the Hudson River at West Point.

We can thank nature for this wonderful ore, but we have to look into the dim past over four hundred years ago when there was a volcanic arc of islands similar to modern Japan that in the process of erupting caused a layer of volcanic ash to rain down on top of the vast coral reefs along the east coast of North America. This was called the Walloomsac formation. Between eruptions the coral reef laid down another thin layer of limestone that then would be buried by the next eruption. This created a formation that was a layer of volcanic ash interbedded with a limestone. The result was a calcareous schist that was rich in iron and magnesium.

Both iron and magnesium are capable of acting as a weak acid under the right conditions. The right conditions were met by the underlying limestone reef. Over years this calcium carbonate extracted both iron and magnesium from the Walloomsac formation. The remarkable iron ore found on Ore Hill was one result of this chemical magic, another was the replacement of some of the calcium in the limestone leaving a new rock called Dolostone.

The combination of iron ore and Dolostone was just what the doctor ordered. One was the ore, and when it was added to the iron furnace the Dolostone acted as a flux making the ore easier to smelt using charcoal.

Tuesday, February 10, 2009

The Iron Ores of Connecticut

The Iron Ores
by John Carter

One must remember that there are many minerals that contain iron, but only those containing enough iron to be profitable can be classified as an ore. In Connecticut there are four iron minerals containing enough of the metal to be profitably mined. These are: bog ore, limonite, magnetite and siderite all of these ores were used at one time or another to produce iron. Of these ores limonite was the most abundant and created a thriving industry in northwestern Connecticut that lasted for almost two centuries.

Bog ore is a form of limonite that precipitates into the bottom of a bog from the action of iron fixing bacteria that are present in the water. It was the earliest iron ore mined in Connecticut and was the ore that was smelted in the furnace that was erected in West Haven by John Winthrop Jr. It also found use in the many forges of Connecticut until the discovery of the Salisbury ore beds in 1734.

Limonite is actually a hydrous iron oxide with the generic formula of FeO(OH)·nH2O. this formula is not especially accurate because at times limonite is apt to contain more oxide than hydroxide. Limonite was discovered in the town of Salisbury, Connecticut in 1734. It has been estimated that the single deposit of ore that was mined on Ore Hill produced over 7,000,000 tons of iron ore from its initial discovery until 1923 when the mine was finally closed. There were other mines producing the same ore found throughout the Tri-state area and as far north as Manchester, Vermont. This particular ore became known as “Salisbury Ore.”

The Salisbury ore is intimately associated with the Walloomsac formation that extends from West Central Vermont under the Taconic range of mountains as far south as the tip of Manhattan. The Walloomsac formation is an iron rich schist interbedded with Dolostone. The ore itself is found at the contact with the Walloomsac formation and the Stockbridge formation that is marble. It is found throughout the length of Manhattan Island intimately associated with the Inwood Marble and underlies Wall Street. Because of this formation you could say that Wall Street and the New York Stock Exchange has a foundation of iron.

Magnetite occurs in two different forms. One form is when it occurs in the sands of rivers and beaches. Its general formula is: Fe2O3. One of the early smith’s in Connecticut mined 85 pounds of the ore on one of the Long Island Sound beaches from which he produced fifty pounds of iron. Another use for this magnetic sand was as a blotting material for wet ink. Because it is naturally porous it was the best material available for this purpose. In use it was sprinkled onto a document while the ink on the paper was still wet.

The other type of magnetite is a solid that is intimately associated with the Precambrian rocks of the Berkshire Highlands complex and other Precambrian rocks in Western Connecticut. Although iron was produced from the ore to a limited extent it was never a real factor in the iron industry as it was too difficult to smelt. There were several mines mentioned producing the ore, but the largest was Tilly Foster mine in Brewster, New York. The only mine in Connecticut that had a name was the Tuttle mine in Winchester. This mine produced about 50 tons of ore that proved to be so difficult to smelt the mine ceased operations and was abandoned. All that remains of this mine today is an open pit in the sight of the Grantville Road that is full of water. Directly under the Gilbert high school at the end of Williams Avenue the aeromagnetic map of the Winchester quadrangle shows an extremely high magnetic anomaly that is probably a large deposit of magnetite. It is interesting to note that in Nova Scotia gold deposits are often found associated with magnetite. The geological terrain of Winsted is very similar to the geological terrain found in Nova Scotia.

Siderite is the last of the four ores to be found in Connecticut. It is found in the town of Roxbury in Mine Mountain. Unlike the other iron ores siderite is composed of iron carbonate, FeCO3. This is also termed “spathic iron ore.” The deposit of this mineral in Roxbury is one of the largest in the world. When it is first dug out of the ground siderite is a cream color but very quickly gathers a crust of limonite when exposed to the air. The siderite of Roxbury gave rise to another company organized to smelt the special ore. This company was named the silver steel company of Bridgeport. The mine in Roxbury was originally opened for silver. There may have been some there but it is doubtful. Many tons of iron ore were mined here, but the company formed to smelt the ore proved to be unprofitable. After the mine was closed it was bought by Columbia University who operated it for years as a field laboratory for their school of mining engineering. You can visit the mine today as a park in the town of Roxbury.

The Salisbury Iron District

Early Iron Work in Connecticut
by John Carter

Connecticut used to be a renowned far and wide for its manufacturing, but what is less well known is that mining was also a very large part of Connecticut's industrial scene. According to the Connecticut Geological Survey in Hartford there are over 600 abandoned mines and quarries within the borders of Connecticut. The iron ore of the Salisbury District gave rise to some of the largest mines in Connecticut. The Salisbury District was not just one town it consisted of seven towns in Connecticut as well as other iron producing towns in Western Massachusetts and Eastern New York. Iron was produced throughout the district that at its peak contained twenty seven blast furnaces producing cast iron as well is innumerable forges capable of producing wrought iron for almost two centuries.

Iron itself was not a monopoly of Salisbury as the first iron furnace in Connecticut was erected in West Haven at the outlet of Lake Saltonstall by the first Governor of Connecticut John Winthrop Jr. Winthrop had been active in setting up several of the furnaces and Eastern Massachusetts including the one located at Saugus. The Saugus Massachusetts furnaces have sense been reconstructed and are now part of the National Park Service.

In 1644 Winthrop petitioned the General Courts of Connecticut for permission to erect an iron furnace in what is now West Haven at the outlet of Lake Saltonstall. In 1651 he finally gained the General Courts permission or wrecked a furnace under some very liberal terms. This was the earliest iron furnace that was located in the State of Connecticut. Winthrop’s furnace continued in production until 1678 to produce “sowes” (pig iron) and pots until the ore bed was presumably depleted. The furnace was allowed to go out of blast, and it wasn't until some 80 years later that another iron furnace was erected in Salisbury, Connecticut. In the intervening 80 years all the iron that was produced in Connecticut came from small forges.