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Earth Science Themed Essays

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Earth Science Themed Essays

Reading the Rocks at Cornwall, Pennsylvania

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By: Brandon
Grade: 10
State: Pennsylvania


 

Cornwall iron furnace is a well-known historical site in Pennsylvania, one that harkens back to the century of coal and iron barons. Built in 1742, the Cornwall iron mine closed in 1972 after approximately 220 years of continuous operation. About 106,000,000 natural net tons of iron were mined during its operation. Cornwall's rich supply of iron ore provided cannon and shot for the Continental Congress during the Revolutionary War. This historic ore deposit lasted until only a couple of decades ago, but even then the importance of this site did not end. Besides being made into a national landmark, the extensive dumps of waste rock from the mines are open to collectors who can take advantage of the rich variety of minerals found there. The foremost of these dumps is the "Big Hill," which is where I collected the following minerals.

brandon_magnetite

Rock with layer of magnetite crystals


Magnetite ores were the main source of iron at Cornwall. At the dumps, plenty of magnetite can be found. The specimen that I have illustrated consists of a nonmagnetic rock (determined by holding a strong magnet from the family refrigerator to it) topped with a thin, highly magnetic layer. Embedded in this layer are small black triangular crystal faces. They are triangular in shape because of the octahedral crystals that magnetite forms. Even these tiny crystals are rare. Most of the magnetite at Cornwall occurs in the massive form, called lodestone. It has a mottled gray color and is very heavy for its size, owing to its high magnetite content. Sometimes lodestone is sold in science stores as "Nature's Magnet." This is not a false claim; lodestone will attract iron filings and other small lodestone pieces.

Rock with garnet crystal


The book Appalachian Mineral and Gem Trails reports that garnets could be found at Cornwall that are "often an inch or more in size, with many crystals being shiny and well developed." My collection contains only one garnet specimen of more than a quarter of an inch in size. To date, it is my largest garnet from Cornwall. The crystal itself is dark reddish-brown with shiny surfaces on three of its faces. Since it is broken at one end, I can see that the cross section of the crystal resembles a hexagon. This specimen and the other incomplete crystals surrounding it are opaque, but I have found seams of tiny transparent garnet crystals in other rocks. The clear crystals aren't as well formed as the opaque sample described, as most were in a thin crust. Garnet is used primarily as an abrasive and also as a gemstone, most popular for being the birthstone of people born in January.

Brandon_serpentite

Serpentinite


At Cornwall, metamorphism is responsible for the wide variety of minerals that occur there. According to the Pennsylvania Geologic Survey, about 90 different minerals have been found at Cornwall. Serpentine is one of them. Thankfully, it was not very hard to identify.
The translucent dark and light green color alone gave me a pretty good idea as to its identity, owing to the commonness of serpentine in Pennsylvania. It has a greasy feel on some of its smooth surfaces, and it is soft enough to scratch with the tip of my metal pen. All these are telltale signs of serpentine. Although there are several varieties (chrysotile and antigorite), the form of serpentine found at Cornwall is the massive mixture of serpentine minerals called serpentinite.

brandon_limestone

Limestone with pocket chalcopyrite crystals


Limestone is one of the two major rock masses that formed the Cornwall site, and where there's limestone, dolomite is present as well. Most dolomite is mixed into the rocks at Cornwall, but some can be found in crystal form. The one I chose to illustrate shows a small (3/4") pocket of dolomite crystals in a limestone matrix. This association helped me with identification, since dolomite commonly occurs with limestone. These crystals are typical of dolomite, being milky-white and rhombohedral in shape. The Audubon Society Field Guide to North American Rock and Minerals tells me that dolomite has a hardness of 31/2-4. Again, the crystals can be scratched using the same ballpoint pen.

brandon_furnace

One of the furnaces at Cornwall


The other major constituent of the Cornwall area is a dark-colored igneous rock called diabase, which is a variety of gabbro. It is composed mainly of pyroxene (by definition, a dark-colored rock- forming mineral) and plagioclase feldspar. At Cornwall, the feldspar in the diabase is a light pink color. That characteristic made it very hard to identify. I had to go through all my rock and mineral books until, luckily, I found a matching picture. The specimen that I found is black with pink splotches and rough in texture. Actually, it is more than meets the eye. Some cavities in the feldspar patches have what appears to be specular hematite in them. Also, on the reverse side of the rock, tiny quartz crystals are studded at one end. Commercially known as "trap rock," diabase is used for crushed stone in concrete, road metal, railroad ballast, roofing granules, and riprap (stone used on an embankment that prevents erosion).

brandon_quartz

Rock with quartz crystal


Quartz is one of the most common minerals, so it seems only natural to find it at Cornwall. I found some exceptionally clear crystals; unfortunately, they are also exceptionally small. The specimen I drew is composed of several small transparent crystals around a larger (3/16") cloudy crystal located in a depression of the matrix. All the crystals are prismatic in shape, characteristic of quartz. Since quartz has a hardness of 7, my pen did not scratch it. Another sample I found has a druse (crystalline crust) of quartz on one corner of the rock.

Limestone with pyrite crystals.


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Rock with chalcopyrite crystals


Pyrite, more commonly known as "fool's gold," is found abundantly at Cornwall. The problem is that chalcopyrite is also. These minerals require careful observations to differentiate. One of my specimens consists of limestone with cubic gold-yellow crystals embedded in veins of calcite. Another sample has a mix of largish coppery crystals on one side of a light-colored magnetic rock. These specimens are alike in some ways, yet definitely not the same mineral. I referred to the Eyewitness Handbook: Rocks and Minerals for information. First of all, pyrite has a pale yellow color, while chalcopyrite has a brassy yellow color, frequently with tarnish (hence the copper-like color on the latter specimen). Second, pyrite has a hardness of 61/2 while chalcopyrite is a relatively soft 31/2-4. The first specimen's crystals were barely scratched by the metal pen, but it took only a mild effort to scratch the crystals of the other. Lastly, pyrite has the cubic (isometric) crystal system, and the crystals of the first sample were cube shaped. Using this evidence, I have concluded that the first mineral is pyrite and the second, chalcopyrite. On a side note, unique to Cornwall, trace amounts of gold and silver can be found in the chalcopyrite, and cobalt occurs similarly in the pyrite.

brandon_calcite

Rock with calcite crystals


One of the most widely known minerals, calcite, is present at the Cornwall mine. The specimen that I have drawn has bands of calcite crystals growing on opposite sides of the matrix. Also mixed in are small seams of chalcopyrite. The calcite crystals are a milky-white color, but they definitely are not quartz because of their rhombohedral shape and softness. A closer look at the border of calcite and groundmass reveals small black metallic crystals. When I held a magnet to the crystals, it was strongly attracted to them even though they are so small. I would say that this is my most interesting mineral specimen in the report because of all the minerals are found in a single rock.

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Talc-rich rock


The following rock had to be my most challenging to identify. It has a crust of magnetite on one side of a hard gray-green rock. The hard rock gradually turns white, soft, and smooth farther down its length. This end is covered with a fine powder and feels almost velvety when stroked. It could be scratched and broken with the least effort. When trying to identify this rock, I had to remember that hardness and brittleness are not the same. The factor that led me on the right track was the property of the powder that, when rubbed between my fingers, had a soapy feel. Soapstone, another name for talc, is noted for this feature. In light of these characteristics, I feel safe concluding that this rock is indeed talc. Interestingly enough, this was the only specimen of this kind that I have found at Cornwall.

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Diabase


As stated above, the two major rock bodies responsible for the ore at Cornwall are diabase and limestone. Diabase is formed when a mass of magma (underground lava) undergoes a slow cooling and crystallization process at depth. The resulting rock body is called a pluton. In the Triassic period, large bodies of magma spread up and into pre-existing Cambro-Ordovician limestone beds all over eastern Pennsylvania. The high temperature of the magma altered some of the sedimentary rocks with which it came in contact. Over a large area, this action is called contact regional metamorphism. The magma then cooled into the diabase we see today. Ore bodies were formed in several other places in Pennsylvania where this sort of intrusion occurred, including Dillsburg, Morgantown, and Cornwall. Geologists have proposed several theories as to where the iron ore at these locations originated:

1. THE SURROUNDING LIMESTONE. Unlikely, because the iron would have had to move a long way from the limestone to where it is.
2. THE ADJACENT DIABASE. Reasons:
• Diabase and ore are often found together.
• Ore is almost always above diabase, which agrees with the theory of an upward release of mineral constituents by diabases.
• The Cornwall diabase is low in iron.
• The concentration of iron is highest at the top of the diabase sheet.
3. A COMPOUND SHEET. More than one diabase sheet released its iron.
4. A PRIMARY MAGMA. One large magma body formed the diabase and iron ore. The iron and diabase concentrated separately out of that.

Although scientists can't rule out any of the above theories, they think that hypothesis number two is most probable.

In conclusion, the Cornwall Furnace area is not just part of Pennsylvania's Historical Trail good for class field trips, but rather it is an interesting locale rich in geologic history.

 

References 
Chesterman, Charles W. Audubon Society Field Guide to North American Rocks and Minerals. New York: Alfred A. Knopf, 1978.

Lapham, Davis M. and Carlyle Gray. Geology and Origin of the Triassic Magnetite Deposit and Diabase at Cornwall, Pennsylvania. Harrisburg: Pennsylvania Geologic Survey, 1973.
Pellant, Chris. Eyewitness Handbooks: Rocks and Minerals. New York: Dorling Kindersley, 1992.

Prinz, Martin and George Harlow, et al. Simon and Schuster's Guide to Rocks and Minerals. New York: The American Museum of Natural History, 1977 & 1978

Socolow, Arthur A. Geologic Map of the Cornwall Area, Commonwealth of Pennsylvania, Department of Environmental Resources, 1973

Van Diver, Bradford B. Roadside Geology of Pennsylvania. Missoula, Montana: Mountain Press:,1990.

Zeitner, June Culp. Appalachian Mineral & Gem Trails. San Diego: Lapidary Journal, 1968.

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