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By:Brandon
Age: 15
Grade: 10
Pennsylvania |
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One of the furnaces at Cornwall |
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.
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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.
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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.
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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.
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Limestone with pyrite crystal |
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
(page 6) 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.
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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).
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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.
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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.
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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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Rock with chalcopyrite crystals |
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. |
Talc-rich rock |
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:
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Limestone with pocket of dolomite crystals
Diabase |
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. |
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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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