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The Oxidation Potential of Mineral Assemblages in Rocks From New York and Washington State

Michael Pic

In 1999, I was in Port Townsend, Washington, on an internship at the Marine Science Center with Libby Palmer. We went to North Beach, where I began collecting my rocks. The actual beach was very narrow, about 5-7 meters wide. At the time I arrived at the beach, high tide had begun to set in. As I walked along the beach, I was very excited when I saw the different colored rocks. In my excitement, I began to collect rocks as soon as I saw them; I ended up with over 36 pounds of rocks. I noticed that they were very different from the rocks I had seen in New York State. The rocks I collected I identified as metamorphic; I came to this conclusion because of the "bands" I saw throughout each rock.

Two weeks later, I spoke with Veronique Robigou of Washington State University. She confirmed that the rocks I had collected were metamorphic, but that the "bands" I saw were actually mineral veins. I had observed that metamorphic rocks from Washington State were different from those in New York State, and I decided to find out why.

As I continued my study through 2000, I collected metamorphic rocks from New York State also. I began drawing my rock samples and identifying the minerals I knew. In 2001, I was able to find out why the rocks from Washington were metamorphic; it was due to their mineral composition. I was able to find this out by making thin sections of my rock samples at Brooklyn College, and identifying the minerals in them with Dr. Powell. According to Dave Knoblach of the Northwest Geological Society, the New York and Washington rocks differ because they originated from different parent rocks and were "subject to different metamorphic histories." Using this information, I found out how the metamorphic rocks in New York and Washington were formed.

Samples of Washington and New York State Rocks.

Samples of Washington and New York State Rocks.


Agents that cause metamorphism are temperature and pressure. High temperatures can change rock by changing the structure of the minerals in the rock and causing new minerals to form (Anderson). Pressure, also known as stress, can be confining or directed. Confining pressure is pressure due to overlying rocks, or compaction. Directed pressure is pressure that is higher in certain places than in others. Tectonic forces cause this type of pressure (Anderson).

The New York and Washington rocks experienced a type of metamorphism called regional metamorphism. According to Blatt and Tracy (1996), regional metamorphism affects a large area of land. Regional metamorphism occurs during orogenies, when tectonic plates collide.

Such an orogeny, called the Greenville orogeny, took place in New York State 1.12 billion years ago, when Proto-North America collided with the Grenville subcontinent. This orogeny formed the metamorphic rocks of the Adirondack Mountains. The product of this collision is known as the Grenville supercontinent (Horenstein, 2001). After many years of erosion and rifting, the Grenville supercontinent split apart. Four hundred fifty million years ago), another orogeny called the Taconic occurred when Proto-North America collided with the Taconic island arc, forming the Taconic Mountains in southeastern New York State (Isachen et al., 2000).

Orogenic events have shaped the West Coast of North America. Two hundred million years ago, after the Acadian orogenic event.the Okanogan subcontinent collided with Proto-North America forming the Okanogan orogeny. (Alt, 1984). About 45 million years ago, the Cascade subcontinent collided with Proto-North America and formed the Cascade Mountains, which extend from British Columbia, Canada, to northeastern Oregon (Tabor, 1996). I learned that both the metamorphic rocks from New York and Washington were formed during orogenies, though at different times in geologic history."

I next wanted to find out why these metamorphic rocks, which had all been formed by the same process, appeared to be made up of different minerals. Minerals that are typically found together in rocks are called mineral assemblages. Mineral assemblages can tell us a lot about the metamorphic processes that formed the rock. According to Blatt and Tracy (1996), mineral assemblages are used to classify a rock‘s grade of metamorphism, name, and facies. 

For example, fine-grained metamorphic rocks suggest a low-grade metamorphism. Coarse-grained metamorphic rocks imply high-intensity metamorphism. The type of foliation in the rock can also determine the grade of metamorphism. Schistosity implies a medium-grade metamorphism, and gneissosity is a characteristic of high-grade metamorphism.

In general, a facies can be defined as a set of metamorphic mineral assemblages that forms under specific temperature and pressure conditions.

The rocks collected from Washington were fined-grained and metabasaltic in composition. It was determined that they were from a low-grade metamorphism called a lower greenschist facies. The composition of the Washington rocks was similar to the Abititi Greenstone belt in Canada (Powell et. al, 1993). The greenschist facies is composed of chlorite, actinolite, epidote, albite, and calcite.

The rocks collected from New York were coarse-grained, and showed evidence of schistosity and gneissosity (Blatt and Tracy, 1996). The rocks could be classified as having amphibolite and granulite facies. The amphibolite facies indicate a medium-grade metamorphism. The groups of minerals that amphibolite facies contain are quartz, chlorite, muscovite, hornblende, garnet, biotite, and epidote. The granulite facies indicate a high-grade metamorphism. Granulite facies contain hornblende, garnet, K-feldspar, Na-feldspar, quartz, and muscovite.

Rocks with the same facies generally contain similar minerals, but they have different amounts of each mineral (Table 1). In order to determine the differences between the New York and Washington rocks, the chemical composition of each mineral was to be recorded (Table 2). It was noted that most of the minerals in the rocks contain elements that have the ability to be oxidized. However, the minerals contain varying amounts of each element  (Table 3) . It can be inferred that rocks have the potential to weather based on the percentage of oxidizing elements found within the minerals. Oxidation is a form of chemical weathering; one example is rust. The purpose of this study is to compare the oxidation potential of Washington and New York rock samples.

Table 1

Table 1


Michael Table 2

Table 2


Tables 3, 4, and 5 (Click to enlarge.)


 

Hypothesis
Based on the mineral assemblages found in the granulite, amphibolite, and greenschist facies, the rocks from New York with granulite facies will have a lower oxidation potential because they contain fewer oxidizing elements. The rocks from Washington containing minerals with a high percentage of oxidizing elements will weather faster.

A Tile Master was used to cut the rocks into sections. The sections were polished on one side using silicon 120, 320, and 600 m (CK) particles. The sections were then glued to glass slides using proxy. The slides were placed into a vacuum chamber to dry for 24 hours. A thin-section cutter was used to cut the rocks into smaller sections. The slides were polished with the thin-section grinder, and then were then sanded down by hand, using the silicon particles to get them as close as possible to 0.03 mm thick.

 

Methods and Materials
A Tile Master was used to cut the rocks into sections. The sections were polished on one side using silicon 120, 320, and 600 m (CK) particles. The sections were then glued to glass slides using proxy. The slides were placed into a vacuum chamber to dry for 24 hours. A thin-section cutter was used to cut the rocks into smaller sections. The slides were polished with the thin-section grinder, and then were then sanded down by hand, using the silicon particles to get them as close as possible to 0.03 mm thick.  

A Nikon Eclipse E600 polarizing microscope was used to identify the minerals within the thin sections, and to estimate the percentages of minerals contained in the rocks. Chemical formulas were used to calculate the percentages of oxidizing minerals within the rock samples (Table 2, above).

In order to calculate the percentage of oxidizing minerals, one counts the total number of metallic elements, and then divides that number by the total number of elements in the mineral. Then one takes the result of that calculation and multiplies it by the total percentage of the mineral in that rock. One repeats these steps for all the minerals in the rock sample, and then adds together the products. The percentage of oxidizing minerals within the rock sample will then be determined  (Table 4, above).

 

Results
Sample A from New York, which was identified as metanorthrosite, contained 60 percent quartz, 20 percent feldspar, 10 percent chlorite, 7 percent opaque minerals, and 3 percent epidote. Sample B from Washington, which was identified as greenstone, contained 70 percent actinolite, 10 percent pumpellyite, 10 percent chlorite, 5 percent quartz, and 5 percent epidote. Sample C, also from Washington, was identified as greenstone. It contained 50 percent chlorite, 15 percent feldspar, 15 percent quartz, 17 percent actinolite, and 3 percent opaque minerals. Sample D from New York was quartzite; it contained 99 percent quartz and 1 percent muscovite. Sample E was determined to be schist, and was composed of 50 percent quartz, 35 percent biotite, 15 percent garnet, 10 percent muscovite, and 2 percent hornblende. Sample F, another greenstone from Washington, contained 70 percent chlorite, 15 percent feldspar, 9 percent opaque minerals, 3 percent quartz, and 3 percent pumpellyite. Sample G, a gneiss from New York, contained 35 percent K-feldspar, 35 percent quartz, 24 percent biotite, 5 percent opaque minerals, and 1 percent actinolite. Sample H, also gneiss from New York, contained 80 percent quartz, 15 percent hornblende, and 5 percent feldspar.

 

Discussion
As shown in Table 5 (above), quartzite had the smallest percentage of oxidizing minerals, 0.34%, which indicates that it is the most resistant to chemical weathering (oxidation). The second most resistant is metanorthrosite, which had 8 percent oxidizing minerals. The gneiss in Sample H would also be less likely to oxidize because it had 7.16 percent oxidizing minerals. The other samples would oxidize more quickly, especially the greenstone in Samples C and F and the schist (Sample E), which was made up of 17.5 percent oxidizing minerals. Samples B and G would not oxidize as quickly as Samples C, E, and F.

 

Figure 1
Taking into account the metamorphic facies each rock sample contained, I found that the greenstones (Samples B, C, and F) had the highest oxidation potential. This means that the rocks with greenschist facies would most likely oxidize before the rocks with amphibolite and granulite facies. However, according to my analysis of the chemicals in the rocks, one sample with amphibolite facies, metanorthrosite (Sample A), had a low oxidation potential, while the schist with the same amphibolite facies (Sample E) had a high oxidation potential. The metanorthrosite is older than the schist and was probably exposed to weathering for a longer period of time, and this caused its metallic minerals to oxidize. The rocks with the granulite facies (Samples G and H) had a low and a medium oxidation potential; this is due to the mineral composition of each rock.

 

Conclusion
Observing the physical features of the rocks on North Beach, first sparked my curiosity about why New York State rocks were different from the rocks of Washington State. Through conducting background research, I found that the rocks from New York and Washington formed in similar metamorphic environments but differed in age. I wanted to know why rocks that formed in similar metamorphic environments looked so different. So I researched their mineral compositions and found that they contained similar minerals. But through further observation, I found that they contained varying percentages of each mineral. From this, I compared the chemical composition of each mineral, and determined that each mineral contained different amounts of metallic elements. Since metallic elements have the ability to oxidize, I used the oxidation potential to compare the metamorphic rocks from New York and Washington.

Based on the results of this investigation, I found that New York State metamorphic rocks have the lower oxidation potential. The New York rocks were made up of granulite facies, which are the least likely to oxidize because they do not contain a high percentage of oxidizing elements. The Washington State rocks were made up of the greenschist facies, which oxidize more. This supports my hypothesis that New York metamorphic rocks have a lower oxidation potential than metamorphic rocks from Washington State. I plan to further this study by conducting a full chemical analysis of the rocks. I also plan to find out about the rocks‘ potential use in construction engineering.

Sample A (Click to enlarge.)


Sample B (Click to enlarge.)


Sample C (Click to enlarge.)


Sample D (Click to enlarge.)


Sample E (Click to enlarge.)


Sample F (Click to enlarge.)


Sample G (Click to enlarge.)


Sample H (Click to enlarge.)


 

References

Alt, David and Donald Hyndman. Roadside Geology of Washington. Missoula, Montana: Mountain Press Publishing Company, 1984.

Anderson, Greg. "Earth Sciences 10." Lecture 13: Metamorphic Rocks. Scripps Institution of Oceanography Retrieved from the World Wide Web on February 10, 2001: http://sorcerer.ucds.edu

Blatt, Harvey and Robert J. Tracy. Petrology: Igneous, Sedimentary, and Metamorphic. New York: WH Freeman and Company,, 1996.

Horenstein, Sidney. Environmental Programs, American Museum of Natural History. Personal communication, February 15, 2001.

Isachsen, Y.W., E. Landing, J. M. Lauber, L.V. Richard, and R. Edulars. Geology of New York: A Simplified Account. Albany, New York: Purple Mountain Press, 2000.

Knoblach, Dave. Webmaster, Northwest Geological Society. Personal communication, December 8, 2000.

Powell, Wayne G., D.M. Carmichael, and C.J. Hodgson. "Thermobarometry in metabasites of the Abitibi Greenstone Belt, Superior Province, Canada." Journal of Metamorphic Geology 11, 1993: 165–178.

Robigou, Veronique, Marine Geologist, University of Washington. Personal communication, September 6, 1999.

Tabor, Rowland and Ralph Haugerud. Geology of the North Cascades: A Mountain Mosaic. Seattle: The Mountaineers Books, 1999.

 

Acknowledgements
I would like to thank the following individuals for their advice and support throughout the duration of my research: my parents, Veronique Robigou of the University of Washington, and members of the Northwest Geological Society: John Whitmer, Emery Bayley, and Dave Knoblach. I would also like to thank Professor Iskander of Polytechnic University, Everton Barrot, the staff of the Yes center, and the staff of the Science Skills Center High School: Michele Williams, Curtis Turney, Yolanda Clayton, and Man Yan. I would also like to thank Lester Orolick of Polytechnic University for teaching me how to us the LAPRO for cutting rock samples, and Mr. Rocha of Brooklyn College for his advice and time in teaching me how to prepare thin sections. I thank Dr. Powell for his advice and time in showing me how to interpret and analyze thin sections.

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