Mineral or Rock Project
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A Building Block Rock: Coquina
Madeline Irwin & Liam Casey
February 8, 2024
Introduction and Rationale
Minerals and rocks provide a template of what changes have occurred over the span of Earth’s existence. Through many different scientific methods, the identification of different minerals and rocks has become a very common process. Identifying both minerals and rocks is extremely important as these identifications lead to an understanding of the composition, formation, and history of the Earth. Additionally, in an article entitled “Collecting Rocks”, Rachel Barker explains that “
By studying how rocks form and change, scientists have built a solid understanding of the Earth we live on and its long history” (Barker, 2022), which leads to the identification of rocks being extremely important in order to understand the history of the Earth.
The sample being analyzed is pictured below in Figure 1. The source does not reveal where the sample originated from. (
Cronin-Geoscience-ed4828, 2020). Based purely on the physical appearance of this specimen (Figure 1), the conclusion that this specimen can be identified as coquina has been gathered. This conclusion has been formed by evaluating the initial appearance of this specimen, which showed a large amount of shell fragments being loosely cemented. Aside from the shells, this specimen appears to be a tan color. The specimen feels very porous with almost a
granular texture. Based on the initial observations and an article entitled “The Impact of Coquina: Unlocking the Mystery Behind the Endurance of the Oldest Fort in the United States” which explains, “
Coquina is primarily made up of crushed shell,
fragmented fossils and coral, limestone, sand, minerals and clay [
1
].” (Subhash, et al., 2015), the observations of this specimen (Figure 1) appear to be coquina.
Figure 1
. Unknown Specimen. This image clearly shows the unknown specimen. Visibly
showing the makeup of shells throughout the specimen and the material surrounding them.
Methodology
Proceeding to identify this rock sample, multiple tests were performed. Based on the findings of texture and composition of the sample, the conclusion of what type of rock this sample (Figure 1) is, was formed. Using Figure 3.16 in the textbook, the name of this rock was discovered (Tarbuck & Lutgens, 2017). Further, testing the specimen’s reaction to hydrochloric acid the mineral group of this rock was discovered. Alternatively, the specimen’s grinded reaction to hydrochloric acid was also tested (
Cronin-Geoscience-ed4828, 2020). Additionally, observing the grain size that the fossils are cemented in by taking a closer look at the specimen, the grain size was found (Figure 2). Knowing the composition, texture, reaction to hydrochloric acid, and the layers, I once again referred to Figure 3.16 in the textbook, and the rock’s name was
then identified.
Figure 2. Closer view of a section of the rock sample shown in Figure 1.
Data Analyses and Results
By first analyzing the texture and composition of the unknown specimen, the conclusion that this specimen (Figure 1) is a sedimentary rock was formed. The original source of this specimen revealed that the composition consists of calcite and biochemical limestone (
Cronin-
Geoscience-ed4828, 2020). I further analyzed the texture by examining the images (Figure 2) closely and observing the shell fragments throughout the specimen that were being loosely cemented. Further, matching my texture and composition findings with the “Chemical, biochemical, and organic sedimentary rocks” graph in the textbook (Figure 3.17), I was able to conclude that this specimen (Figure 1) is sedimentary (Tarbuck & Lutgens, 2017). The matrix of this specimen (Figure 2) is fine-grained and consists of sediments that are silt or clay-sized particles that hold the fossils together (
Cronin-Geoscience-ed4828, 2020). Further identification, such as the cleavage and streak color, of this specimen is unclear. Although the specimen’s primary minerals reacted weakly to hydrochloric acid, with grinding of the specimen the reaction
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became a bit stronger (Figure 3 & Figure 4), which shows that this specimen’s mineral group is calcite or calcium carbonate (
Cronin-Geoscience-ed4828, 2020). Figure 3. Specimen having a weak reaction to hydrochloric acid in a natural state.
Figure 4. Specimen having a stronger reaction with hydrochloric acid after being grinded prior.
The textbook also revealed that the variety of limestone consisting of shell fragments means that the specimen has a biochemical origin and was formed through the deposition and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water, at the Earth’s surface (Tarbuck & Lutgens, 2017). This specimen (Figure 1) was also revealed to have a layered fabric (
Cronin-Geoscience-ed4828, 2020). Referring to Figure 3.16 in the textbook again, after finding the composition, texture, reaction to hydrochloric acid, and layers, the rock’s name was then identified as coquina. Through the composition, texture, reaction to hydrochloric acid, and layers it was determined that these characteristics did not fit any other rock. The major coquina deposits around the world are located in the United States, specifically
along the coasts of Florida and North Carolina, Australia, Brazil, Mexico, and the United Kingdom (Hausman, 2022). Coquina’s main human usage goes towards architecture. Focusing on Florida, Coquina has been used extensively for over four centuries and is common in many colonial buildings. Coquina was also used in the construction of nearly every East Florida plantation and sugar mill (Hausman, 2022). Crushed stone made from coquina rock is also used to fix unpaved roads due to how well the material drains and is used as a base material in many construction projects. Walls, monuments, and buildings are the most common human uses of coquina, yet coquina is not extremely durable and tends to crumble over time, so it typically is not used in these ways unless other materials are unavailable (Hausman, 2022).
A scientific research study entitled “The Impact Response of Coquina: Unlocking the Mystery Behind the Endurance of the Oldest Fort in the United States” created by Sanika Subhash and others, focuses on Castillo de San Marcos fort in St. Augustine, FL, and the fort’s endurance against impacting cannonballs that was attributed to coquina. The Castillo de San
Marcos fort has held up against multiple wars and hurricanes for over 300 years and during a military siege, the discovery that walls did not shatter upon impact, but instead absorbed the cannonballs was found. This discovery led to extensive research on the main materials of the fort, coquina, in which they discovered that “the microstructure of coquina does not lend itself to macroscopic brittle fracture, but absorbs impact energy by progressive crushing similar to a cellular structure, and thus allows for a high specific energy absorption” (Subhash, et al., 2015). In an attempt to explore the extreme durability and endurance of the fort, research found that this
was due to the fort walls being made of coquina. Coquina was found to absorb strong impacts without causing large cracks or catastrophic failure of the wall and instead left cannonballs embedded several inches deep into the wall, but still left no cracks or harsh damage. This study conducted multiple experiments under three loading conditions: (1) quasistatic uniaxial compression, (2) dynamic uniaxial compression, and (3) low-velocity ball impact. Since coquina
is a partially lithified sedimentary rock and is found abundantly along the coast of Florida, this became a perfect building material as it can be easily shaped for structural usage due to the many
pores and softness when quarried (Subhash, et al., 2015). This study concluded by holding that “The energy of the impact was instead absorbed locally due to progressive crushing of the coquina. These mechanisms contributed to the ability of the fort to endure sieges and the inclement Florida weather (hurricanes and tropical storms) over the past 300 years” (Subhash, et al. 2015), giving a real-life example of the benefits and usage of coquina.
Conclusions, Limitations, and Implications
Overall, the unknown specimen pictured in Fig. 1 has been identified as coquina. Through the composition, texture, reaction to hydrochloric acid, and layers it was determined that these characteristics could not fit any rock besides coquina. Texture analysis by examining
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the images (Figure 2) closely and observing the shell fragments throughout the specimen that were being loosely cemented, looking at the matrix of this specimen (Figure 2), which is fine-
grained and consists of sediments that are silt or clay-sized particles that hold the fossils together,
and observing the specimen’s primary minerals reacting weakly to hydrochloric acid, but with grinding of the specimen the reaction becoming a bit stronger (
Cronin-Geoscience-ed4828, 2020), all supported the findings of the unknown specimen being coquina. Based purely on the initial physical appearance of the specimen (Figure 1), the conclusion that this specimen can be identified as coquina was gathered, leaving us to accept the hypothesis. The newfound understanding of coquina is beneficial to me as I had not previously known how useful this rock was, but have most likely encountered this rock multiple times. I find it fascinating how coquina is used in so many different colonial buildings and the study of the Castillo de San Marcos fort was something I would definitely be interested in exploring. This rock is impactful to different architects and environments, yet is not as thoroughly researched or written about as I thought it would be. I believe that more extensive research and analyses should be created and published on
coquina, as this rock is extremely important.
References:
Barker, R. (2022). Collecting Rocks. USGS.
https://pubs.usgs.gov/gip/collect1/collectgip.html
Cronin-Geoscience-ed4828 [Username]. (2020, September 7). Unknown 9122 vc [Video]. YouTube. https://www.youtube.com/watch?v=xX-Q3p-fDm0
Hausman, T. (2022, August 9). Coquina Rock Definition, Formation & Characteristics. Study.com. https://study.com/academy/lesson/coquina-rock-shells-
types.html#:~:text=Crushed%20stone%20made%20from%20coquina,to%20build
%20walls%20and%20monuments
.
Subhash, S.G., Jannotti, P. & Subhash, G.
(2015, November 5) The Impact Response of Coquina:
Unlocking the Mystery Behind the Endurance of the Oldest Fort in the United States.
J. dynamic behavior mater.
1, 397–408. https://doi.org/10.1007/s40870-015-0035-1
Tarbuck, E.J., Lutgens, F.K. & Tasa, D. (2017). Earth Science. Pearson.