Introduction
Cedar Creek is situated within the basin-shaped Samford Valley. This creek is made up of a granitic intrusion that has been slowly weathered and eroded over time (SERF, 2012).This creek system and its water flow seeps into the surrounding rock and slowly weathers this creating deeper runs of water and wider bends etc. Chemical weathering also affects this creek system with different minerals (feldspar in this case) in the granodiorite being broken down causing different chemical compositions.
Porous weathered rocks are important in the sense that they store nutrients that can be passed onto invertebrates and plants within the creek system. Before calculating the porosity of the rocks in Cedar Creek we are able to hypothesize that the porosity directly correlates with how fast the rock will weather, which essentially means these rocks are more easily eroded due to their porosity. During the field trip we examined the rocks and their placements and saw the importance of weathering in the way that the creek system flows perfectly and is able to hold an abundance of invertebrates as well as having the ideal riparian zone.
Through experimentation the basanite was weighed before and after being exposed to water (erosion). This study was able to help us calculate the initial and final mass of the basanite and the rate at which the erosion occurred. This also helped determine the porosity of the basanite. In this report, the CaSO4 + H2O system will be used to analyse the hydration related reactions. This reaction is used as its significantly faster than the reactions observed directly in the field.
Methods
To conduct this experiment, multiple stations were rotated through to gain data. These stations consisted of:
Station 1:
A piece of weighing paper is placed on a set of scales and the scales are then tared to zero. The powdered basanite (plaster of paris) is then measured out to between 0.98 and 1.02 grams and the sample weight is recorded.
Station 2:
The basanite powder is then pressed into a pellet press. Pressure is then applied to this and a maximum pressure is then recorded and a disk is received.
Station 3:
Calipers are tared and the thickness of the pellet is recorded from 3 different locations. The average thickness is then recorded.
Station 4:
To make sure the weight of the pellet has not changed through handling, it is to be weighted a second time. A foil dish is to be labeled and weighed to 3 decimal places. The pellet is then placed in this foil dish and the weight is recorded to 3 decimal places. The mass is then calculated.
Station 5:
A pipette is prepared with 0.5mL of water and is slowly dripped onto the pellet across a one-minute interval (timed through a stopwatch). Any pellet breakages and water spilt is recorded.
Station 6:
The sample is then placed in a drying oven to dry off non reacted water.
Station 7a:
The sample is then reweighed to calculate the change in mass between the initial pellet and the pellet after hydration. This is done by weighing it in the foil dish and recording to 3 decimal places.
Station 7b:
A mortar and pestle are cleaned and the disk is powdered with it. This is then placed in a zip lock bag and labeled.
Station 8:
The sample is analysed with the Olympus portable XRD. The brand name and model and the x-ray tube type is recorded.
Station 9:
Data is ensured to be 100% accurate and shared across any group members.
Results
During the lab the pellet directly had a lot of water run from its surface which would have affected how the basanite pellet was able to react. When fully reacted there was not a lot of mass change and the porosity of the individual pellet was not changed drastically. Although with the class data as seen in the graphs below it can be seen that the more porous the pellet the more gypsum it can become.
Figure 1: Gypsum VS Porosity
A general linear trend seen here. As porosity goes up, the percentage of gypsum goes up.
Figure 2: Porosity of rock VS Change of mass in final product
This is showing the change in porosity during the experiment and how it increases the mass as the porosity rises.
Figure 3: Porosity percentage in relation to pressure applied to the pellet
It is seen that the higher the pressure the lower the porosity.
Table 1: Results directly from lab
Station 1
Station 2
Station 3
Station 4
Station 5
Station 6
Station 7
Station 8
Grams of plaster of paris?
1g
Tonnes of pressure used?
8
Micrometer measurement of pellet thickness (mm)? (1,2,3)
3.47, 3.57, 3.43
Average pellet thickness:
3.49
Volume of pellet:
463.74
Grams of petri dish and filter paper?
1.055
grams of petri dish + plaster of paris disk
2.058
grams of pellet
1.003
has the pellet broken?
no
When applying water did it spill?
Splashed everywhere
What temp is the oven at?
105
how long is the pellet in the oven?
15min
Grams of petri dish + pellet post drying?
2.115g
grams pellet post drying
1.06g
XRD details:
Terra-164-olympus cobalt (tube type)
The results showed the pellet only grew 0.06g post drying which shows it’s not overly porous. The more porous the pellet the more gypsum it was able to become which will be discussed further in the discussion.
Discussion
It was hypothesized that the higher the porosity the faster the rock was to be eroded. This was proven directly through the experiment and analysis of the class data. It can be seen in figure 1 that the data directly shows the porosity rising and the gypsum being created through the reaction. There was an obvious interaction found between the relationship of basanite and gypsum vs porosity. When the H2O is added to the reaction of the basanite it is seen that the porosity rises as well (as seen in figure 1) which is what causes gypsum to occur.
Through analysis it can also be seen that the pellets that had been created under lower pressure were seen to have a higher porosity. This can be seen in figure 3 where the trend shows directly that this is correct and that the lowest pressure correlates with the highest porosity. This also links back to the initial statement of the higher the porosity, the higher the gypsum as the water needed to infiltrate the basanite would not need to be as high if the porosity is high.
This porosity is also seen in the Samford Valley when we had attended the field trip. Plagioclase feldspar is directly related as we saw that the higher along the bank the rock was situated, the higher the porosity and therefor in turn an increase in hydraulic weathering.
Boyles law outlines that the product of volume and pressure must remain constant which shows that the changing of the tonnes of pressure in the experiment showed this law to be correct. When looking at porosity, it refers to the amount of space not occupied within the item (in this case pellet). When looking into the space not occupied by the gypsum or basanite it can be seen that if more pressure is added initially to the pellet, there will be less volume left to be occupied therefor a lower porosity.
There were also a few outliers which would have been caused by human error through the experimental testing. One main issue was the data being skewed from the wrong heat used by the ovens for the first practical which essentially ruled that data as unusable. There were also a few groups with incorrectly displayed data which is what may have caused some of these outliers.
When looking directly at the mass of the pellets pre and post experiment it was noticed that the mass changed in regards to the porosity directly. In figure 2 it can be seen that as the porosity percentage increases, it in turn increases its mass. This would be due to the reaction causing the basanite to hold the molecules of water and bonding them causing the mass to increase. It can also be seen that the more porous pellets held more of the water which was due to their higher porosity.
Another observation made was that the higher the porosity of the pellet the smaller the amount of gypsum being observed by the XRD. This can happen when the higher porosity gives more room for interaction between the reactants in the experiment. This in turn leads to a higher product yield. This was then concluded that the pellets that were exposed to more pressure were lacking the same porosity as the pellets under minimal pressure, which would then in turn have a higher amount of unreacted basanite with the lowest pressures having the most gypsum.
The overall aim of this experiment was to determine whether a higher porosity leads to a faster rate of hydration weathering. This was found to be related directly to the pressure being applied to the pellet itself. It was seen that when the pellet was placed under higher pressure, the pellet then has a lower porosity which caused a slower rate of hydration weathering. Which essentially proved the aim right, that the higher porosity, higher weathering rate. Which was seen physically in the plagioclase rocks in the high banks of Cedar Creek that had been weathered quite a lot. Although it was seen that overall there was not a lot of assumptions that could be made about the rocks themselves and that essentially they were seen to have low porosity, especially the ones in contact with the water.
If this experiment was to be repeated it would be ideal to have more options for different amounts of water, as well as more pellets under a more diverse range of pressures tested. It would also be interesting to see different substances tested to show different reactions of rock under these circumstances. Also if less human error was to occur, this would give clearer data. Essentially, the first practical could have been carried out more accurately so that in turn the data would not have to be ruled out completely.