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Boulders on the side of the river
Boulders on the side of the river
Welcome to your fifth field location. The GPS coordinates for this site are 0235082 6278298. It is important to note that the rocks in this location are not in situ so that means that they were deposited here after breaking away from their original location and transported by the river during a large flood event from site 10 further upstream. Site 10 is marked by the black star in the yellow unit on the right side of the 'Geology Field Map' at the top of each site webpage.
At first glance in the field, this rock appears quite weathered and pock-marked, requiring a fresh surface to get a better look. You can get a better look by using a rock hammer to smash open the rock, exposing the fresh interior. It is important to be careful when swinging a hammer to make sure there is no one in the firing line of rock chips that may fly off and that you are wearing appropriate eye protection.
Examine the model of a hand sample from site 5 in Pedestal3D below. Click on the < button to hide the side menu and give you more space to view the sample. Choose 'High' instead of 'Medium' from the drop down menu to maximise the quality of the sample image.
Task: Please choose an informative view direction in the Pedestal3D model and make a sketch in your field note book. You may zoom into interesting features and make multiple sketches rather than sketching the whole hand sample if needed. Please use the tools in Pedestal3D to measure grain size etc. for your sketches. It is important to recognise the difference between a fresh and weathered face of a sample. Sometimes weathering can conceal important features and the fresh face gives the best view, however the opposite can also be true. Study both faces in these samples to see which gives you the best view of the rock's microstructures.
About this rock
About this rock
This rock is another contact metamorphic rock, but the protolith was originally deposited as a fossiliferous sandstone or limestone in a shallow marine environment. What can you see in the photograph that helped us to determine this?
This rock formed during contact metamorphism following the intrusion of a very hot magma. The reactions inferred in this rock suggest that silica-rich fluids invaded the protolith and reacted with the existing mineral assemblage.
The fossiliferous material in the original sediment formed calcite in the sedimentary rock. When the Si-bearing fluid interacted with the calcite it experienced a chemical reaction to form the metamorphic mineral wollastonite (CaSiO3).
Wollastonite is typically colourless or white with elongate crystals that commonly form radiating or fanning aggregates. It has two cleavages at 90° and the hardness of this mineral is measured at 4.5–5.0 on the Mohs scale.
The wollastonite within this rock fills or copies the shape of the white curved crescents. The contact metamorphic event allowed for wollastonite to pseudomorph the material that originally filled those crescents.
Other minerals that may be found within this rock include garnet and diopside (clinopyroxene).
Crescent moon holes
Crescent moon holes
The weathered exterior of the wollastonite-rich rock can be seen here. You will notice that the appearance is very different from that of the fresh interior. The hollow crescent moon shaped holes are representative of the material that was replaced following metamorphism.
Question 1: Given the description of the rock, can you guess what the crescent shapes within the rock used to be?
Mineral habit & Sulphide minerals
Mineral habit & Sulphide minerals
A minerals habit refers to the general appearance or any term you would use to describe the general appearance of the mineral.
Some terms you would be familiar with in using to describe a minerals habit include prismatic, platy, radiating, fanning, and micaceous.
Inspect the white wollastonite grains in the figure. Can you think of an appropriate way to describe the habit?
Question 2: What terms would you use to describe this mineral habit?
Chemical Reactions
Chemical Reactions
Wollastonite which has the chemical formula CaSiO3 is formed from the reaction between quartz (SiO2), and calcite (CaCO3).
Given this information, an equation can be written to express this chemical reaction.
Quartz + Calcite --> Wollastonite + Gas
Quartz and Calcite react to create wollastonite and gas. This equation needs to be balanced so that the chemical components on one side of the reaction match the other. Given this information can you figure out the appropriate chemical reaction?
SiO2 + CaCO3 = ? + ?
Question 3: What component is missing here that would balance the equation?
Because the calcite within this rock has been replaced by wollastonite, testing the rock with HCI will not see any reaction if all the calcite was consumed. Before metamorphism, however, splashing some HCI on the calcite bearing fossiliferous sandstone would have seen a fizzing reaction between the acid and calcite.
Economic Minerals
Economic Minerals
Metamorphic events can often lead to the formation of economic minerals, particularly through hydrothermal activity. The sulphidic inclusions within the Si-rich metamorphic fluid that penetrated into the protolith rock allowed for the precipitation of this economic mineral.
This is a sulphide economic mineral and its gold appearance can be confused for gold. An important characteristic of this mineral that can help with its identification is its cubic habit.
Question 4: What is this sulphide mineral?
Following the metamorphism, the rock is no longer referred to as a fossiliferous sandstone. The new metamorphic name describes the heat altered protolith and draws upon the texture of the rock and the new mineral assemblage.
Question 5: What would the metamorphic name of this rock be called?
In metamorphic rocks, specific terms are used to describe large crystal grains. It is important to note that the term used to describe large crystals in igneous and metamorphic rocks is different. For a metamorphic rock, the term 'porphyroblast' is used to describe the large grains. The wollastonite in some some samples is much coarser than the other minerals in the rock and in these samples we could describe the porphyroblastic texture.
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