This is an impressive and busy public space in the heart of Leeds with interesting statues, and lots and lots of Granite!
We can use some of the examples here to compare the different minerals present in granite and explain why they are different colours.
GZ will place you in front of one of the many statues of nymphs holding lanterns in the square. Have a look at the base of the statue.
1) Describe the largest, cube shaped, piece of granite - what colours can you see, what size are the crystals. What mineral(s) contribute to this colour?
2) The large circular base (with inset lights) is a different type of granite. What colours can you see, what size are the crystals. What mineral(s) contribute to this colour?
3) The other granite is immediately above and below the cube shaped granite from Question (1). What colours can you see, what size are the crystals. What mineral(s) contribute to this colour?
4) Compare the large cube on the statue from Question (1) with the similarly sized block at the end of the wall to the left as you face the statue. These are the same types of granite, but do you think they have the same source? If not, which do you think cooled more slowly, and why?
5) Take a photo of yourself at GZ, or an identifying item, being careful not to reveal any of the answers to the questions.
Please submit your answers via message though the Geocaching website or by sending me an email - there is no need to wait for a response before logging your find.
Formation of granite
Molten rock containing a high content of silica, minerals, and alkali metal oxides seeps into gaps in sedimentary rock layers before slowly cooling and solidifying underground over the course of millions of years, creating granite.
As the molten rock gradually cools over time, crystals form at different periods of time from the various minerals that are present which alters the final appearance of the granite.
One piece of granite typically consists of 20-60 percent quartz, 10-65 percent feldspar, 5-15 percent mica, and minor or trace amounts of amphiboles and other minerals.
What gives granite it's colour?
Below is a list outlining the primary colour of common minerals so that we can understand what colour the granite is likely to be if a high ratio of that mineral is present.
- Quartz: Milky White or Colourless
- Feldspar: Off-White
- Potassium Feldspar: Light Pink to Red
- Biotite: Black or Dark Brown
- Muscovite: Light Gold or Light Yellow
- Blue Labradorite: Blue
- Amphibole: Black or Dark Green
Black Granite
Black granite gets its colour and glassy appearance when it’s made up of around 20 percent quartz along with various other minerals, like biotite and/or black mica.
White Granite
White granite contains a high percentage of quartz and feldspar, which gives it a milky off-white and somewhat opaque appearance.
Many white granite memorials also contain black spots of different sizes which is likely due to the presence of small amphibole grains.
Black and White Granite
Some granite has an equal blend of both black and white, this combination is due to an equal amount of quartz, feldspar and amphibole.
Grey Granite
Grey granite gets its colour from a high ratio of colourless quartz.
Pink Granite
Pink granite gets its salmon tone from a high percentage of the potassium feldspar mineral.
Within the material of pink granite, you can often see small specks of semi-transparent milky quartz, dark specks of amphibole, and opaque white feldspar.
Crystal Size and Cooling in Granite
In igneous rocks such as granite, the size of crystals is intricately tied to the rate at which the molten magma solidifies. Consider it akin to the process of cooling a liquid to form ice. Swift cooling leads to the development of minute crystals, resembling the rapid freezing of small water droplets in an ice tray. On the other hand, gradual cooling affords more time for crystal growth, resulting in larger, well-defined crystals akin to the formation of sizable ice crystals in a slow-freezing process.
If magma cools quickly, for example when lava erupts from a volcano, then many crystals form very quickly, and the resulting rock is fine-grained, with crystals usually less than 1mm in size.
If magma is trapped underground , it cools slowly because it is insulated by the surrounding rock. Crystals have more time to grow to larger size and crystals 2mm and larger can form.
For example, the picture below is Shap Granite, formed in the Lake District - the coin for scale here shows a slow cooling crystal as it's large in size.
