On Moor Lane in Lancaster is a distinctive fountain dedicated to Thomas Johnson, commemorating his contributions to the youth of Lancaster in the 19th century.
It's constructed from granite, a popular material for building memorials. Igneous rocks like granite are formed at extremely high temperatures and are extremely durable. The differing minerals present during this process can give different colours to the granite, which is what we want to look at here.
1) Describe the fountain - compare the different sections, mentioning the crystal sizes and colours you can see in each
2) The majority of the fountain is one type of granite. Using your description and the information below, which type of granite do you think this is. What mineral is likely to give it this colour?
3) The four inset panels of the fountain are another type of granite. What type of granite is this? What minerals are present to give it this colour?
4) 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.