The granite stone that make up seating area adjacent to parliment house are a form of granite known as "Kingstone Blue Granite", as it was mined from an area not far from Kingston SE in the 1970s. This granite has an interesting feature imbedded in the stone: feldspar crystals.
Feldspar crystals are created when a process called crystallization, which occurs when magma cools down and solidifies. Feldspar is a common mineral found in many types of igneous rocks, such as granite, syenite, and diorite.
During the cooling process the magma begins to solidify from the outside in, with the outer layer cooling more quickly than the inner layer. As the magma cools and solidifies, minerals in the magma start to crystallize out in a specific order based on their melting points.
Feldspar crystals typically form early in the crystallization process, before other minerals such as quartz, mica, and amphiboles.
As feldspar crystals begin to form and grow, they can remove certain elements and minerals from the remaining liquid magma because they selectively incorporate some while excluding others.
The main elements incorporated by the growing feldspar crystals are sodium, potassium, silica and aluminum, this leads to changes in the chemical composition of the remaining magma as the above elements are removed. These changes in the chemical composition of the magma can influence the types of minerals that form during the later stages of solidification. For example, if feldspar crystals remove too much silica from the magma, it may become more difficult for minerals such as quartz to form.
Feldspar crystals can form in a variety of shapes and sizes depending on the conditions under which they are formed. When feldspar crystals form under slow-cooling conditions, such as in a large intrusive rock like granite, they tend to form larger crystals that can be several centimeters in size. However, when feldspar crystals form under faster-cooling conditions, such as in a volcanic rock like basalt, they tend to form smaller crystals that may be only a few millimeters in size.
Also visible in the stone are xenoliths, these do not originate from the original magma like the feldspar does. Xenoliths are introduced from elsewhere, most commonly other stones. These stones are torn off their host rock by the magma as it rises to the surface.
Logging requirements:
View the specimen at GZ and answer the following questions:
Q1. How large are the feldspar crystals in the stone? Give an estimation of the size, not just "big" or "small".
Q2. What does the size of the crystals say about the conditions in which the rock formed?
Q3. Describe the xenoliths. Including the colour, size and texture.
Q4. Is there other minerals in the stone? What is missing and why?
Q5. Why do you think the feldspar forms before the other minerals?
Q6. Take a photo of you, your stamp or personal item with the rock behind you or in the background.
Please send answers via the Geocaching message centre or to my email, answers posted in the logs will be deleted.
You can log the find immediately, if answers are supplied to CO within 14 days, or you log will be deleted.
Happy caching!
I have reached the GSA's highest level:
