This EarthCache is focusing on a granite specimen with rare geological features known as orbicules embedded in the stone.
Orbicules are a layered, sphere shaped structures that are rarely seen within stones.
Orbicules are formed by a process known as nucleation, which takes place in magma chambers beneath the Earth’s surface. The nucleation process starts forming orbicules around a grain (made from a shell fragment or fossil) in the magma chamber, orbicules have a central core which have formed around the grain in the stone.
The speed at which nucleation occurs depends on the temperature beneath the surface where the process is taking place. Hotter temperatures increase the growth rate of the crystals while also slowing down the nucleation rate, which is shown in the image below:
(source: https://www.researchgate.net/)
This means higher temperatures result in larger crystals and cooler temperatures result in smaller crystals.
The granite at GZ is known as "Boogardie orbicular granite" and was discovered in Mount Magnet on Boogardie Pastoral Station, roughly 500 kilometres North-East of Perth. The stone at GZ and other rocks originating from the Mount Magnet region have been dated at 2.6 billion years old, making them one of the oldest examples of orbicular granite stones in the world.
The orbicules in the stone are made up of hornblende and plagioclase feldspar.
Hornblende is not recognised mineral in its own right but is a general term referring to dark amphibole.
Plagioclase is also a general term for silicate minerals within the feldspar group.
Logging requirements:
Observe the stone at the given coordinates and answer the following questions:
Q1. Did the orbicules form in higher or lower temperatures? How did you get to this conclusion?
Q2. What does your answer suggest about the nucleation process?
Q3. What factors of the hotter temperatures do you think cause the slow nucleation rate?
Q4. Did you spin the rock?
Q5. Take a photo of you, your stamp or personal item with buildings to your south behind you included in the image. NOT the granite stone, due to spoilers. Alternatively you can send the image to my inbox with your answers.
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, however, if answers are not supplied to CO within 14 days, your log will be deleted.
Happy caching!
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