At Noosa Granite Bay, a Quartz Diorite is exposed along the shore for about 250m. Granite and Diorite are similar in their formation – both are magmatic rocks. Another similarity is that they have both a very similar texture with minerals of similar grain size.
The big difference is the mineral composition and therefore the chemistry of the rocks. Per definition, granites have a high percentage of alkali feldspar (which makes the brownish colour), at least 20% quartz as well as plagioclase (sodium/calcium feldspar). This rock here, the Quartz Diorite is quite different in its mineral composition with about 50% plagioclase, 20% quartz and 10% hornblende. Some alkali feldspar and accessory minerals make the rest.
1: Have a close look at the Quartz Diorite you are standing on and try to identify the three different minerals – plagioclase (feldspar), quartz and hornblende.
What is the colour and crystal shape of each of these minerals?
2: While walking around these boulders, you will see many light coloured veins crossing the rocks.
What minerals do you think are those veins made of? What is the width of the veins?
3. You will also see some brownish/rusty spots covering the Quartz Diorite.
What process do you think has caused this and which chemical elements are involved?
4. Optional: Take a Photo of the beach or yourself at GZ. Please avoid any details that could help solving this earth cache at home.
Please send me the answers including the cache name and you are welcome to log your visit straight after visiting this place. I will get in contact with you if there are any issues.