'Mortlake - Australia's olivine capital'. That's what the signs say as you drive into town, but what is olivine and why is there so much of it around Mortlake? Take a short drive to the Mt Shadwell Quarry to find out more - beware of vehicles as this is a working quarry!
What is Olivine?
Olivine is actually very common - in fact it's one of the Earth's most common minerals, and exists in many different forms - it's even been found on meteorites and other objects from space. In the case of Mortlake, the olivine would've formed around 70 - 80 kilometers below the ground and was brought to the surface during volcanic eruptions with the immense amounts of gas shoting up the volcano flinging it far and wide.
Much of the olivine around Mortlake is contained in what are called 'bombs' which look like tear drop shaped lumps of rock. These 'bombs' range in size from something as small as a pigeon's egg to ones as big as a car. When fossicking for olivine it's merely a matter of finding a 'bomb' and then cracking it open with a hammer.
About 90 per cent of olivine is an olive green colour but sometimes you'll find goldy coloured pieces as well which are more valuable. Most of the olivine you'll find around Mortlake looks like tightly packed green sugar and will crumble away if you rub it. While a lump of small green olivine crystals can look quite pretty, the real value is in the gem quality olivine - and at the Mt Shadwell quarry, if you're lucky, you might find a larger piece of olivine which can be cut into a gem stone, or peridot.
Logging the Cache
Your task is to answer a few simple questions:
1. Looking at the quarry, describe what you see (layers, colours etc.)
2. How many kilometres below ground did the olivine form?
3. What colours does olivine come in?
Once you complete the Earthcache requirements you can post your find without delay, as per the guidelines. You will also need to verify your find by sending me an email providing answers to the questions. This can be through the message centre or via gemmasiemensma@gmail.com
Thanks for visiting this Earthcache. I hope you enjoy it.