Jilakin Rock is a granite outcrop located approximately 15 km east of Kulin in the central Wheatbelt. It overlooks Jilakin Lake and is a prominent feature in the area, rising out of the surrounding flat plains. The rock is an inselberg, similar to Uluru and Mount Augustus and is situated in the south western part of the Yilgarn Craton.
The town of Kulin was gazetted as Jilakin in 1913, but the name was changed in 1914 when the railway to the town was completed.
There is a parking area and picnic tables at the base of the rock.
The area around the rock is also the home of the Blazing Swan, an annual Burning Man event that has been held in the area since 2014 at Easter time, attracting around 2,500 attendees. When I was researching this cache the festival was on Day Three. On top of the rock were two naked attendees standing with arms outstretched soaking up the energy. Definitely one of the odd things I have seen while geocaching.
Earthcache Lesson
Today's lesson is all about phenocrysts and their shape. A phenocryst is a large crystal in an igneous rock that is bigger than the surrounding groundmass. Large crystals or phenocrysts indicate the molten rock formed deep in the Earth's crust has cooled slowly allowing the crystals to grow.
Pictured below is an example of hornblende phenocrysts in an andesite. Hornblende is dark coloured mafic mineral rich in iron and here it forms large crystals set in a pale and finer grained groundmass. Take a closer look at the centre crystal. All the crystal faces have straight edges.
Now, we understand the phenocryst is a large crystal set in a finer groundmass, it's time to explore how geologists describe these large crystals.
Colour is very important as it gives an insight into the probable chemical composition of the mineral. Light colours may mean the mineral is rich in potassium, sodium or silica. Dark colours may mean the mineral has manganese, iron and magnesium in it's composition.
Shape of the phenocryst is very important way to determine the mineral's identity and describing the rock. There are three terms used to describe phenocryst shape: euhedral (all straight sides), subhedral (some straight sides) or anhedral (a blobby looking crystal).
The picture below is a stylised example of crystal shapes. A regular crystalline shape has all straight edges and is called euhedral. The hornblende crystal pictured above is euhedral. If the crystal has some straight edges and some irregular edges then it can be described as subhedral. Anhedral has no regular crystalline shape and the phenocrysts look like blobs.
Earthcache questions
At GZ, there is a small circle of loose rocks. Once you have found this, get down low and look at the granite outcrop. There are some large phenocrysts of feldspar in the Jilakin Rock granite. Find the largest crystal in this circle and based on the lesson above, let's describe it.
Q1. Describe the colour of the largest phenocryst in this rock. What clues does this give you about the constituent minerals?
Q2. How long and wide is the largest phenocryst (measure or guesstimate in centimetres)? Where does this suggest the material here was when it cooled?
Q3. This is tough one but fundamental to the lesson. Is the largest phenocryst euhedral (all straight sides), subhedral (some straight sides) or anhedral (a blobby looking crystal)?
Q4. Well, a task really. Please attach a photo of a view from Jilakin Rock, preferably showing you or a personal object in it
Please log the cache straight away. You can submit the answers to the questions within a few days but logs without answers will be discreetly deleted within 14 days.
The cache was placed to celebrate the 10th anniversary of International Geocaching Day in 2021.
