Friday, 1 February 2013

Really, really thin slices of rock

One of my most keen interests in the geology of our area is about the difference between the 'basalts'. The Lismore Basalt, the Kyogle Basalt and the Alstonville Basalt. Most of the time it is impossible to tell the difference if you are holding a piece in your hand. The size of the crystals that make up the rock are usually too small to see but occasionally you can see the characteristic twinning of a larger feldspar crystal (called a phenocryst) or even some olivine. But 95% of the time you'll just say "this is a black rock - probably a basalt".

Since there is very little in the way of field techniques to determine the difference between the regions 'basalts' other techniques need to be used. The best technique is geochemistry, looking at the actual chemical composition of the rock itself. Alas, this only works for the freshest, most unweathered rocks and of course it costs money for each sample. The second technique is called petrography (no usually nothing to do with petroleum), whereby a piece of rock is ground down to a thickness of around 30 microns, light shone through it and its properties determined by a specialised type of microscope (one with polarizing light filters). This can often give you a qualitative assessment of the rock geochemistry.

Kyogle Basalt in Plane Polarized Light
I'm not very good at making these myself but it doesn't stop me from trying. Sometimes I can make some good ones. With some assistance and advice from a technical officer at the University of Ballarat and a fellow club member who is a Senior Visiting Fellow at the University of Wollongong, I've recently had some more success.

The photos are of a piece of Kygole Basalt that I've made into thin sections. The sample was obtained from a quarry in the Afterlee area on the way to Toonumbar. The first is what the sample looks like under plane polarized light. The larger crystals are olivine which in come cases have been slightly reabsorbed creating an embayed texture on their edges. They are also slightly altered to a clay mineral of some kind around the edges and in some of the cracks. The dark black minerals are iron or titanium oxides and if you look closely you'll see little  prism like minerals which is feldspar.

Kyogle Basalt in Cross Polarized Light
The next photo is the same field of view with a second polarizer inserted at 90 degrees to the first to reveal wonderful interference patterns. The interference patterns are characteristic from mineral to mineral. The olivine clearly showing what are called high order interference colours. In this you can also see little accumulations of bright colours that are different from the olivine crystals. These are pyroxenes, probably the variety called augite. The prismatic crystals of feldspar are more evident in their straw colour and the grey 'background' is nepheline.

Overall the chemistry of the constituent minerals this rock is called a alkaline basalt. This is because it contains abundant 'alkali' metals (sodium (Na) and potassium (K)). Alkaline rocks are common in the Kyogle Basalt of the Focal Peak Volcano. Remember though, that the term Alkaline does not refer to a rock type but a suite of rock types. If this rock sample had remained in the magma chamber for a longer period of time it would factionate or evolve to rocks called nephelenites or similar. This is a different process to what goes on with the other major suite of rocks called tholeiites which tend to evolve towards dacites and rhyolites (such as the Nimbin Rhyolite that that we see on top of the Lismore Basalt - which is mainly tholeiitic in composition). But more about that another day.

Hopefully this weekend I get the chance to make some more thin sections. Which reminds me - this is the way I do it, if you have a polarizing microscope and are keen to learn the art yourself give it a go.


  1. Thank you for visiting my blog Rod. Your comments are very true and greatly appreciated. Just a look at photos from North Bundaberg makes very clear what could have been.

    1. Thanks for dropping by again. I always enjoy your blog. Incredible photos you take as part of it too.