Wordless Wednesday – Any geologists around?

by Deb on March 28, 2012

Layered rock

The girls found this rock on our walk. Usually I put strict limits on what I’m willing to drag back to the house, but I found this one intriguing as well. I did some geology but that was 20 years ago so I’m asking for a little help on this one 🙂

The white layers are obviously quartz. I’m going to stick my neck out and say I think they’re a later intrusion into the dark layers, although I’m not that confident.


Inside the dark layers there are some small quartz crystals and a few sections of these tiny crystals, plus it looks a little moth-eaten. My first thought on seeing the red and black is haematite.


It also has some extremely straight corners and angles where it is broken.

I have found a geological map of my area, and as far as I can tell I’m either in the little orange part, which is porphyry, or in the Warramunga formation with ‘tuffaceous turbiditic rocks.’ Unfortunately, they both fit my limited geological memories. I’m leaning to the Warramunga but I think that might be familiarity.

Any geologists out there willing to take a guess and tell me how it formed?

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{ 5 comments… read them below or add one }

alexander March 28, 2012 at 7:43 am

Porphyry, its full of potasium rich feldspar. Which was the stuff that broke in to squareish bits. The silverly grey stuff could be hemetite or Moly, if it makes a brown streak on a ceramic surface its hemeite. If a siliver streak and very soft, Moly.


Deb March 28, 2012 at 8:48 am

Thank you, I’ll try the streaks with my kids to see what it is.
Is the whole chunk porphyry or is the quartz a later intrusion? At least now I know what part of the map we are on.


sannah March 28, 2012 at 9:04 am

The tiny bit of Geology I did at uni isn’t any help I’m afraid, but it is a really fabulous rock – my boys love rocks, and would have been so stoked to find something like that. What a wonderful find for your little ones!!
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Tamara March 28, 2012 at 4:04 pm

Oh boy look at that rock. I am not much help with further knowledge about it though
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Trish April 1, 2012 at 9:35 am

How interesting Deb.
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