crushing rocks for fun and profit

There comes a time in the lives of most graduate students when the scholarship funding has run out, but the writing isn’t over yet. Fortunately for us, many geology departments have people who are busy enough with their own research that they are better off paying us students to do some easily delegate-able task than to do it themselves. As a result, this week I have been crushing rocks for fun and profit. The goal for this week is turn rocks into coarse sand so that zircons can be separated from the sand. As I mentioned in my previous post on that topic, the easy part is the first two steps. Those two steps are what I’ve been assigned this week. I've taken photos of the process to share with all of you, but most especially, for my friend in Alaska, who wanted to see the power tools with which I am playing.

Rock dust being a nasty thing to breath, the first thing one does when entering the crushing room is turn on the ventilation system. It is recommended that in addition to that one wear a breathing mask. Once the ventilation system is functioning it is very important to clean all of the work surfaces and tools carefully, to be certain that there isn’t any remaining dust from previous samples—it is important that the zircons we collect at the end of the separation process actually came from the sample we think they did!

The hydraulic crusher, cleaned and ready to use:

A sample in the crusher, ready to crush:

A clean sheet of paper (scrap, rescued from the recycle bin upstairs) is placed under the sample to make it easier to transfer the chips to the mill in the next step. Alas, the paper does tend to get destroyed, and it requires a bit of scrubbing to remove the tiny bits of it which get squished onto the underlying metal surface, but it really does make the process easier, so is worth that extra effort.

With a push of a button, the hydraulic crusher lowers the upper head inexorably onto the sample, and, eventually, there is brittle failure of the rock.

The chips are then put into the ring mill, which had first been completely cleaned and completely dried:

the mill is taken to the milling machine, set into place, and the hydraulic clamp lowered to hold it securely. (The compressed air system in use here is essential for nearly every step of the process!)

Then the lid closed, and the machine is turned on for 3 to 5 seconds (depending on how hard this sample is).

While the machine is on the mill is shaken in such a way as to cause the internal rings to rotate and move about inside the mill, crushing the rock chips into sand.

Once the mill stops shaking one takes the sand out, pours it into a clean bag, with the sample number clearly marked upon it, and then the real work begins: scrubbing the mill, the counters, the crusher, and anything else which might have come into contact with the dust/sand from the sample. One of the challenges is cleaning the rubber collar from the hydraulic crusher, because it has many small cracks on the internal edge, from years of sometimes having been crushed along with some of the samples. It is very important to make certain that no sand remains trapped in these cracks.

Once everything is clean, and fast-dried with the compressed air gun (which will, it is hoped, blow away any tiny zircons which might have stuck to the surface during the washing process) it is time to start again on the next sample. So far I’ve turned 16 samples into sand,

with an average time of 30 minutes per sample, 3 seconds of which is the actual transformation process. There are five more samples to go on this job. One of the joys of crushing someone else's samples is that I have no idea where they come from, or what their significance is, so that I'm free to just appreciate each one on its own merit, admiring the igneous crystallization in this one, the fine banded metamorphic texture in that one, or the rich red hue of the sand of a third.

Next week I will turn rocks into powder for XRF chemical analysis. This will take much longer—the powder needs to be quite fine grained, so I will have to give it 15 seconds in the mill, instead of just 3 to 5. The cleaning process before and after each sample, however, will be unchanged. The need to avoid contamination is just as great for chemical analysis as it is for zircon separation.

Always double check!

I collected most of my samples for my PhD project in March of 2006, consequently their field numbers start 06___. Those samples all came from the northern fault block of the Collingwood River Metamorphic Complex. In November of 2007 my advisor and I did one additional trip, to the southern fault block of the Collingwood River, in search of the long-lost Tasmanian whiteschist. We found it, and collected a number of other samples from the area as well. Whilst we were out, we also stopped by the northern block and picked up another couple of oriented samples, just for good measure.

In October of 2008 we decided that there was just enough funding left to do some detrital zircon dating for two samples from the Collingwood River. I was told to select "sandy" samples as being most likely to contain zircons. So I picked out half a dozen from both the northern and southern blocks of the Collingwood River which were sandier than the others, though not necessarily sandy enough, and brought them along to my lesson on rock crushing. My teacher du jour narrowed it down to three likely candidates, two with 06___ numbers, and one with a 07___ number. So we, thinking that there might be value in having one from each fault block, chose one of each and we commenced crushing.

Since then I've been thinking in terms of having crushed one from each fault block, so was not too surprised when their detrital zircon patterns were somewhat different from one another. Today, at long last, I decided to make the figure showing the location these two samples were collected from. Yup, you guessed it. They are both from the north block! They were collected from locations not more than 250 meters apart from one another along the highway.

However, since the other 07___ samples were all rejected as not being sandy enough to bother, I can't really complain that I don't have one from each block. There is just that lingering embarrassment that one gets when one realizes that the assumption under which one has been operating doesn’t actually apply. The best cure for that? Admit the mistake publicly—then it won’t ever be repeated!

And always consult the map...