30% is ever so much more than I think it is...

One of the things I am meant to be doing today is estimating the percentage of the minerals in this sample so that I can multiply that times the composition of each mineral to obtain an estimate of the composition of the rock to use in calculations. This is because the composition we obtained when we crushed a chunk of this rock and sent it to the lab for analysis isn't working with the modelling program to predict the existence of garnet of the composition whcih is actually present. This is not terribly surprising--while rocks can be homogeneous, they are not often so, and it is very likely that the chunk of rock crushed is not of the same composition as the thin section cut from that sample.

So, how does one estimate the quantity of each mineral? One method it to compare it to the handy % charts that have been published, showing examples of little flecks of black in a white circle, one circle each for 1%, 2%, and so on through the small numbers, and then jumping by 10's for the larger numbers. However, the books I have with such diagrams in them are at uni. The diagrams are also usually circles, because they were created in the day when one looked at the thin section through a microscope. These days we use expensive cameras with automated stages to take lots of photos of the thin section and "stitch" them together into one huge photo with such good resolution that we can zoom in to see the smallest minerals present more clearly than if we were looking in the microscope. But the photos are square, or rectangular, not circles. While I did find a reasonable old-fashioned circle-based % chart by doing a Google image search, I decided that I would rather make my very own.

So I opened up Corel Draw and made a square 100 x 100 mm, and then another 10 x 10 mm. I then zoomed in to the 10 mm square and filled it with tiny randomly shaped objects. As it turns out , 23 shapes, many of which are triangles, but some have four or five sides completely filled my square. I then deleted the small square, and randomly spread the small shapes out across the area of the large square. Voila, a diagram showing 1%. Copy-paste that, and copy the collection of shapes again, re-arrange & re-distribute in the new square, and I've got a 2% diagram. I repeated this step a bunch more times, and wound up with randomly (largely) oriented shapes up to 40%, but it was getting rather hard to squeeze in the larger shapes into gaps for the larger diagrams. I started doing one for 50%, but decided that it was getting too full and that I don't really need one with that quantity.

However, the result is really much more appropriate for igneous rocks--when you have molten magma cooling slowly near the surface of the earth, where there isn't much pressure, the crystals tend to have somewhat random orientations. My rocks are metamorphic--the crystals all grew in response to changes (increasing) in temperature and pressure, and the pressure component tends to mean that minerals with a long axis are likely to wind up aligned, more or less, in the same direction. So, back to the drawing board--copy the 10% chart, and this time arrange the "grains" to follow a pattern and be in clumps, more like is seen in my rocks. Repeat for 20% and 30%. Decide that I've wasted enough time playing with an art project in the name of science. Time to see if these charts help me come up with estimates for the % of each mineral in this thin section that actually translates to a whole-rock composition which, when modelled, will predict the presence of the minerals which are here!



If anyone wants a copy, let me know I can e-mail it to you as either a jpg, CorelDraw file, or pdf...

30% is ever so much more than I think it is...

One of the things I am meant to be doing today is estimating the percentage of the minerals in this sample so that I can multiply that times the composition of each mineral to obtain an estimate of the composition of the rock to use in calculations. This is because the composition we obtained when we crushed a chunk of this rock and sent it to the lab for analysis isn't working with the modelling program to predict the existence of garnet of the composition whcih is actually present. This is not terribly surprising--while rocks can be homogeneous, they are not often so, and it is very likely that the chunk of rock crushed is not of the same composition as the thin section cut from that sample.

So, how does one estimate the quantity of each mineral? One method it to compare it to the handy % charts that have been published, showing examples of little flecks of black in a white circle, one circle each for 1%, 2%, and so on through the small numbers, and then jumping by 10's for the larger numbers. However, the books I have with such diagrams in them are at uni. The diagrams are also usually circles, because they were created in the day when one looked at the thin section through a microscope. These days we use expensive cameras with automated stages to take lots of photos of the thin section and "stitch" them together into one huge photo with such good resolution that we can zoom in to see the smallest minerals present more clearly than if we were looking in the microscope. But the photos are square, or rectangular, not circles. While I did find a reasonable old-fashioned circle-based % chart by doing a Google image search, I decided that I would rather make my very own.

So I opened up Corel Draw and made a square 100 x 100 mm, and then another 10 x 10 mm. I then zoomed in to the 10 mm square and filled it with tiny randomly shaped objects. As it turns out , 23 shapes, many of which are triangles, but some have four or five sides completely filled my square. I then deleted the small square, and randomly spread the small shapes out across the area of the large square. Voila, a diagram showing 1%. Copy-paste that, and copy the collection of shapes again, re-arrange & re-distribute in the new square, and I've got a 2% diagram. I repeated this step a bunch more times, and wound up with randomly (largely) oriented shapes up to 40%, but it was getting rather hard to squeeze in the larger shapes into gaps for the larger diagrams. I started doing one for 50%, but decided that it was getting too full and that I don't really need one with that quantity.

However, the result is really much more appropriate for igneous rocks--when you have molten magma cooling slowly near the surface of the earth, where there isn't much pressure, the crystals tend to have somewhat random orientations. My rocks are metamorphic--the crystals all grew in response to changes (increasing) in temperature and pressure, and the pressure component tends to mean that minerals with a long axis are likely to wind up aligned, more or less, in the same direction. So, back to the drawing board--copy the 10% chart, and this time arrange the "grains" to follow a pattern and be in clumps, more like is seen in my rocks. Repeat for 20% and 30%. Decide that I've wasted enough time playing with an art project in the name of science. Time to see if these charts help me come up with estimates for the % of each mineral in this thin section that actually translates to a whole-rock composition which, when modelled, will predict the presence of the minerals which are here!



If anyone wants a copy, let me know I can e-mail it to you as either a jpg, CorelDraw file, or pdf...

and progress plods along

...or perhaps "fits and starts" would better describe it. As I mentioned earlier today, I happily wiled away some hours playing with art in the name of science, until I finally realized that I could procrastinate no more, that the day was fading away and I'd not yet begun the task I'd set myself for the day. I understand why I was delaying, it is actually a scary thing to decide to base one's calculations upon an *estimate* which one makes *visually*. But since the chemical (XRF) analysis made by crushing a bit of this sample and sending it off to the lab is clearly not representative of the thin-section cut from the same rock (as can be told by comparing the estimates of what minerals should be present and at what composition with those which are there--indeed, the huge garnet present isn't even possible if the entire rock was the composition measured from that bit of powered sample). So it is is clearly time to try another technique.

My first attempt at visual estimating the amount of each mineral present resulted in a calculated whole-rock composition which is very, very similar to that obtained via the XRF analysis for most of the elements (ok, oxides). But three of them were *very* different. One of those, MnO, *needed* to be different--the XRF whole-rock composition had it so low that it shouldn't be possible for there to be this much garnet in the sample (well, at least not if it has this composition). I'd already determined that MnO needed to be at least three times higher for this much garnet to be present, and, indeed, my visual estimates resulted in it being about three times higher. This is encouraging. However, the results of the calculations predicting what minerals should be present only came out a tiny bit better than that obtained with the XRF whole-rock analysis.

Tomorrow I start playing with limiting the amount of H2O available for reactions, to see if it helps. Today I managed to write up 549 words for the thesis and create three new tables/figures to illustrate them, basically summarizing (a bit more technically) what I said above. So little progress, when what I want to be doing is finishing the writing, but without the calculations,there is naught to say, so the work goes on...

But now it is late (How does it get to be so late, so quickly?!) and I want to go for a walk before doing yoga and heading off to sleep.

and progress plods along

...or perhaps "fits and starts" would better describe it. As I mentioned earlier today, I happily wiled away some hours playing with art in the name of science, until I finally realized that I could procrastinate no more, that the day was fading away and I'd not yet begun the task I'd set myself for the day. I understand why I was delaying, it is actually a scary thing to decide to base one's calculations upon an *estimate* which one makes *visually*. But since the chemical (XRF) analysis made by crushing a bit of this sample and sending it off to the lab is clearly not representative of the thin-section cut from the same rock (as can be told by comparing the estimates of what minerals should be present and at what composition with those which are there--indeed, the huge garnet present isn't even possible if the entire rock was the composition measured from that bit of powered sample). So it is is clearly time to try another technique.

My first attempt at visual estimating the amount of each mineral present resulted in a calculated whole-rock composition which is very, very similar to that obtained via the XRF analysis for most of the elements (ok, oxides). But three of them were *very* different. One of those, MnO, *needed* to be different--the XRF whole-rock composition had it so low that it shouldn't be possible for there to be this much garnet in the sample (well, at least not if it has this composition). I'd already determined that MnO needed to be at least three times higher for this much garnet to be present, and, indeed, my visual estimates resulted in it being about three times higher. This is encouraging. However, the results of the calculations predicting what minerals should be present only came out a tiny bit better than that obtained with the XRF whole-rock analysis.

Tomorrow I start playing with limiting the amount of H2O available for reactions, to see if it helps. Today I managed to write up 549 words for the thesis and create three new tables/figures to illustrate them, basically summarizing (a bit more technically) what I said above. So little progress, when what I want to be doing is finishing the writing, but without the calculations,there is naught to say, so the work goes on...

But now it is late (How does it get to be so late, so quickly?!) and I want to go for a walk before doing yoga and heading off to sleep.