Scratching minerals

In our Marrickville science classes we have been looking at rocks, minerals and crystals.

We started off by doing an activity from the CSIRO Primary CREST booklet: mineral scratch tests. These tests are very useful for working out the hardness of minerals. Mohs Scale of Hardness is a scale devised in 1812 by the German geologist Friedrich Mohs.

It is a relative scale, not a linear scale. This means, for example, that a mineral with hardness 10 will not be twice as hard as one with hardness 5. However, it is still useful for working out hardness of minerals because we know where other common materials are placed on the scale:

fingernail – hardness 2.5

copper coin – 3

glass – 6

steel file – 7

sandpaper – 7.5 to 9 depending on what material is used for the grains

We can use these items to see if they can scratch the minerals, and then work out where on the scale the minerals come.

Our groups each had a box of seven different minerals (bought from Australian Geographic): quartz, chalcopyrite,  calcite, fuchsite, pyrite, fluorite, red jasper. They tried scratching them with the items listed above. If the mineral was scratched by a soft item (e.g. a fingernail) they didn’t have to scratch them with anything harder.

They then put the minerals in order from softest to hardest.

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The groups didn’t end up with the same list of minerals. Everyone agreed about the softest mineral (red jasper) and the hardest (quartz). The one that caused the most disagreement was iron pyrite. The pyrite samples were all made up of a combination of smaller cubic crystals. It was pretty easy to break off small crystals but it was hard to tell if the smooth surface of the crystals had actually been scratched.

Based on my internet searches, I came up with the following order of hardness for the minerals in our kit.

Fuchsite 2.5 to 4

Calcite 3

Chalcopyrite 3.5 to 4

Fluorite 4

Pyrite 6 to 6.5

Red jasper 6.5 to 7

Quartz 7

What is surprising about this list? I was particularly surprised to see Red Jasper as the second hardest, since all our groups agreed it was the softest. Perhaps we should have followed different instructions for carrying out the scratch tests. I would quite like to the scratch test again on the red jasper from our kits, and see whether it really was scratched or if the marks were due to something else.

It looks like pyrite comes pretty high up on Mohs scale as well. Did you know that pyrite is also known as Fool’s Gold? Pyrite specimens often have the beautiful cubic crystals we saw last Thursday, but sometimes they have different shaped crystals. Other times they are shapeless lumps, a little more like a gold nugget. I found a web page with recommendations for gold prospectors who might not be sure if they have pyrite or real gold. It says at the bottom, that if scratched by a steel knife, pyrite will flake off instead of scratching. That must have been what was happening with our specimens.

You can read more about mineral properties, including hardness and scratch tests, on the Rocks for Kids website. This doesn’t have any pictures but has loads of information. The Oxford Museum of Natural History website has information plus pictures if that’s more your style.

Geologists and materials scientists today will use specialist equipment to determine the properties of the material they are looking at. But I was heartened to read that a scratch test is recommended as the first test for geologists in the field to try to identify the mineral they have found. (This is right at the bottom of the Rocks for Kids page I linked to above.)

Colours and dyeing

Last term our little science club in Manly had some great activities exploring colours and dyeing.

We started off with a packet of synthetic dye and some squares of cotton. We left the fabric in the dye and used tongs to take out one piece every 5 minutes, until the last piece had been in the dye for one hour. We used the leftover dye to do some tie dyeing and put in some of my little girl’s clothes too.

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The square we removed after 60 mins looked lots darker than that for 5 mins but we did wonder if that was perhaps because it was wetter.

We left all the fabric squares for a week to dry out, then shook them up with warm detergent solution to see how much dye would actually stay fixed onto the fabric.

I was hoping that we would be able to see by eye how much dye had gone back into solution, but we couldn’t really tell. (Perhaps a spectrophotometer would have come in handy at this point, but we don’t have access to a lab and it’s rather outside the budget for our club!) However, the 5 min piece of fabric looked loads paler than the 60 min, so we could at least deduce that more dye molecules had attached themselves onto the fabric that had been in there for longest.

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The children and I thought about what we were trying to find out in this experiment. We discussed what is meant by the scientific method and the idea of a fair test. They were really good at suggesting what variables there might be in the experiment and how we could try to keep all of them constant except the one we were testing.

If doing this again, I would reduce the number of samples to six (as suggested in the instructions) and allocate more than one pair of disposable gloves per child, because they took them off in between turns. I had hoped it would be more obvious which fabric samples had been left in the dye solution for longest. Perhaps we could experiment with the concentration of dye solution to give more of a contrast.

This activity came from the booklet of ideas for the CREST award scheme (run by CSIRO). My two young scientists-in-the-making were not so keen on the technology activity (which personally I thought was predominantly design and very little to do with technology). I think this has been a good example of making sure the activities follow the children’s interests. Furthermore, it seems to me that successful inventions or designs come from perceiving a need or a gap in the market, rather than just doing an activity that someone else has set for you.  I’m not going to set them an activity we all think is pointless just so they can get a certificate. If the children decide that there is a design activity they want to do which revolves around the use of colour, I will encourage them in this.

Anyway, we were all very interested in natural dyes and the ochres that have been used for thousands of years by the Australian aborigines. I borrowed a selection of books from our library all about natural dyes and a few about design (and tried to stop myself from taking on more projects than I have time for). We boiled up some chopped red cabbage and water in one saucepan and some onion skins and water in the other. As it was the last week of term we didn’t have enough time to dye fabric (or yarns) and wait for it to dry. Instead, we used the natural dyes like watercolours. I made some strong tea and coffee for brown colours. Dickie Turpin painted a beautiful landscape but didn’t want me to take a photo of it!

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The same day, we ground up a rock we had picked up on a bush walk, mixed it with oil and used this to paint with. This was a lot easier than I had expected and produced some great results.

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The project has been very successful so far and still has plenty of potential. I would like to show the children the molecular structure of some dyes so that they can think about what the common features might be that contribute to the colour. We have done some subtractive colour mixing in our homeschooling before (with paint) but it would be fun to get some colour filters and show them how additive mixing works. This might or might not lead to a discussion about the electromagnetic spectrum, waves, frequency, wavelength etc. We could move on to talking about our eyes and how we see colours. (I have a particular interest in this because Pokemon Boy has some red-green colour deficiency. He’s fine with bright colours but finds it a lot harder with muted colours.)

We all want to do some more work with natural dyes and I must remember to order some alum to use as a mordant so we can see what difference that makes. (Mordants help the dye molecule to bind to yarn or fabric.) In the school holidays the children attended an activity (not run by me) where they used cabbage water as an indicator, so we could talk more about pH and indicator solutions. All of this is only if the children want to! I’m happy to be led by their interests.