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.

P1010313

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.)

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