Wild West Yorkshire, Friday 12 November 2010
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THE CRYSTALS of calcite, calcium carbonate, CaCO3, can take many forms, such as 'nail-head' and 'dog-tooth'. It is number 3 on the Mohs scale of hardness which means you can scratch it with a pen-knife. Steel ranges from 5.1 for a pen-knife to 6.5 for the steel used in a file, so it would always scratch calcite. A finger-nail is around 2.2-2.5, so you'd never make an impression on the crystal with your finger-nail. It might seem like vandalism to scratch this attractive specimen but I chose a small face of a crystal amongst the chalky-looking matrix that the crystal has grown from and the knife made a scratch. This proves that the crystal isn't quartz which is harder than steel at 7 on the Mohs scale.
Calcite fizzes when you put a drop of cold dilute hydrochloric acid on it, but I haven't got any to try on this specimen.
Like the galena I drew yesterday, it forms in hydrothermal vents.
I was drawing this at visiting time in the hospital, with sketchbook and mineral balanced on my knee, and the toddler visiting his grandma in the next bed was curious about what the strange object might be:
'Is it food?' he asked.
No, not food.
'It's a butterfly.' he suggested.
Richard Bell, illustrator
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