Minerals make rocks. One definition is: ‘A structurally homogeneous solid of definite chemical composition, formed by the inorganic processes of nature’. But it’s not a watertight definition; for example, mercury is classed as a mineral. As of July 2024, the International Mineralogical Association recognized 6,062 official mineral species, but other sources list more. So here’s a brief guide to differentiating between them in the filed:
There’s lots of us – we’re minerals.
You’ll have seen us in the rocks
When you’ve ventured out on field trips
With hard hats and woolly socks.
So how should you describe us?
What shape do you perceive?
Our colour, lustre, density,
How we fracture, how we cleave?
See what colour mark we make
When you ‘streak’ us on a plate.
Add a drop of HCl
To test for carbonate.
Where are we on Herr Mohs’ scale*?
Try scratching us; and then,
If your sample can’t be scratched at all,
It’s diamond, hardness 10!
For other tests, you’ll need a lab
Does UV make us glow?
Are we birefringent?
Radioactive – yes or no?
(Some quick field tests were used once,
Like taste, or bite, or smell;
But they’re not now recommended:
They could make you quite unwell…)
So use the simple tests above –
There’s nine of them to try;
And when you’re done you’ll have a name
To call your mineral by.
*Carl Friedrich Mohs (1773–1839), who devised a 10-point scale of relative hardness to help distinguish one mineral from another.