The hardness of minerals is measured on the Mohs Hardness Scale. Reference minerals are used with which any other mineral can be compared. The softest reference mineral is talc (from which talcum powder is produced) and the hardest is diamond. The scale compares the hardness of a given mineral by seeing the effect of scratching it with the ten standard minerals in the table to the left.
This is a useful way of identifying a particular mineral because, although any given mineral might vary in colour or texture, its hardness tends to be very consistent. A good example of a mineral which can vary in appearance is number 9 on the hardness scale; corundum. Common corundum is highly abundant but the gem forms are less so.
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The pieces of rock above are all corundum and share the basic chemical formula Al2O3 – aluminium oxide. As well as being very hard and resistant to chemical action, it also forms hexagonal shaped crystals. In the picture above, the first piece is common corundum from South Africa; the middle is a piece from India in the gem form known as ruby; and the third piece is also in a gem form known as sapphire and is from Sri Lanka. The gem forms can be cut and polished to produce valuable gem stones. Rubies are always red in colour (caused by traces of the metal chromium) but sapphires can vary in colour, although blue is the most prized of these.
When you see these numbers next to the chemical symbols for elements they indicate the atomic number of the element and the atomic mass. The top figure (the atomic number) is their number on the Periodic Table and it corresponds to the number of protons it has in its nucleus. The lower number is the atomic mass of the atom (relative to hydrogen with a mass of 1). This is the number of protons in the atomic nucleus added to the number of neutrons in its nucleus. Sometimes you will see this as a decimal fraction, which will be the average mass allowing for occasional atoms that have extra neutrons. These rogue atoms are called isotopes and they are very exciting!
The gem forms of corundum form as a result of extreme heat and pressure under the ground. The best quality and most valuable rubies come from Myanmar.
Corundum can be made artificially, even in the form of gem stones, so the value of rubies and sapphires is lower than its relative rarity in nature would imply. Artificial corundum is also used to make scratch-proof lenses, windows for space craft and lasers. The first working laser used a piece of ruby corundum. A bright light was shone on the ruby, this caused some of its electrons to become excited and emit photons of laser light.
Questions…
- Between which two metals does aluminium sit in the reactivity series?
- Why does aluminium usually behave as if it is unreactive despite its position on the reactivity series?
- Of what non-metal element does diamond consist?
- What is the chemical formula for gypsum?
- What non-metal element is found in fluorite?
- What are the three most abundant elements in the Earth’s crust?
- How many atoms are there in a molecule of aluminium oxide?
- How many electrons are there in the outer shell of (a) aluminium (b) oxygen?
- What is the relative molecular mass of aluminium oxide?
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