Science Clock Series: Part VII

Today's number comes from geology, and is given by:


\[\text{Quartz (Mohs scale, SiO}_2)\] The Mohs scale of mineral hardness is a scale developed by the German geologist Friedrich Mohs in 1812 to help classify minerals based their relative hardness. Its purpose is to show which minerals can scratch other minerals, and it doesn't represent the absolute hardness difference between them. (There are ten levels on the Mohs scale, but the absolute hardness difference between level one and level ten is a factor of 1600.) For any mineral, it can (in theory) be scratched by anything higher on the scale than it, and can scratch anything lower on the scale than itself.

There are ten levels on the Mohs scale, defined by ten specific minerals. The ten levels of the Mohs scale are defined by:
\begin{align}1&\dots\dots \text{Talc}\\
2&\dots\dots \text{Gypsum}\\
3&\dots\dots \text{Calcite}\\
4&\dots\dots \text{Fluorite}\\
5&\dots\dots \text{Apatite}\\
6&\dots\dots \text{Orthoclase Feldspar}\\
7&\dots\dots \text{Quartz}\\
8&\dots\dots \text{Topaz}\\
9&\dots\dots \text{Corundum}\\
10&\dots\dots \text{Diamond}
\end{align}As you can see, the hardness of quartz is the definition of hardness level 7 on the Mohs scale. For comparison, your fingernails have a hardness of about 2.2–2.5 on the Mohs scale, a copper penny is about 3.2–3.5, a pocket knife is about 5.1–5.5, and a steel file about 6.5.

As an interesting aside, the enamel your teeth are made of is basically a variant of apatite (called hydroxyapatite) which, as you can see, is the definition of hardness level 5. This suggests that you probably don't want to be scratching your teeth with anything higher on the scale than a 5. (Tooth enamel also happens to be the hardest substance in the human body, in case you were wondering.)

Quartz itself is an interesting mineral. It's the second most abundant mineral in the Earth's continental crust after feldspar, and has been used in jewelry and handicrafts throughout history. It is made up of silicon dioxide (also known as silica) with the chemical formula \(\text{SiO}_2\). Silica can solidify in many different arrangements, or even mixtures of them in an amorphous structure; two of these arrangements are known as \(\alpha\)- and \(\beta\)-quartz.

Quartz/silica is a pretty tough material and is relatively resistant to erosion (it's number 7 on the scale after all). Being the second most common mineral in the Earth's crust, silica shows up in many places. Most sand in inland deserts is made up of tiny particles of silica, and the type of glass used to make up windows and drinking glasses for the last few centuries (soda-lime glass) is composed of about 75% silica. Many marine organisms construct skeletons or homes for themselves out of it (sponges and diatoms [single-celled plankton] in particular). Silica even has a slight connection with our number 4: it is used to help extract DNA due to its ability to bind to it.

Anyway, check back next time for a look at another number from astronomy! Click here to jump directly to it.