Mohs Hardness Scale

Lesson notes

Overview

Minerals are ranked by which can scratch which, from soft talc to diamond. This topic is rated beginner and is number 10 of 98 in Earth & Space. Core ideas and first intuitions. No prior knowledge needed.

What you are seeing

The panel above shows a bar chart that compares values on one scale, so differences in size are easy to see. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.

In depth

The Mohs scale ranks minerals by their ability to scratch each other, from 1 for talc to 10 for diamond: gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8 and corundum 9. A fingernail is about 2.5, a copper coin about 3 and a steel knife about 5.5. The scale is ordinal, not linear: diamond is far harder than corundum. It is a simple field test for identifying minerals.

Reading the relationship

The expression "scratch test" is a compact summary of the idea. It uses the symbols s, c, r, a, t, h; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.

A guided exploration

1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.

Try this

Start by only moving one slider at a time. Say aloud what you expect, then check.

Common pitfalls

Earth and space systems work on very different scales of time and distance. Check which scale a number refers to before comparing it with another.

How it connects

It builds on “The Layers of the Earth”, the topic before it in Earth & Space. Once you are comfortable, continue with “Geological Time”.

Why it matters

Earth and space science helps us understand our planet, its climate and the wider universe.