Activity 1. Mineral hardness scale
The Mohs Hardness Scale was developed in 1812 by the German geologist and mineralogist Friedrich Mohs. It is used to determine and compare the relative hardness of minerals. The 10 reference minerals in the scale are ranked in order of increasing hardness. Any harder material can scratch minerals of lower hardness, but not those of higher hardness. The diamond that has the highest hardness of 10 can scratch e.g. quartz (hardness 7), but quartz cannot scratch diamond. This method is still used today to estimate the relative hardness of minerals. This scale gives no information about other properties of minerals such as density, colour or chemical composition.
How is the Mohs scale used?
We can determine the hardness of a mineral by scratching it with a mineral from the Mohs scale. In case it can be scratched, it means that it has a lower hardness than the one on the scale. Otherwise, it has a higher hardness.
A mineral sample of a certain hardness will scratch other minerals of the same hardness and samples from the rock of lower hardness. For example, if you can scratch a sample with your fingernail, you know that its hardness is less than 2.5. If you can scratch the sample with a steel file but not with your fingernail, you know that its hardness is between 2.5 and 7.5.
The hardness of a mineral is one of its most characteristic qualities. Hardness is usually given in a range, but it is not a completely accurate value.
Minerals are equal in hardness if they do not scratch each other or if the minerals to be tested and the minerals of the reference rock scratch each other. If the mineral to be tested cannot scratch the reference rock mineral, but the rock mineral can scratch the mineral sample to be tested, this means that the hardness of the test mineral is equal to the hardness of the mineral from the lower grade of the rock increased by 0,5.
Source: The Mohs Scale | Education (thenaturalsapphirecompany.com)
- Independently analyze the Mohs scale data showing the hardness of 10 reference minerals.
- Answer the following questions in writing:
- How many degrees are there in the Mohs scale for the hardness of minerals?
- Which is the hardest mineral?
- Which is the softest mineral?
- Compare the hardness of calcite and topaz.
- From the description given, determine/identify the mineral on the Mohs scale.
We found that we cannot scratch it with a fingernail, but we can carve it with a nail. The unknown mineral will not make a groove in a topaz or diamond but may leave a dent in an orthoclase. What is this mineral?
- From the description given, determine/identify the mineral on the Mohs scale.
We figured we couldn’t scratch it with a fingernail, but we could make dents with a copper coin. The unknown mineral won’t make a dent in apatite, but it can make a groove in calcite. Decide which mineral it is.
- Read the additional information given below. How will you use it in your daily life? What will it be useful for? Give specific examples.
Gold, silver and platinum are relatively soft, with Mohs scores between 2.5 and 4. Because gemstones can scratch each other and their settings, each gemstone jewel should be individually wrapped in silk/velvet fabric or paper. They should not be treated with commercial cleaners as they may contain abrasives that can damage gemstones.
Based on this physical property, hardness is also roughly the limit at which a “stone” is considered precious – this threshold is the magic “seven” of quartz. Typically, a gemstone is considered to have a hardness of 8 or more, but this is not a requirement. Some may consider amber and malachite to be gemstones, but their hardness is low, which matters in jewellery because the mineral’s polished surface wears and scratches. After a while, this stone is not as shiny and beautiful as it was when you bought it in the jewellery store. For this reason, low-hardness minerals are used in jewellery where there is not as much risk of scratching, for example in neck pendants, earrings etc.
Household items can also be examined and compared for hardness. This can be useful when selecting appropriate cleaners, selecting tools for a particular household activity, or selecting glue or other materials to repair a particular item.
Here are some approximate Mohs scale values for some of the most common materials used in the household:
- Glass: About 5-6
- Ceramics: About 6-7
- Metal (most): About 5-6
- Marble: About 3
- Granite: About 6-7
Care should always be taken to avoid scratching valuable jewellery, and damaging and injuring household items.
