Earth Science: Mineral Identification with Mohs Hardness Scale
Using scratch tests to compare mineral hardness
Earth Science: Mineral Identification with Mohs Hardness Scale
Using scratch tests to compare mineral hardness
Earth Science - Grade 6-8
- 1
A student tests an unknown mineral. It scratches gypsum but does not scratch calcite. Based on the Mohs hardness scale, what range of hardness could the mineral have?
A mineral can scratch another mineral only if it is harder.
The mineral has a hardness greater than 2 and less than 3. It can scratch gypsum, which has a hardness of 2, but it cannot scratch calcite, which has a hardness of 3. - 2
An unknown mineral scratches glass. Glass has a hardness of about 5.5. What can you conclude about the mineral's hardness?
The mineral has a hardness greater than 5.5 because it is hard enough to scratch glass. - 3
A mineral can be scratched by a copper penny. A copper penny has a hardness of about 3. What does this tell you about the mineral?
If the tool leaves a scratch on the mineral, the tool is harder than the mineral.
The mineral has a hardness less than 3 because the copper penny is able to scratch it. - 4
Use these test results to identify the most likely mineral: The unknown mineral scratches calcite, does not scratch fluorite, and has no metallic luster. Calcite has hardness 3, and fluorite has hardness 4.
The unknown mineral most likely has a hardness between 3 and 4. It is harder than calcite but softer than fluorite, so it could be a nonmetallic mineral in that hardness range. - 5
Order these minerals from softest to hardest using the Mohs hardness scale: quartz, talc, apatite, gypsum.
Use the Mohs scale values: talc 1, gypsum 2, apatite 5, quartz 7.
The minerals from softest to hardest are talc, gypsum, apatite, and quartz. Their Mohs hardness values are 1, 2, 5, and 7. - 6
A student claims that a mineral with hardness 6 can scratch quartz, which has hardness 7. Is the student correct? Explain.
The student is not correct. A mineral with hardness 6 is softer than quartz, so it cannot scratch quartz. - 7
An unknown mineral is scratched by quartz but not by apatite. Quartz has hardness 7, and apatite has hardness 5. What is the possible hardness range of the unknown mineral?
Compare the unknown mineral to each testing mineral separately.
The unknown mineral has a hardness greater than 5 and less than 7. It is harder than apatite because apatite cannot scratch it, but it is softer than quartz because quartz can scratch it. - 8
Why should you use a fresh, clean surface when doing a scratch test on a mineral?
You should use a fresh, clean surface because dirt, weathering, or old marks can make the test results inaccurate. A clean surface helps show whether a true scratch is made. - 9
A mineral leaves a powder mark on a porcelain streak plate but does not scratch the plate. The streak plate has a hardness of about 7. What does this show about the mineral's hardness?
A streak mark and a scratch are not the same thing.
The mineral has a hardness less than 7 because it does not scratch the streak plate. The powder mark is a streak, not evidence that the mineral is harder than the plate. - 10
Two minerals look very similar, but Mineral A scratches glass and Mineral B does not. What property can help tell them apart, and which mineral is harder?
Hardness can help tell the minerals apart. Mineral A is harder because it scratches glass, while Mineral B does not. - 11
Use the data table to identify which mineral is likely quartz: Mineral W is scratched by a fingernail, Mineral X scratches glass, Mineral Y is scratched by a copper penny, and Mineral Z is scratched by calcite. Quartz has hardness 7.
Quartz is harder than glass.
Mineral X is most likely quartz because it scratches glass, which has a hardness of about 5.5. The other minerals are too soft based on the test results. - 12
Explain why the Mohs hardness scale is useful but does not give the exact hardness of every unknown mineral.
The Mohs hardness scale is useful because it ranks minerals by their ability to scratch one another. It does not always give an exact hardness because scratch tests often show a range, such as harder than calcite but softer than fluorite.