The mineral that fits this description is Calcite.
Understanding Mineral Hardness
Mineral hardness is a fundamental property used for identification, often measured using the Mohs Hardness Scale. This scale ranks minerals from 1 (softest, like Talc) to 10 (hardest, like Diamond) based on their ability to scratch other minerals or be scratched by common objects. The principle is simple: a harder material will scratch a softer material, but a softer material cannot scratch a harder one.
The Role of Common Objects in Hardness Testing
Geologists and enthusiasts often use everyday objects to estimate a mineral's hardness. These objects serve as a practical set of reference points on the Mohs scale:
- Fingernail: A fingernail has a hardness of approximately 2.5. This means any mineral with a hardness less than 2.5 can be scratched by a fingernail.
- Copper Penny: A copper penny has a hardness of approximately 3. Minerals with a hardness less than 3 can be scratched by a penny.
Why Calcite Fits the Criteria
Calcite has a Mohs hardness of 3. Let's break down why this specific hardness makes it the answer to the question:
- Not scratched by a fingernail: Since Calcite's hardness (3) is greater than a fingernail's hardness (2.5), a fingernail will not leave a scratch on Calcite. Instead, the Calcite would scratch the fingernail if enough pressure is applied, or no mark would be left on the mineral.
- Scratched by a copper penny: Because Calcite's hardness (3) is equal to a copper penny's hardness (3), a penny can scratch Calcite. When two materials have the same hardness, they can scratch each other, but the penny is often used as the scratching tool in this scenario, confirming that Calcite is not harder than the penny.
This unique combination of properties makes Calcite a key mineral to identify when performing simple field tests.
Hardness Comparison Table
To visualize these relationships clearly, consider the following table:
| Material/Mineral | Mohs Hardness | Interaction with Calcite (Hardness 3) |
|---|---|---|
| Fingernail | 2.5 | Calcite not scratched |
| Copper Penny | 3 | Calcite is scratched |
| Calcite | 3 | - |
Identifying Minerals Using Hardness Tests
Hardness testing is a crucial step in mineral identification. It helps narrow down possibilities and is often combined with other tests, such as streak, luster, and crystal habit.
- Practical Steps:
- Always use a clean, fresh surface of the mineral for testing.
- Apply firm, consistent pressure when attempting to scratch.
- Check for a true scratch (a groove) rather than just a powdery residue from the softer material.
- Repeat the test in different spots if unsure.
Understanding these simple tests can provide valuable insights into the geological world around us. For more information on mineral properties, consider exploring resources on mineral identification or the Mohs Hardness Scale.
