Mohs Hardness Test: Can *You* Scratch These Minerals?

The Mohs scale, a qualitative ordinal scale characterizing the scratch resistance of minerals, ranks materials from talc (1) to diamond (10). Determining a mineral’s hardness often involves performing a mohs hardness scale scratch test procedure, where an unknown mineral is scratched against minerals of known hardness. Mineralogists frequently utilize the hardness picks, a set of tools with points corresponding to specific Mohs hardness values, for consistent and controlled scratch testing. Understanding the American Geological Institute’s (AGI) guidelines for mineral identification is crucial for accurately interpreting the results of a mohs hardness scale scratch test procedure and properly identifying minerals based on their scratch resistance.

Image taken from the YouTube channel Rock Identifier App , from the video titled Identifying rocks & minerals using the Mohs hardness test .

Mohs Hardness Test: Can You Scratch These Minerals?

Understanding the relative hardness of minerals is crucial in geology, mineralogy, and various industrial applications. The Mohs hardness scale provides a qualitative measure of scratch resistance, allowing for identification and characterization of different materials. This article details the Mohs hardness scale scratch test procedure.

What is the Mohs Hardness Scale?

The Mohs hardness scale, developed by German mineralogist Friedrich Mohs in 1812, ranks minerals on a scale of 1 to 10, based on their ability to scratch softer materials. A mineral can scratch any mineral with a lower Mohs hardness number. Importantly, the Mohs scale is ordinal, meaning the difference in absolute hardness between consecutive minerals is not uniform. For example, the difference in hardness between diamond (10) and corundum (9) is significantly larger than the difference between gypsum (2) and talc (1).

Preparing for the Mohs Hardness Scale Scratch Test Procedure

Before conducting the scratch test, proper preparation is essential to ensure accurate and reliable results. This includes preparing the samples and acquiring the necessary tools.

Sample Preparation

• Clean the Samples: Ensure the surfaces of the mineral samples are clean and free from dirt, dust, or any other contaminants. Use a soft cloth or brush to gently remove any debris.
• Smooth Surfaces: The surfaces being tested should be as smooth as possible. Avoid testing highly weathered or irregular surfaces as they may yield inaccurate results.
• Adequate Size: The samples should be of sufficient size to allow for a clear and visible scratch to be made. Small or fragmented samples can be difficult to work with.

Required Tools and Materials

To perform the Mohs hardness scale scratch test procedure effectively, you’ll need:

• Reference Minerals: A set of reference minerals corresponding to the Mohs hardness scale (Talc [1], Gypsum [2], Calcite [3], Fluorite [4], Apatite [5], Orthoclase [6], Quartz [7], Topaz [8], Corundum [9], Diamond [10]). Alternatively, materials of known hardness can substitute, such as a steel knife (5.5) or a glass plate (5.5).
• Hand Lens or Magnifying Glass: A hand lens or magnifying glass will aid in observing the scratch and distinguishing between a true scratch and a powder streak.
• Cleaning Cloth: A soft cloth for cleaning the mineral surfaces.

The Mohs Hardness Scale Scratch Test Procedure: Step-by-Step

The Mohs hardness scale scratch test procedure is relatively straightforward. The following steps outline the process:

1. Start with the Softest Mineral: Begin with the softest reference mineral, typically talc (1), and attempt to scratch the unknown mineral. Apply moderate pressure.
2. Observe the Result: Carefully examine the surface of the unknown mineral using a hand lens or magnifying glass. Look for a visible scratch.
3. Distinguish Scratch from Powder Streak: A true scratch is a groove or indentation on the surface. A powder streak is simply a deposit of material that can be wiped away. Try wiping the surface with your finger or a clean cloth. If the mark disappears, it was likely a powder streak.
4. Progress to Harder Minerals: If talc does not scratch the unknown mineral, proceed to the next hardest mineral (gypsum [2]) and repeat the process. Continue this process, using increasingly harder minerals until a scratch is observed.
5. Determine the Hardness Range: The hardness of the unknown mineral lies between the hardness of the last mineral that did not scratch it and the first mineral that did scratch it. For example, if the unknown mineral is scratched by quartz (7) but not by orthoclase (6), its hardness is between 6 and 7.
6. Refine the Hardness (If Possible): If you have materials with known hardness values between the reference minerals, you can further refine the hardness determination. For example, a steel nail can test between apatite and orthoclase.

Interpreting the Results

The result of the Mohs hardness scale scratch test procedure provides a relative hardness value for the unknown mineral. While it doesn’t provide an absolute measurement of hardness, it offers a useful means of identification and comparison. Consider the following during interpretation:

• Subjectivity: The scratch test is subjective and relies on visual observation. Different individuals may interpret the results slightly differently.
• Inhomogeneity: Mineral samples can exhibit variations in hardness across their surface due to impurities or internal structures. Multiple tests on different areas of the sample are recommended.
• Grain Size: For fine-grained materials, the individual grains may influence the scratch test result.

Common Errors and Considerations

Avoiding these common errors will improve the accuracy of the Mohs hardness scale scratch test procedure:

• Applying Excessive Force: Applying too much pressure can cause even soft minerals to appear to scratch harder minerals. Use moderate and consistent pressure.
• Misinterpreting Powder Streaks: As mentioned earlier, carefully distinguish between true scratches and powder streaks.
• Contamination: Ensure the surfaces of both the reference minerals and the unknown mineral are clean to prevent contamination from influencing the results.
• Using Damaged Reference Minerals: Reference minerals with pre-existing scratches or imperfections can affect the accuracy of the test. Regularly inspect and replace damaged reference minerals.
• Testing Weathered Surfaces: Weathered surfaces are often softer than the underlying material and can lead to inaccurate results.
• Incorrect Identification of Reference Minerals: Accurate identification of the reference minerals is crucial. Double-check your reference samples to avoid errors.

Using a Hardness Table to Document Tests

A table is a useful way to document the results of each scratch test conducted when trying to identify an unknown sample using the Mohs hardness scale scratch test procedure.

Based on the table, the sample has a hardness between 5 and 6 on the Mohs Hardness Scale. More detailed testing with materials in that range, such as a steel knife, would help refine the sample’s hardness.

So, grab your hardness kit and get scratching! Hopefully, this cleared up the mohs hardness scale scratch test procedure for you. Happy identifying!