Mineral hardness measures a rock or mineral’s resistance to being scratched or having its surface permanently altered by abrasion. Determining this physical characteristic is an important step in the basic identification of geologic materials. Since many minerals look similar, relative hardness provides a quick, non-chemical test to narrow down possibilities. Identification is performed using a comparative method that utilizes materials with known hardness values.
Understanding the Mohs Hardness Scale
The most widely accepted method for comparing mineral hardness is the Mohs scale, which is an ordinal scale ranging from 1 to 10. Developed in 1812 by German mineralogist Friedrich Mohs, the system uses ten specific reference minerals. These minerals are arranged in order of increasing hardness, starting with talc (1) and ending with diamond (10).
The scale operates on the principle that any substance can be scratched by a material that is harder than itself. Conversely, a material will remain unscratched if the testing tool is softer. This allows for a simple, relative comparison; a mineral with a hardness of 7 will scratch all minerals with a value of 6 or less.
The ten reference minerals provide fixed points for comparison:
- Talc (1)
- Gypsum (2)
- Calcite (3)
- Fluorite (4)
- Apatite (5)
- Orthoclase (6)
- Quartz (7)
- Topaz (8)
- Corundum (9)
- Diamond (10)
Note that the scale is not linear; the difference in absolute hardness between Corundum (9) and Diamond (10) is much greater than the difference between Talc (1) and Gypsum (2).
Performing the Physical Scratch Test
To begin the test, select a clean, fresh surface on the specimen, free of dirt or weathering. The test involves attempting to scratch the unknown sample with a material of known hardness. Apply firm, steady pressure using the sharp point or edge of the testing tool. It is advisable to begin with softer tools and progress to harder ones until a scratch is successfully made.
A set of common household and field objects can be used as reliable testing tools, each corresponding to a specific Mohs value. Your fingernail is the softest tool (2 to 2.5). A copper penny is rated at approximately 3 to 3.5.
A piece of glass, such as a microscope slide, is rated at 5.5, which is the same approximate hardness as a steel knife blade. A hardened steel nail or file typically falls between 5.5 and 6.5. For harder samples, a piece of Quartz (7) is often carried in field kits.
After attempting the scratch, you must carefully examine the sample to determine if a true scratch has been formed. A genuine scratch is an etched groove that cannot be rubbed away, indicating that the testing tool successfully broke the bonds of the mineral surface. If the test leaves a white, chalky line, this is merely powder residue from the softer testing tool and should be wiped away before proceeding.
Analyzing Results and Assigning a Hardness Value
The analysis of the physical scratch test results relies on observing one of three outcomes to determine the sample’s relative hardness.
Sample is Softer
The testing tool leaves a permanent, etched line on the sample, meaning the rock is softer than the tool. For instance, if a copper penny (3.5) scratches the sample, the sample’s hardness is less than 3.5.
Sample is Harder
The testing tool is completely ineffective and leaves no mark, even when firm pressure is applied. In this case, the sample is harder than the tool, requiring a tool with a higher Mohs value for the next test. If the penny (3.5) does not scratch the sample, the rock’s hardness is greater than 3.5.
Hardness is Approximately Equal
This outcome occurs when the sample and the tool appear to scratch each other, indicating their hardness values are very close. Once you identify a tool that scratches the sample and a tool that does not, you can bracket the rock’s hardness value. For example, if the sample is scratched by glass (5.5) but not by a copper penny (3.5), the Mohs hardness is between 3.5 and 5.5.