Determining the hardness of a mineral is a fundamental procedure in geology and mineralogy, serving as a primary step in identifying unknown specimens. Hardness is defined as the resistance of a mineral to scratching or abrasion, a physical property linked to the strength of its chemical bonds. Assessing this resistance quickly allows geologists and enthusiasts to narrow down identification possibilities significantly. The most recognized method for this purpose is the Mohs scale of hardness, which provides a straightforward way to conduct field and preliminary laboratory testing.
The Relative Nature of the Mohs Scale
The Mohs scale, developed in 1812 by German mineralogist Friedrich Mohs, is an ordinal scale ranking the scratch resistance of minerals from one to ten. It is considered relative because the intervals between the numbers are not equal measures of hardness. For instance, the difference between a mineral rated 9 and one rated 10 is far greater than the difference between minerals rated 1 and 2.
The scale is anchored by ten specific reference minerals, ranging from the softest, Talc (1), to the hardest known natural substance, Diamond (10). The index minerals include Gypsum (2), Calcite (3), Fluorite (4), Apatite (5), Orthoclase Feldspar (6), Quartz (7), Topaz (8), and Corundum (9). The underlying principle is that any mineral on the scale can scratch any mineral with a lower number, but it cannot be scratched by a mineral with a lower number.
This comparative method allows for hardness determination by bracket-testing an unknown sample against these known standards. If a mineral sample is scratched by Apatite (5) but not by Fluorite (4), its Mohs hardness is approximately 4.5. Because the test relies on simple observation of which material scratches which, it remains an efficient and widely used technique for quick identification in the field.
Step-by-Step Scratch Testing
Determining a mineral’s hardness involves systematically attempting to scratch the unknown sample with materials of known hardness. For field testing, common, accessible tools can substitute for the ten index minerals, allowing for a close approximation of the Mohs number. These tools provide convenient checkpoints along the scale, simplifying the process.
A person’s average fingernail has a Mohs hardness of approximately 2.5, easily scratching Talc (1) and Gypsum (2). A copper penny registers around 3.5, sufficient to scratch Calcite (3). Moving up the scale, a common steel nail or knife blade typically falls around 5.5, similar to a piece of window glass. A steel file can reach a hardness of about 6.5, which is softer than Quartz (7) but harder than Orthoclase Feldspar (6).
To perform the test, hold the mineral specimen firmly against a stable surface. Apply the testing tool with a sharp point, using firm and consistent pressure across a flat, representative surface. The goal is to see if the tool leaves a permanent groove or indentation. If the tool is scratched or damaged, the mineral is harder; if the mineral is scratched, the mineral is softer.
Ensuring Accurate Hardness Determination
Achieving an accurate hardness determination requires careful observation to avoid common errors. The most frequent mistake is confusing a true scratch, which is a permanent groove, with powdered residue left by the softer testing tool. A true scratch appears as a deep furrow that cannot be rubbed away, while residue wipes off easily, revealing an unscratched surface beneath.
It is advisable to test a fresh surface of the mineral, away from weathered or altered areas, which may give a falsely low hardness reading. Weathering processes can soften the exterior of a specimen, making it easier to scratch than the true interior hardness indicates. Testing a small, inconspicuous area is also recommended to avoid damaging a display-quality surface.
A challenge arises when testing minerals that exhibit cleavage, the tendency to break along planes of structural weakness. Applying excessive pressure to a cleavable mineral might cause it to flake or break rather than produce an abrasion mark, potentially mimicking a scratch. For very soft minerals, such as Talc, the testing material may leave a mark that appears more like a greasy streak or powder on the sample, indicating a low Mohs number. Testing small or irregular samples demands patience and precision, often requiring the sample to be secured to ensure stability during pressure application.