Mineral hardness is defined as a mineral’s resistance to scratching or abrasion. This characteristic results directly from the atomic structure and the strength of the chemical bonds holding the mineral together. Determining a mineral’s hardness is a fundamental initial step in mineralogy, providing a quick and reliable way to begin the identification process. The most accessible method for measuring this property is through a comparative technique known as the scratch test, which utilizes a standardized set of reference materials.
The Mohs Scale of Mineral Hardness
The Mohs Scale of Mineral Hardness, developed in 1812 by German mineralogist Friedrich Mohs, is the most widely used system for characterizing this property. This scale is a relative, qualitative measure ranging from one to ten, based entirely on the ability of one mineral to physically scratch another. A mineral with a higher Mohs number will scratch any mineral with a lower number, while two minerals of the same hardness will scratch each other.
The scale uses ten index minerals:
- Talc (1)
- Gypsum (2)
- Calcite (3)
- Fluorite (4)
- Apatite (5)
- Orthoclase Feldspar (6)
- Quartz (7)
- Topaz (8)
- Corundum (9)
- Diamond (10)
The scale is not linear, meaning the difference in absolute hardness between Talc and Gypsum is much smaller than the difference between Corundum and Diamond. Diamond, for example, possesses an absolute hardness many times greater than Corundum, despite being only one number higher on the Mohs scale.
Practical Steps for Performing the Scratch Test
The scratch test is performed by attempting to mar the surface of an unknown mineral with a material of known hardness. First, firmly hold the specimen against a stable surface to prevent movement. Using a sharp point or edge of the known material, a firm, deliberate pressure must be applied while dragging the point across a clean, unscratched area of the specimen.
When conducting the test, it is often most practical to use common objects as proxies for the index minerals, as they have approximate Mohs values. A human fingernail has a hardness of about 2.5, a copper penny is around 3.5, and a steel knife blade or a piece of common glass is approximately 5.5. A steel file or masonry nail provides a comparative hardness of roughly 6.5, which is just below that of Quartz.
After attempting the scratch, the surface must be carefully examined to confirm whether a true groove has been created. A softer material may leave a powdery residue or chalk-like mark that can be mistaken for a scratch. This residue should be wiped away, and a fingernail can be run across the surface to feel for a permanent indentation. If the mark rubs away, the unknown mineral is harder than the test tool; if a permanent furrow remains, the unknown mineral is softer.
Testing should progress systematically, starting with a softer tool, such as a fingernail, and moving to progressively harder materials until a scratch is achieved or until all available tools have failed to leave a mark. This bracketed approach allows for the hardness of the unknown mineral to be pinpointed to a specific narrow range on the Mohs scale.
The Role of Hardness in Mineral Identification
The result of the scratch test is a highly diagnostic property that significantly aids in mineral identification. Hardness is particularly useful for differentiating between minerals that might appear visually similar but have distinct internal structures. For instance, the mineral Calcite has a hardness of 3, meaning it is easily scratched by a copper penny, whereas Quartz has a hardness of 7 and will readily scratch glass.
Because hardness is governed by the strength of the atomic bonds, it is a consistent and reliable physical property, unlike color, which can be altered by chemical impurities. Knowing a mineral’s hardness helps a geologist narrow down possibilities significantly when using a mineral identification key. Moreover, the property of hardness has considerable industrial relevance, influencing how minerals are used in manufacturing and technology.
Harder minerals, such as Corundum and Diamond, are utilized as abrasives in grinding wheels and as cutting edges in drill bits due to their superior resistance to wear. Conversely, softer minerals, like Talc, are employed as fillers in paint or as lubricating powders. Thus, the measurement of scratch resistance provides a tool for scientific classification and indicates a mineral’s practical durability and application.