Mineral Hardness

Hardness (H) is the resistance of a mineral to being scratched, and it is one of the most diagnostic physical properties for identification. Every mechanical cohesion property - hardness included - is ultimately controlled by the strength of the chemical bonds holding the mineral together. Stronger bonds are harder to disrupt, and a harder mineral resists the scratching action of a softer one because it takes more force to break or displace surface bonds. ^[1]

The Mohs Scale

The most widely used hardness measure in mineralogy is the Mohs scale of hardness - a unitless, ordinal, 10-point scale in which each position is defined by a reference mineral. The defining minerals in order are: ^[1]

| Hardness | Reference Mineral | | ------------ | --------------------- | ----------------- | | 1 | Talc | | | 2 | Gypsum | | | 3 | Calcite | | | 4 | Fluorite | | | 5 | Apatite | | | 6 | Orthoclase | | | 7 | Quartz | | | 8 | Topaz | | | 9 | Corundum | | | 10 | Diamond | |

The scale is ordinal, not linear - the intervals between steps are unequal. Each reference mineral will scratch any mineral with a lower number and be scratched by any mineral with a higher number. Hardness is reported to the nearest half unit. A mineral that falls between calcite (3) and fluorite (4), for example, is assigned a hardness of 3½.

Field Testing

Formal hardness test kits containing scribes of the reference minerals are available for laboratory use. In the field, common objects provide a practical substitute. A fingernail scratches at ~2+, a copper penny at ~3, a steel knife blade at ~5, window glass at ~5½, and a chip of quartz at 7. These approximations are sufficient for most identification tasks. ^[1]

When testing, it is important to work on a fresh surface - weathering and surface alteration can lower the apparent hardness significantly. A softer mineral may rub off onto the surface of a harder material and leave a mark that looks like a scratch but wipes away; always confirm a true scratch by checking whether the mark survives light cleaning. ^[1]

Directional Variation of Hardness

Scratch hardness is not always the same in every direction on a crystal surface. Because chemical bonds are not uniformly strong in all crystallographic directions, the resistance to scratching varies with orientation. This variation is significant in some minerals and barely perceptible in others. ^[1]

Kyanite (Al₂SiO₅) is the most striking example. Its crystals are elongate blades parallel to chains of edge-sharing Al octahedra. Scratching parallel to the length of the blades gives a hardness of 5, while scratching at right angles gives 7. The difference reflects weaker bonding between adjacent chains than along the length of each chain - the direction of strongest bonding is the hardest to scratch. ^[1]

The kyanite example is diagnostic: encountering an apparent hardness of 5 in one direction and 7 in another on the same grain is a strong indicator of kyanite. No other common mineral shows this extreme anisotropy. Reported hardness values for kyanite depend on which direction was tested, so a single number without a directional qualifier is incomplete.

Directional hardness variation is a general consequence of crystal symmetry, not a peculiarity of kyanite alone. All non-isometric minerals should show some directional variation in principle, though it may be too small to detect easily. Even isometric minerals can display a small variation - halite, for instance, is slightly softer when scratched parallel to a cleavage trace than at 45° to it. In all cases, the pattern of hardness variation is consistent with the point group symmetry of the mineral. ^[1]

Vickers Indentation Hardness

The Mohs scale is ordinal and comparative - it ranks minerals relative to one another but assigns no absolute physical value to the intervals. A more quantitative alternative is indentation hardness, measured by pressing a shaped plunger into a polished mineral surface under a constant load and measuring the size of the indentation left behind. ^[1]

The most widely reported indentation scale is Vickers hardness, which uses a pyramid-shaped diamond indenter with a square cross-section. The Vickers Hardness Number (VHN) equals the applied load divided by the surface area of the indentation, expressed in kg/mm². Vickers hardness is commonly measured alongside petrographic analysis of opaque minerals using a reflected-light microscope, because the polished grain surface needed for optical study is also the surface needed for indentation. ^[1]

VHN is not a fixed single value for a mineral - it varies with crystallographic direction and applied load size. Despite this, VHN increases systematically from Mohs 1 through 9. The jump from Mohs 9 to 10 (diamond) is far larger than any other step, reflecting the exceptional hardness of covalently bonded diamond. ^[1]

References

1. Nesse, W. D. (2018). Introduction to Mineralogy, 3rd ed. Oxford University Press.

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