Hematite is a widely distributed iron oxide mineral with the chemical formula Fe2O3, making it the world’s most significant ore for iron production. This compound, which is essentially the mineral form of rust, is present in various geological settings across the globe. For geologists and mineral enthusiasts, the identification of hematite relies on measurable physical characteristics such as color, luster, and density. Among these characteristics, the resistance a mineral exhibits to scratching, known as hardness, is a fundamental tool for identification. Analyzing a mineral’s response to scratching allows for placement on a comparative scale, which helps distinguish hematite from other visually similar minerals.
The Mohs Hardness Value of Hematite
Hematite exhibits a hardness that falls within a moderate range on the Mohs scale, typically registering between 5.5 and 6.5. This range reflects the variation depending on the sample form; highly crystalline specimens tend toward the upper end, while massive or earthy varieties test closer to 5.5.
A hardness value of 5.5 to 6.5 places hematite as generally harder than common glass (approx. 5.5) and comparable to common objects like a steel knife blade or a standard file. Because it is only moderately hard, hematite is easily scratched by minerals that rank higher, such as quartz (7). This characteristic helps in quickly differentiating hematite from harder minerals that might share a similar external color or appearance.
Understanding the Relative Hardness Scale
The Mohs scale of mineral hardness is a system developed in 1812 by German geologist Friedrich Mohs. This scale is an ordinal, qualitative measure that ranks a mineral’s resistance to abrasion or scratching. It consists of ten reference minerals, each assigned an integer value from one (softest) to ten (hardest).
The fundamental concept is simple: any mineral on the scale can be scratched by a mineral with a higher number, and conversely, it can scratch any mineral with a lower number. For example, Gypsum (2) can scratch Talc (1), but it cannot scratch Calcite (3). This arrangement provides a quick, comparative method for identifying unknown minerals.
The ten reference minerals that anchor the Mohs scale are:
- Talc (1)
- Gypsum (2)
- Calcite (3)
- Fluorite (4)
- Apatite (5)
- Orthoclase Feldspar (6)
- Quartz (7)
- Topaz (8)
- Corundum (9)
- Diamond (10)
Other Key Physical Properties for Identification
While hardness provides a strong diagnostic clue, other physical properties are necessary to confirm hematite’s identity, especially its streak. The streak is the color of a mineral when it is finely powdered, and for hematite, this is an unfailingly diagnostic reddish-brown or “rust-red” color. This property is particularly useful because the external color of hematite itself can vary dramatically from silvery-gray to black in crystalline forms.
Another important property is the mineral’s luster, which describes how light reflects from its surface. Hematite can display a metallic luster in its crystalline variety, often called specular hematite, giving it a shiny, mirror-like appearance. However, in fine-grained or massive forms, the luster may be dull or earthy.
The density of hematite is also a significant identifying factor due to its high iron content. The mineral has a high specific gravity, typically measuring around 5.26 g/cm3. This density makes a specimen of hematite feel noticeably heavy compared to a piece of rock of the same size, assisting in field identification.