Hematite is an iron oxide mineral (Fe₂O₃) and one of the most abundant iron-bearing minerals found on Earth. It serves as the most important ore of iron used for global steel production. Its powdered form is responsible for the deep red color in many soils and rocks. To identify a specimen definitively, one must rely on distinct physical properties, primarily the color of the mineral’s powder, known as its streak.
Visual Characteristics
The external appearance of hematite is highly variable, often confusing those new to mineral identification. Specimen color ranges from a mirror-like, metallic silver-gray or black in its crystalline form to a dull, earthy red or reddish-brown in its massive state. This wide spectrum means that visual inspection alone is insufficient for positive identification.
Hematite’s luster changes significantly depending on its form. The metallic, crystalline variety, often called specular hematite, exhibits a brilliant shine, appearing like polished metal. Conversely, the finer-grained, massive forms, such as red ochre or kidney ore, display a much duller, earthy luster. Kidney ore is characterized by its smooth, rounded, bubbly surface texture, resembling a kidney.
The micaceous variety of hematite, which is composed of tiny, flaky crystals, can appear to shimmer due to light reflection. These different habits—micaceous, massive, or botryoidal—are all composed of the same Fe₂O₃ and possess identical foundational properties. The density of hematite is high, typically between 5.0 and 5.3 g/cm³. This high density makes a specimen feel noticeably heavy for its size.
Performing the Definitive Streak Test
The streak test is the most reliable method for identifying hematite because its powder color remains constant regardless of the specimen’s external appearance. To perform the test, you need a streak plate, which is a piece of unglazed porcelain tile. The plate has a Mohs hardness of about 6.5 to 7.0, making it abrasive enough to grind the mineral into a fine powder.
The procedure requires firmly holding the specimen and drawing it across the streak plate, similar to drawing with chalk. The mark left behind is the mineral’s true color in its powdered form, known as the streak. Every variety of hematite, from shiny metallic gray to dull reddish-brown, consistently produces a rusty red or reddish-brown streak.
This consistent result is due to the chemical composition of hematite, which is iron(III) oxide. The fine powder reveals the inherent color of the iron oxide, which is not obscured by the crystal structure or surface impurities of the larger specimen. This single property is often sufficient to distinguish hematite from nearly all other minerals that share a similar appearance. Minerals harder than the streak plate will not leave a powder, instead scratching the plate itself.
Hardness and Magnetic Properties
Assessing the mineral’s relative hardness and magnetic response provides secondary confirmation for identification. Hematite has a Mohs hardness ranging from 5.5 to 6.5, placing it in the moderately hard category. This means a steel knife blade (hardness 5.5) may or may not be able to scratch the specimen.
A practical field test involves attempting to scratch the specimen with a piece of ordinary window glass, which is approximately 5.5 in hardness. Hematite is typically able to scratch the glass, but it is softer than common quartz (hardness 7). The mineral’s high density further supports identification, making the specimen feel disproportionately heavy in the hand.
The magnetic properties of hematite are a significant characteristic, specifically its lack of strong magnetism. Pure hematite is considered non-magnetic or only very weakly magnetic, meaning it will not be attracted to a common handheld magnet. This property is useful in distinguishing it from its close chemical cousin, magnetite.
Distinguishing Hematite from Similar Minerals
Applying the physical tests allows for clear differentiation between hematite and other similar-looking minerals. The most common confusion occurs with magnetite, another iron oxide mineral that is black or dark gray. Magnetite is strongly attracted to a magnet, which immediately separates it from the non-magnetic nature of pure hematite.
The streak test also differentiates them, as magnetite leaves a black or dark gray streak, contrasting sharply with hematite’s reddish-brown streak. Iron-bearing minerals like goethite and limonite, which are iron oxyhydroxides, can be confused with earthy hematite. Goethite and limonite are distinguished by their yellowish-brown or ochre-yellow streak, unlike the red streak of hematite.
Another potential look-alike is galena, a lead sulfide mineral that shares a similar metallic gray color and high density with specular hematite. Galena is much softer, with a Mohs hardness of only 2.5. This means it can easily be scratched with a fingernail or a copper penny. Comparing the streak color, hardness, and magnetic response allows confident isolation of hematite from deceptive mineral specimens.