Hematite is a common iron oxide mineral with the chemical formula Fe2O3. It is one of the most abundant minerals on Earth’s surface and is the primary source of iron used in industry. Because of its variable appearance, visual identification requires attention to its surface characteristics, its powdered color, and its physical structure.
Surface Appearance: Color and Sheen
Hematite presents itself in two dramatically different surface appearances: a metallic form and an earthy form. The metallic variety often appears as a steel-gray to silver-gray material, sometimes even nearly black. This form exhibits a bright, highly reflective metallic luster, leading to it being called specular hematite or specularite.
Conversely, the earthy variety has a much duller look and can range in color from dark reddish-brown to bright red. This form, often called red ochre, is fine-grained and lacks the shiny reflection of the metallic type.
The Crucial Identification Test: Streak
Despite the wide variation in its surface color, all specimens of hematite share a single, consistent, and identifying feature: its streak. The mineral streak is the color of the mineral when it is ground into a fine powder, typically tested by scraping the specimen across an unglazed porcelain plate. Hematite consistently produces a reddish-brown to rusty-red streak, which is a definitive way to distinguish it from other minerals.
This red streak is so unique that it led to the mineral’s name, derived from the Greek word for blood, haima. This test immediately rules out similar-looking minerals like magnetite or ilmenite, which both leave a black streak.
Physical Forms and Habits
The physical form, or habit, of hematite can vary significantly, dictating the overall shape of the specimen. One distinct form is the botryoidal habit, where the mineral grows into rounded, bubbly masses. This appearance is often described as resembling a bunch of grapes or a kidney, which gives it the common name “kidney ore.”
Another common habit is specular hematite, or specularite, which typically forms as micaceous, platy, or flaky crystals. These highly metallic flakes give the specimen a shimmering, silver-like appearance, often found in hydrothermal veins. Hematite is also found in massive forms, which are dense, non-crystalline masses that serve as important iron ore. Additionally, oolitic hematite is composed of small, spherical or oval grains cemented together, frequently appearing in reddish-brown sedimentary ironstones.