Hematite, a common iron oxide mineral, is one of the most abundant minerals found on Earth’s surface and shallow crust. Its name, derived from the Greek word haima (meaning “blood”), reflects the characteristic deep red color of its powdered form. This mineral has maintained historical significance for millennia, serving as one of the earliest known pigments used by ancient cultures. Today, hematite is recognized as the world’s most important source of iron, making it fundamental to modern industry.
Defining Hematite: Chemical Composition and Classification
Hematite is chemically classified as ferric iron oxide, with the chemical formula Fe2O3. This composition means that, by weight, the mineral is comprised of approximately 70% iron and 30% oxygen. It belongs to the oxide mineral group, which includes compounds where metal atoms are bonded with oxygen.
The mineral crystallizes within the trigonal crystal system, often forming a structure similar to that of the mineral corundum (Al2O3). This shared rhombohedral structure makes hematite the most thermodynamically stable form of iron oxide under typical surface conditions. While pure hematite is only weakly magnetic, its high iron content gives it a noticeable density, distinguishing it from many other rock-forming minerals.
Identifying Features: Physical Properties
The external appearance of hematite is remarkably variable. Its color can range dramatically from a metallic, silvery-gray to a dull, earthy red or brown. The metallic variety, often called specular hematite, exhibits a bright, splendent luster and sometimes appears as a mass of silvery flakes.
Other common forms include reniform or “kidney ore,” which displays a rounded, botryoidal structure with a metallic to submetallic sheen. The more earthy forms, known as red ochre, are dull and non-metallic, appearing as fine-grained masses.
Hematite consistently produces a reddish-brown streak when rubbed across an unglazed porcelain plate. This diagnostic feature is the most reliable way to distinguish hematite from other metallic-looking minerals. Hematite ranks between 5.5 and 6.5 on the Mohs hardness scale, a moderate hardness that means it can scratch glass but can also be scratched by common quartz.
Geological Formation and Global Occurrence
Hematite’s largest and most economically significant deposits formed in ancient marine environments. The world’s vast reserves are predominantly found in Banded Iron Formations (BIFs), which are layered rock deposits from the Precambrian era. These formations record a time when oxygen, produced by early microbial life, reacted with dissolved iron in the oceans, causing iron oxides to precipitate and settle on the seafloor in distinct bands.
Hematite also forms through hydrothermal activity, where hot, iron-rich fluids cool and deposit the mineral in veins and cavities. It can also be a product of contact metamorphism or the widespread weathering of iron-bearing rocks. The mineral is globally abundant, contributing to the reddish color of many soils and even the distinct rust-red hue of the Martian landscape.
Primary Industrial and Commercial Applications
Hematite’s most important role is as the predominant source of iron for the global steel industry. Iron ore, primarily hematite, is mined and processed to create the raw iron needed for manufacturing steel. This material is foundational to construction, transportation, and infrastructure worldwide, underscoring hematite’s economic significance.
Beyond metallurgy, hematite’s characteristic color makes it invaluable as a pigment. The earthy, fine-grained variety, red ochre, has been used historically in art and painting and continues today in the production of durable red, brown, and purple pigments. Additionally, the dense, metallic varieties are popular in lapidary work, where they are polished into cabochons, beads, and ornamental carvings.