Hematite frequently presents in massive, rounded forms, appearing more like a blob of metal or a chunk of rust than a traditional geometric crystal. This appearance leads many to question whether the mineral, with its metallic sheen and heavy feel, possesses an ordered internal structure. The answer is yes, but understanding why requires looking beyond the mineral’s outward shape to its fundamental atomic arrangement.
What Defines a Crystalline Solid
A crystalline solid is defined by a long-range, repeating pattern in the arrangement of its atoms, molecules, or ions. This organized structure is known as a crystal lattice, and the smallest repeating unit is called the unit cell. This order extends throughout the entire material, providing a predictable structure at the microscopic level.
This long-range order differentiates crystalline solids from amorphous solids, which have a random, irregular arrangement of particles. The highly organized nature of a crystal dictates many of its physical properties, including its melting point. Crystalline solids melt at a specific, sharp temperature because all the bonds break simultaneously due to the uniform arrangement. The presence of this repeating atomic structure means that any mineral, regardless of its external shape, is a crystal.
The Internal Structure of Hematite
Hematite, iron(III) oxide (\(\text{Fe}_2\text{O}_3\)), is a crystal with a highly organized internal structure. It belongs to the trigonal crystal system, often classified within the rhombohedral lattice, placing it in the same group as corundum (\(\text{Al}_2\text{O}_3\)).
At the atomic level, the structure consists of iron ions (\(\text{Fe}^{3+}\)) and oxygen atoms (O) arranged in a precise stacking pattern. The oxygen atoms are arranged in a hexagonal closest-packing framework. The iron ions occupy two-thirds of the available octahedral sites within this framework.
Each iron ion is coordinated by six oxygen atoms, forming a distorted octahedron. This repeating arrangement defines the unit cell. The strong bonds within this dense lattice are responsible for hematite’s hardness and high specific gravity.
Key Physical Properties Resulting from Structure
The internal crystalline structure and chemical composition of hematite directly influence its observable physical properties. Its most diagnostic property is its streak, the color of the mineral in powdered form, which is always a bright, reddish-brown. This color results directly from the iron(III) oxide composition, revealing the inherent rust-red color associated with ferric iron.
Hematite exhibits a luster ranging from splendent metallic (steel-gray or black) to a dull, earthy appearance in its massive forms. The ordered, dense packing contributes to its high hardness, rating between 5.5 and 6.5 on the Mohs scale.
Despite its crystalline nature, hematite lacks cleavage, meaning it does not break along smooth, flat planes. Instead, the strong, interlocking bonds of the trigonal lattice cause it to exhibit an uneven to subconchoidal fracture when broken. The high specific gravity, around 5.3, also results from the dense arrangement of the heavy iron and oxygen atoms.
Common Forms and Real-World Applications
Hematite is often mistaken for a non-crystal because of its common habits, or the ways it grows in nature. It frequently occurs in massive forms where the mineral grains are so fine and intergrown that they obscure the underlying crystal structure. The botryoidal, or “kidney ore,” variety is a common example, forming rounded, grape-like masses.
In contrast, specularite (specular hematite) grows as well-formed, flat, platy crystals with a brilliant metallic luster, clearly indicating a crystalline structure. Regardless of the external habit, all these forms are composed of the same \(\text{Fe}_2\text{O}_3\) crystal lattice.
The primary application of hematite is its role as the most important ore for iron, which is extracted for steel production. It is also widely used in the production of pigments. The red ochre variety has been used since prehistoric times to create reddish-brown paints and dyes.
Other Uses
Historically, hematite has been used as a polishing agent and is valued as an ornamental stone in jewelry.