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Hematite is an iron oxide mineral with the chemical formula Fe2O3. The botryoidal habit, often called 'kidney ore' due to its resemblance to a kidney, is characterized by smooth, rounded, intergrown masses that resemble a bunch of grapes. These masses can range in size from a few millimeters to several tens of centimeters. The surface is typically smooth and can have a metallic to dull luster, often with a reddish-brown to black color. When scratched, all forms of hematite produce a characteristic reddish-brown streak, which is a key diagnostic property.
How to Identify
- Color
- Steel-gray to black, reddish-brown, or silver-gray. Botryoidal forms are typically dark gray to black on the surface.
- Luster
- Metallic to submetallic, sometimes dull or earthy. Botryoidal forms often have a submetallic to dull luster.
- Texture
- Smooth, rounded, mammillary, or grape-like masses. The surface can feel slightly rough or granular in some specimens.
- Crystal Form
- Botryoidal (grape-like aggregates), mammillary, reniform (kidney-shaped), massive, micaceous (specular hematite), tabular, or rhombohedral crystals.
- Cleavage
- None. Hematite exhibits no true cleavage, but may show parting on {0001} and {1011}.
- Geological Environment
- Hydrothermal veins, sedimentary iron formations (BIFs), contact metamorphic deposits, and as an alteration product in igneous and metamorphic rocks. Botryoidal forms are common in low-temperature hydrothermal deposits and as concretions in sedimentary rocks.
Key Facts
- Hardness: 5.5-6.5 on the Mohs scale
- Specific Gravity: 5.26 (pure)
- Crystal System: Trigonal
- Color: Steel-gray, black, reddish-brown, silver-gray
- Luster: Metallic, submetallic, dull, earthy
- Transparency: Opaque
- Fracture: Uneven to subconchoidal
- Cleavage: None (parting on {0001} and {1011})
- Composition: Iron oxide (Fe2O3)
Quick Check
- Color: Dark gray to black (botryoidal surface), reddish-brown (when earthy)
- Luster: Submetallic to dull
- Streak: Reddish-brown
Physical Characteristics
- Crystal Habit: Botryoidal, mammillary, reniform, massive, micaceous, tabular, rhombohedral
- Cleavage Type: None, but parting on {0001} and {1011} is common
- Fracture Type: Uneven to subconchoidal
- Tenacity: Brittle
- Luster Type: Metallic to submetallic, dull, earthy
Formation
Hematite forms in a variety of geological environments. Botryoidal hematite, often referred to as 'kidney ore', typically forms in low-temperature hydrothermal veins, sedimentary environments, and as a weathering product. It can precipitate from iron-rich solutions in cavities or on surfaces, gradually building up the characteristic rounded, grape-like masses. This habit is common in environments where iron-rich fluids interact with oxygen or other oxidizing agents.
Usage
Hematite is the most important ore of iron, used extensively in steel production. Finely ground hematite is used as a red pigment (ochre) and as a polishing agent (jeweler's rouge). Botryoidal specimens are highly prized by mineral collectors for their aesthetic form.
Age Distribution
Found in rocks of all ages, but particularly abundant in Precambrian banded iron formations (BIFs) dating back 2.5 to 1.8 billion years ago.
Where to Find
Cumbria, England
Famous for producing excellent botryoidal hematite specimens, often referred to as 'kidney ore', from iron ore mines.
Lake Superior Region, USA (Michigan, Minnesota, Wisconsin)
Major source of iron ore, including hematite, from vast banded iron formations. While massive hematite is more common, botryoidal forms can be found in some localities.
Minas Gerais, Brazil
Known for high-quality hematite, including some botryoidal and specular varieties.
Elba Island, Italy
Historically significant locality for well-formed hematite crystals and masses.
Morocco
Various localities produce botryoidal hematite, often associated with other iron minerals.
Finding Tips
Look for Reddish-Brown Streak
Always perform a streak test. Even metallic-looking hematite will produce a characteristic reddish-brown streak on an unglazed porcelain plate. This is the most reliable diagnostic test.
Examine for Botryoidal Form
Specifically for botryoidal hematite, look for rounded, bubbly, or grape-like masses in cavities or on rock surfaces. These forms are quite distinctive.
Check for Density
Hematite is relatively dense (specific gravity 5.26), so specimens will feel heavier than similarly sized common rocks.
Test for Magnetism (or lack thereof)
Pure hematite is not magnetic, unlike magnetite. However, some hematite can be weakly magnetic due to inclusions of magnetite or martite (pseudomorphs of hematite after magnetite).
Observe Associated Minerals
Botryoidal hematite often occurs with quartz, calcite, barite, or other iron oxides/hydroxides in veins or sedimentary concretions.
Similar Rocks
Goethite
Goethite (FeO(OH))
Also known as: Bog Iron Ore
Limonite
Limonite (a mixture of hydrated iron oxides, primarily goethite)
Also known as: Brown Iron Ore
Magnetite
Magnetite (Fe3O4)
Also known as: Lodestone
Scientific Classification
- Mineral Class
- Oxides
- Group
- Hematite Group
- Crystal System
- Trigonal (hexagonal scalenohedral class)
- Chemical Formula
- Fe2O3
- Composition
- Iron(III) oxide
Explore Hematite (botryoidal)
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