Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Iron-stained quartzite is a non-foliated metamorphic rock predominantly composed of quartz (SiO2), typically greater than 90%. Its characteristic coloration, ranging from yellow, orange, red, to brown, is imparted by the presence of iron oxides and hydroxides, such as hematite (Fe2O3), goethite (FeO(OH)), and limonite (a mixture of hydrated iron oxides). These iron minerals can be primary constituents of the original sandstone protolith or introduced later through hydrothermal alteration or weathering processes. The rock exhibits a granular, often sugary texture due to the interlocking quartz grains, which are so tightly fused that the rock typically fractures across the grains rather than along their boundaries. It is extremely hard and resistant to chemical and physical weathering.
How to Identify
- Color
- Typically yellow, orange, red, reddish-brown, or brown due to iron oxide staining. Can also be white, gray, or pink if iron content is low or absent.
- Luster
- Vitreous (glassy) to greasy on fresh surfaces, dull on weathered surfaces.
- Texture
- Granular, sugary, interlocking mosaic of quartz grains. Non-foliated. Grain size typically fine to medium.
- Crystal Form
- Anhedral (lacking well-formed crystal faces) interlocking quartz grains.
- Cleavage
- None, as quartz itself lacks cleavage. The rock fractures conchoidally or irregularly across grain boundaries.
- Geological Environment
- Forms in regional metamorphic settings from the metamorphism of quartz-rich sandstones. Found in ancient cratons, mountain belts, and areas subjected to significant burial and tectonic activity. Often associated with other metamorphic rocks like schists and gneisses.
Key Facts
- Hardness: 7 (Mohs scale)
- Specific Gravity: 2.65 - 2.70 g/cm³
- Crystal System: Trigonal (for quartz)
- Color: Variable, commonly yellow, orange, red, brown due to iron oxides; also white, gray, pink.
- Luster: Vitreous to greasy.
- Transparency: Opaque to translucent.
- Fracture: Conchoidal to irregular, breaking across grains.
- Cleavage: None (for quartz and the rock as a whole).
- Composition: Primarily quartz (SiO2), with varying amounts of iron oxides (hematite, goethite, limonite) and minor accessory minerals like mica, feldspar, zircon, rutile.
Quick Check
- Color: Yellow, orange, red, brown (due to iron oxides); can also be white or gray.
- Luster: Vitreous to greasy.
- Streak: White (for quartz), but may be reddish-brown if significant iron oxides are present and abraded.
Physical Characteristics
- Crystal Habit: Anhedral, interlocking granular mosaic.
- Cleavage Type: Absent.
- Fracture Type: Conchoidal to irregular.
- Tenacity: Brittle.
- Luster Type: Vitreous to greasy.
Formation
Iron-stained quartzite forms from the metamorphism of quartz-rich sandstones (quartz arenites) that contain varying amounts of iron-bearing minerals (e.g., hematite, goethite, limonite) or where iron-rich fluids have subsequently infiltrated and stained the rock. The metamorphic process involves recrystallization of quartz grains under high temperature and pressure, leading to a dense, interlocking mosaic of quartz crystals. The iron oxides are typically disseminated throughout the rock or concentrated along bedding planes and fractures.
Usage
Due to its hardness and resistance to weathering, iron-stained quartzite is used as a construction aggregate, road material, railway ballast, and dimension stone. Its often attractive coloration can also make it suitable for decorative purposes, including landscaping and architectural facings. Historically, some iron-rich quartzites have been associated with iron ore deposits.
Age Distribution
Precambrian to Cenozoic, depending on the protolith and metamorphic event.
Where to Find
Appalachian Mountains, USA
Extensive quartzite formations, often iron-stained, are found throughout the Appalachian orogen, particularly in Pennsylvania, Maryland, and Virginia.
Lake Superior Region, USA/Canada
Associated with Precambrian iron formations and banded iron formations (BIFs), where quartzites can be heavily iron-stained.
Brazil
Significant deposits of high-quality quartzite, including iron-stained varieties, are found in various states, often used as dimension stone.
India
Widespread occurrences in various geological terrains, utilized for construction and decorative purposes.
Australia
Found in ancient cratonic areas and mountain ranges, often associated with iron ore deposits.
Finding Tips
Look for Metamorphic Terrains
Focus on areas known for regional metamorphism, such as ancient mountain belts, shield areas, and cratons. Quartzite is a common component of such terrains.
Identify Protolith Clues
While highly metamorphosed, sometimes relict sedimentary structures like bedding or cross-bedding can be faintly preserved, indicating a sandstone protolith.
Test Hardness
Quartzite is extremely hard (Mohs 7). It will scratch steel and cannot be scratched by a knife or glass. This distinguishes it from softer sandstones.
Examine Fracture
Unlike sandstone, which tends to break around individual sand grains, quartzite fractures through the grains, creating a smoother, often conchoidal fracture surface.
Observe Coloration
The distinctive yellow, orange, red, or brown hues are key indicators of iron staining. Look for variations in intensity and distribution of these colors.
Similar Rocks
Sandstone
Quartz Sandstone
Also known as: Quartz Arenite
Chert
Cryptocrystalline Quartz
Also known as: Flint
Jasper
Opaque Cryptocrystalline Quartz
Also known as: Red Jasper
Gneiss
Gneiss
Also known as: Banded Gneiss
Scientific Classification
- Mineral Class
- Silicate (for quartz)
- Group
- Metamorphic Rock
- Crystal System
- Trigonal (for quartz)
- Chemical Formula
- SiO2 (main component, quartz) + Fe2O3, FeO(OH) (iron oxides)
- Composition
- Predominantly silicon dioxide (SiO2) with iron oxides and hydroxides.
Explore Iron-stained Quartzite
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.