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Ferruginous quartzite is a hard, non-foliated metamorphic rock composed primarily of quartz (SiO2) with a significant proportion of iron oxides, typically hematite (Fe2O3) and/or magnetite (Fe3O4). The iron content gives the rock its characteristic reddish-brown, purplish, or sometimes black coloration. It often exhibits a granular texture due to the recrystallization of quartz grains. In some instances, it can display banding, reflecting the original sedimentary layering of the protolith, particularly when derived from banded iron formations.
How to Identify
- Color
- Typically reddish-brown, purplish-red, dark red, or sometimes black due to the presence of iron oxides. Can also have gray or white quartz bands.
- Luster
- Vitreous (glassy) to dull, depending on the grain size and abundance of iron oxides. Hematite-rich varieties may have a submetallic sheen.
- Texture
- Granular, interlocking quartz grains. Non-foliated, though relict bedding (banding) may be preserved. Fine-grained to medium-grained.
- Crystal Form
- Individual quartz crystals are anhedral (lacking well-formed faces) due to recrystallization and interlocking growth. Iron oxides may be disseminated or form distinct layers.
- Cleavage
- None (quartz lacks cleavage). Fracture is conchoidal to irregular.
- Geological Environment
- Found in ancient cratonic shields and metamorphic belts, often associated with large-scale iron ore deposits. Occurs in regions that have undergone regional metamorphism of iron-rich sedimentary protoliths.
Key Facts
- Hardness: 7 (Mohs scale, due to quartz)
- Specific Gravity: 2.65 - 3.0 (varies with iron oxide content; pure quartz is 2.65)
- Crystal System: Trigonal (for quartz component)
- Color: Reddish-brown, purplish-red, dark red, black, often banded with white or gray.
- Luster: Vitreous to dull, submetallic if rich in hematite/magnetite.
- Transparency: Opaque to translucent (thin edges of quartz).
- Fracture: Conchoidal to irregular/splintery.
- Cleavage: None (quartz).
- Composition: Primarily SiO2 (quartz) with significant Fe2O3 (hematite) and/or Fe3O4 (magnetite).
Quick Check
- Color: Reddish-brown, purplish-red, dark red, or black, often with lighter quartz bands.
- Luster: Vitreous to dull, sometimes submetallic.
- Streak: White (for quartz) to reddish-brown (for hematite) or black (for magnetite), depending on the dominant iron mineral and how much is abraded.
Physical Characteristics
- Crystal Habit: Granular, massive, interlocking anhedral grains.
- Cleavage Type: None (quartz).
- Fracture Type: Conchoidal to irregular/splintery.
- Tenacity: Brittle.
- Luster Type: Vitreous to dull, submetallic.
Formation
Ferruginous quartzite forms from the metamorphism of iron-rich chert or banded iron formations (BIFs) that have undergone silicification and subsequent regional or contact metamorphism. The original sedimentary protoliths were typically deposited in marine environments where iron and silica precipitated from seawater, often in association with microbial activity. During metamorphism, quartz grains recrystallize, and iron oxides (primarily hematite and magnetite) are concentrated or disseminated within the quartz matrix.
Usage
Historically, ferruginous quartzite has been a significant source of iron ore, particularly when it occurs as part of banded iron formations. It is also used as an aggregate in construction, for road building, and as a decorative stone due to its attractive banding and color variations. High-purity varieties can be used in refractory materials.
Age Distribution
Predominantly Precambrian (Archean to Proterozoic), but can occur in younger metamorphic terrains.
Where to Find
Lake Superior Region, USA and Canada
Extensive deposits of banded iron formations, which are protoliths for ferruginous quartzite, are found in the Mesabi Range (Minnesota), Marquette Range (Michigan), and parts of Ontario, Canada.
Hamersley Basin, Western Australia
Home to some of the world's largest and richest iron ore deposits, largely derived from metamorphosed banded iron formations, including ferruginous quartzite.
Minas Gerais, Brazil
Significant iron ore provinces, where metamorphosed iron formations and associated ferruginous quartzites are abundant.
Kola Peninsula, Russia
Contains ancient Precambrian terrains with iron-rich metamorphic rocks, including ferruginous quartzite.
India (e.g., Odisha, Jharkhand)
Major iron ore belts with extensive occurrences of banded iron formations and their metamorphic equivalents.
Finding Tips
Look for Iron Ore Deposits
Ferruginous quartzite is often found in close association with major iron ore deposits, particularly those derived from banded iron formations. Geological maps showing BIFs are good indicators.
Examine Metamorphic Terrains
Focus on ancient cratonic shields and areas that have experienced regional metamorphism, as these are the typical environments for its formation.
Check for Reddish Staining
The presence of iron oxides often leads to reddish or brownish staining on the rock surface and surrounding soil, which can be a visual clue.
Test Hardness
Quartzite is very hard (Mohs 7), so it will scratch glass and steel. This helps differentiate it from softer iron-rich sedimentary rocks.
Observe Texture
Look for a granular, interlocking texture of quartz grains. The presence of distinct reddish or purplish bands or disseminated iron minerals within a quartz matrix is characteristic.
Similar Rocks
Banded Iron Formation
Banded Iron Formation
Also known as: BIF
Chert
Chert
Also known as: Flint, Jasper
Jasper
Jasper
Also known as: Red Jasper
Hematite
Hematite
Also known as: Specular Hematite, Kidney Ore
Scientific Classification
- Mineral Class
- Silicate (for quartz), Oxide (for iron minerals)
- Group
- Quartz Group (for quartz), Hematite Group/Spinel Group (for iron minerals)
- Crystal System
- Trigonal (quartz), Hexagonal (hematite), Isometric (magnetite)
- Chemical Formula
- SiO2 (quartz) + Fe2O3 (hematite) and/or Fe3O4 (magnetite)
- Composition
- Silicon dioxide with varying amounts of iron oxides.
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