Ferruginous Sandstone with Quartz

Sedimentary Rock (Sandstone) with Hydrothermal Vein

Also known as: Iron-stained Sandstone with Quartz, Hematitic Sandstone with Quartz, Limonitic Sandstone with Quartz

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Description

Iron-rich sandstone with a quartz vein is a clastic sedimentary rock characterized by a significant proportion of iron oxide cement, giving it reddish, brownish, or yellowish hues. The primary framework grains are typically quartz. The rock is cut by a distinct vein composed predominantly of crystalline quartz, which represents a later infilling of a fracture. The vein material is usually lighter in color (white, gray, translucent) and has a different texture and hardness compared to the host sandstone.

How to Identify

Color: Sandstone matrix: Red, reddish-brown, brown, yellow, or orange due to iron oxides. Quartz vein: White, gray, translucent, or clear.
Luster: Sandstone matrix: Dull to earthy. Quartz vein: Vitreous (glassy).
Texture: Sandstone matrix: Clastic, granular, often gritty to the touch. Grain size typically sand-sized (0.0625 mm to 2 mm). Quartz vein: Crystalline, often massive or showing euhedral to subhedral quartz crystals filling the fracture.
Crystal Form: Sandstone matrix: Detrital quartz grains are typically sub-rounded to angular. Iron oxide cement is cryptocrystalline or amorphous. Quartz vein: Anhedral to euhedral quartz crystals, often forming interlocking mosaics.
Cleavage: Sandstone matrix: No true cleavage, but may break along bedding planes or grain boundaries. Quartz vein: Quartz itself lacks cleavage, exhibiting conchoidal fracture.
Geological Environment: Sedimentary basins, often associated with continental or shallow marine depositional environments where iron-rich sediments accumulate. The quartz veins indicate subsequent tectonic activity and hydrothermal fluid circulation, often related to faulting, folding, or magmatic activity.

Key Facts

Hardness: Sandstone matrix: Varies (2-7 on Mohs scale, depending on cementation and grain hardness). Quartz vein: 7 on Mohs scale.
Specific Gravity: Sandstone matrix: 2.6 - 2.9 g/cm3 (higher with more iron oxides). Quartz vein: 2.65 g/cm3.
Crystal System: Sandstone matrix: Constituent minerals (e.g., quartz) are trigonal. Iron oxides can be various systems. Quartz vein: Trigonal (for quartz).
Color: Red, brown, yellow (sandstone); white, gray, clear (quartz vein).
Luster: Dull to earthy (sandstone); vitreous (quartz vein).
Transparency: Opaque to translucent (sandstone); translucent to transparent (quartz vein).
Fracture: Irregular to conchoidal (sandstone); conchoidal (quartz vein).
Cleavage: None (quartz); some iron oxides may have poor cleavage. Sandstone breaks along grain boundaries.
Composition: Predominantly quartz (SiO2) grains and cement, with significant iron oxides (e.g., Fe2O3 - hematite, FeO(OH) - goethite, limonite) as cement in the sandstone. The vein is almost pure SiO2 (quartz).

Quick Check

Color: Red, brown, yellow (sandstone) with white, gray, clear (quartz vein)
Luster: Dull/earthy (sandstone) to vitreous (quartz vein)
Streak: Reddish-brown to yellowish-brown (for iron oxides in sandstone); white (for quartz)

Physical Characteristics

Crystal Habit: Sandstone: Detrital grains, often sub-rounded to angular. Quartz vein: Massive, granular, or euhedral crystals (e.g., prismatic with pyramidal terminations).
Cleavage Type: None for quartz. Sandstone breaks along grain boundaries.
Fracture Type: Conchoidal for quartz. Irregular to sub-conchoidal for sandstone.
Tenacity: Brittle.
Luster Type: Vitreous (quartz), earthy to dull (iron oxides in sandstone).

Formation

Ferruginous sandstone forms from the lithification of sand-sized grains (typically quartz) cemented by iron oxides (e.g., hematite, goethite, limonite). The iron content can be primary (deposited with the sand) or secondary (introduced later by diagenetic fluids). Quartz veins form when silica-rich hydrothermal fluids circulate through fractures in the pre-existing sandstone, precipitating quartz (SiO2) as the fluids cool or undergo pressure changes. The presence of a quartz vein indicates a later stage of fluid flow and mineralization after the sandstone's initial formation.

Usage

Historically, some highly ferruginous sandstones have been used as low-grade iron ore, building stone, or aggregate. The quartz veins themselves are generally not economically significant in this context unless they contain other valuable minerals (e.g., gold, base metals), which is not implied by the general description. As a geological specimen, it illustrates diagenetic processes, fluid flow, and structural deformation.

Age Distribution

Can occur in sedimentary sequences of all ages, from Precambrian to Cenozoic, wherever iron-rich sediments and subsequent quartz veining events have taken place.

Where to Find

Colorado Plateau, USA

Extensive Mesozoic sandstones (e.g., Navajo Sandstone, Entrada Sandstone) are often highly ferruginous, exhibiting vibrant red and orange colors. Quartz veins can be found in areas of structural deformation.

Pilbara Craton, Western Australia

Ancient sedimentary sequences, including some iron-rich sandstones, are present, often cut by quartz veins associated with tectonic events and gold mineralization.

Brazil (Minas Gerais)

Regions known for extensive iron ore deposits often have associated ferruginous sedimentary rocks and quartz veins related to regional metamorphism and fluid flow.

United Kingdom (e.g., Jurassic sandstones)

Many Mesozoic sandstones, such as those in the Jurassic sequence, are iron-rich and can exhibit quartz veining in areas of structural complexity.

Finding Tips

Look for Color Contrast

The most obvious indicator is the color difference between the reddish/brownish sandstone and the lighter, often white or translucent quartz vein cutting through it.

Examine Texture and Hardness

The sandstone will feel gritty and may be friable, while the quartz vein will be much harder (scratching steel) and have a glassy, crystalline texture.

Identify Fracture Patterns

The quartz vein will typically show conchoidal fracture, whereas the sandstone will break irregularly or along grain boundaries.

Observe Geological Context

Look for these rocks in areas with exposed sedimentary sequences, especially near faults, folds, or areas of past hydrothermal activity.

Similar Rocks

Quartzite

Also known as: Metaquartzite

Ironstone

Also known as: Siderite Ironstone, Oolitic Ironstone

Banded Iron Formation (BIF)

Banded Iron Formation

Also known as: BIF

Scientific Classification

Mineral Class: Silicate (for quartz), Oxide (for iron oxides)
Group: Tectosilicate (for quartz), Oxide (for iron oxides)
Crystal System: Trigonal (for quartz)
Chemical Formula: SiO2 (quartz) + Fe2O3, FeO(OH) (iron oxides)
Composition: Silicon dioxide (quartz) and various iron oxides/hydroxides (e.g., hematite, goethite, limonite) as cement and/or coatings.

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