Quartz Vein in Basalt

Composite Rock (Igneous with Hydrothermal Vein)

Quartz (SiO2) and Basalt

Also known as: Basalt with Quartz Infillings

Got a photo? Identify rocks instantly with the Rockby app

Description

This describes a geological occurrence where veins of quartz (a common silicate mineral) are found cutting through a basaltic host rock. Basalt is a fine-grained, dark-colored, mafic igneous rock. The quartz veins typically appear as lighter-colored, often white or translucent, linear or anastomosing structures contrasting sharply with the dark basalt. The texture of the quartz within the veins can range from massive to crystalline, sometimes exhibiting euhedral quartz crystals if open space was available during formation.

How to Identify

Color: Basalt is typically dark gray to black. Quartz veins are usually white, colorless, or translucent, sometimes stained by iron oxides to yellow or brown.
Luster: Basalt is dull to sub-vitreous. Quartz is vitreous (glassy).
Texture: Basalt is fine-grained to aphanitic, sometimes porphyritic with phenocrysts of olivine or pyroxene. The quartz veins will have a crystalline texture, ranging from fine-grained to coarse-grained, often massive or drusy.
Crystal Form: Basalt typically shows no macroscopic crystal form due to rapid cooling. Quartz in veins can be anhedral (massive) or euhedral (well-formed hexagonal prisms with pyramidal terminations) if space allowed.
Cleavage: Basalt minerals (pyroxene, plagioclase) have cleavage, but the rock itself does not exhibit prominent cleavage. Quartz has no cleavage, exhibiting conchoidal fracture.
Geological Environment: Common in areas of volcanic activity, mid-ocean ridges, oceanic islands, and continental flood basalt provinces where subsequent hydrothermal alteration has occurred. Often found in fault zones or areas of intense fracturing within basaltic flows.

Key Facts

Hardness: Basalt (overall rock) is 6-7 on Mohs scale (due to constituent minerals). Quartz is 7 on Mohs scale.
Specific Gravity: Basalt: 2.8-3.0 g/cm³. Quartz: 2.65 g/cm³.
Crystal System: Basalt is a rock, not a single mineral, but its constituent minerals (e.g., plagioclase, pyroxene) are triclinic and monoclinic, respectively. Quartz is trigonal.
Color: Basalt: Dark gray to black. Quartz: Colorless, white, or translucent.
Luster: Basalt: Dull to sub-vitreous. Quartz: Vitreous.
Transparency: Basalt: Opaque. Quartz: Transparent to translucent.
Fracture: Basalt: Uneven to conchoidal. Quartz: Conchoidal.
Cleavage: Basalt: No distinct cleavage as a rock, but constituent minerals have cleavage. Quartz: None.
Composition: Basalt: Primarily plagioclase feldspar, pyroxene, and often olivine, with minor magnetite and ilmenite. Quartz: Silicon dioxide (SiO2).

Quick Check

Color: Dark gray to black basalt with white to colorless veins.
Luster: Dull to sub-vitreous for basalt, vitreous for quartz.
Streak: White for quartz; basalt typically has a grayish-white streak, but is often not practical to streak due to its hardness and fine grain.

Physical Characteristics

Crystal Habit: Basalt: Aphanitic to porphyritic. Quartz: Prismatic, massive, granular, or drusy.
Cleavage Type: Basalt: Not applicable as a rock. Quartz: None.
Fracture Type: Basalt: Uneven to conchoidal. Quartz: Conchoidal.
Tenacity: Basalt: Brittle. Quartz: Brittle.
Luster Type: Basalt: Dull to sub-vitreous. Quartz: Vitreous.

Formation

Basalt is an extrusive igneous rock formed from the rapid cooling of magnesium-rich and iron-rich lava exposed at or very near the surface of a terrestrial planet or moon. Quartz veins form later through hydrothermal processes. Hot, silica-rich fluids circulate through fractures and fissures within the pre-existing basalt. As these fluids cool or undergo pressure changes, dissolved silica (SiO2) precipitates out, filling the open spaces and forming quartz veins. The silica can be derived from the basalt itself through alteration, or from deeper magmatic or metamorphic sources.

Usage

Primarily of geological interest for understanding hydrothermal alteration, fluid flow, and mineralization processes in mafic igneous rocks. Occasionally, if the quartz veins contain economic minerals (e.g., gold, copper sulfides), they can be targets for mineral exploration. Basalt itself is widely used as aggregate in construction, road building, and as a raw material for basalt fiber.

Age Distribution

Varies widely depending on the age of the basaltic host rock and the timing of hydrothermal activity. Basalts can range from Archean to recent, and quartz veining can occur at any point after the basalt's formation.

Where to Find

Columbia River Basalt Group, USA

Extensive flood basalts in the Pacific Northwest, where hydrothermal alteration and quartz veining can be observed in road cuts and river exposures.

Deccan Traps, India

Another large igneous province where basaltic flows are widespread, and secondary mineralization, including quartz veins, can be found.

Iceland

A volcanically active island with abundant basaltic rocks, where geothermal activity leads to widespread hydrothermal alteration and vein formation.

Mid-ocean ridges and ophiolite complexes

Submarine hydrothermal systems associated with basaltic crust often produce quartz veins, which can be observed in uplifted ophiolite sequences on land.

Finding Tips

Look for contrasts

The white or translucent quartz veins will stand out against the dark basalt. Look for linear features or networks of lighter material cutting through the darker rock.

Examine fractured zones

Hydrothermal fluids preferentially flow through fractures and faults. Focus your search in areas where the basalt is visibly fractured or sheared.

Check for associated alteration

The basalt immediately adjacent to the quartz veins may show signs of hydrothermal alteration, such as discoloration, softening, or the presence of other secondary minerals (e.g., chlorite, epidote, zeolites).

Use a hand lens

A hand lens will help distinguish the glassy luster and crystalline nature of quartz from other infilling minerals like calcite or zeolites.

Similar Rocks

Calcite Vein in Basalt

Basalt with CaCO3 Veins

Also known as: Basalt with Calcite Infillings

Zeolite Vein in Basalt

Basalt with Zeolite Veins

Also known as: Basalt with Zeolite Infillings

Chalcedony Vein in Basalt

Basalt with Cryptocrystalline SiO2 Veins

Also known as: Basalt with Chalcedony Infillings

Scientific Classification

Mineral Class: Quartz: Tectosilicate. Basalt: Igneous Rock.
Group: Quartz: Quartz Group. Basalt: Mafic Extrusive Igneous Rock.
Crystal System: Quartz: Trigonal. Basalt: Not applicable (rock).
Chemical Formula: Quartz: SiO2. Basalt: Complex silicate mixture (e.g., (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 for pyroxene, NaAlSi3O8-CaAl2Si2O8 for plagioclase).
Composition: Quartz: Silicon dioxide. Basalt: Predominantly plagioclase feldspar, pyroxene, and often olivine, with minor accessory minerals.

Explore Quartz Vein in Basalt

Identification Localities & Collecting Look-alikes

Identify rocks anywhere

Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.

Download on theApp Store Get it onGoogle Play Continue onWeb

Open in app