Quartz in Basalt

Description

Quartz in basalt refers to the occurrence of the mineral quartz (silicon dioxide, SiO2) within a basaltic rock matrix. Basalt is a fine-grained, dark-colored extrusive igneous rock, typically composed of plagioclase feldspar, pyroxene, and olivine, with a low silica content (45-52 wt% SiO2). Primary quartz is rare in basalt due to its low silica saturation. Therefore, quartz found in basalt is almost exclusively secondary, forming as infillings of vesicles (amygdules), fractures, or other voids. These quartz infillings can range from microscopic crystals to macroscopic masses, often appearing as clear, milky, or colored (due to impurities) crystalline quartz, or as cryptocrystalline varieties like chalcedony, agate, or jasper. The contrast between the dark basalt and the lighter quartz can be visually striking.

How to Identify

Color

Basalt is typically dark gray to black. Quartz inclusions can be colorless, white, milky, gray, or various colors if impurities are present (e.g., purple amethyst, yellow citrine, red jasper, banded agate).

Luster

Basalt is dull to sub-vitreous. Quartz is vitreous (glassy) to waxy (chalcedony).

Texture

Basalt is typically fine-grained to aphanitic, sometimes porphyritic or vesicular. Quartz inclusions will have a crystalline or cryptocrystalline texture within the vesicles or fractures.

Crystal Form

Quartz can form euhedral to anhedral crystals, drusy coatings, or massive to botryoidal (chalcedony/agate) aggregates within the basaltic matrix. Basalt itself has a granular or interlocking texture of its constituent minerals.

Cleavage

Basalt minerals (pyroxene, feldspar) have cleavage, but the rock as a whole does not exhibit prominent cleavage. Quartz has no cleavage, exhibiting conchoidal fracture.

Geological Environment

Found in extrusive igneous environments, specifically in basaltic lava flows, sills, and dikes that have undergone secondary alteration by silica-rich hydrothermal fluids. Common in amygdaloidal basalts.

Key Facts

• Hardness: Basalt (variable, typically 5-6 on Mohs scale for constituent minerals); Quartz (7 on Mohs scale)
• Specific Gravity: Basalt (2.8-3.0 g/cm³); Quartz (2.65 g/cm³)
• Crystal System: Basalt (polycrystalline, constituent minerals are triclinic, monoclinic, or orthorhombic); Quartz (Trigonal)
• Color: Dark gray to black (basalt); Colorless, white, milky, or various colors due to impurities (quartz)
• Luster: Dull to sub-vitreous (basalt); Vitreous to waxy (quartz)
• Transparency: Opaque (basalt); Transparent to translucent (quartz)
• Fracture: Conchoidal to uneven (basalt); Conchoidal (quartz)
• Cleavage: Absent in quartz; Basalt minerals have cleavage but not prominent in the rock mass.
• Composition: Basalt (Plagioclase feldspar, pyroxene, olivine, magnetite); Quartz (SiO2)

Quick Check

• Color: Dark gray to black (basalt) with colorless, white, or colored inclusions (quartz)
• Luster: Dull to sub-vitreous (basalt) with vitreous to waxy (quartz) inclusions
• Streak: White (quartz) on a streak plate; basalt typically has a dark gray to black streak, but the quartz inclusion itself will be white.

Physical Characteristics

• Crystal Habit: Basalt: Aphanitic to porphyritic, massive. Quartz: Prismatic, massive, granular, cryptocrystalline (chalcedony, agate), drusy.
• Cleavage Type: Quartz: None. Basalt: Constituent minerals have cleavage (e.g., pyroxene: two at ~90°, plagioclase: two at ~90°).
• Fracture Type: Conchoidal (quartz); Uneven to sub-conchoidal (basalt).
• Tenacity: Brittle (both basalt and quartz).
• Luster Type: Vitreous to waxy (quartz); Dull to sub-vitreous (basalt).

Formation

Quartz (SiO2) is generally not a primary mineral in basalt, which is an iron- and magnesium-rich, silica-poor mafic igneous rock. The presence of quartz in basalt typically indicates secondary mineralization. This occurs when silica-rich fluids, often derived from the alteration of volcanic glass or other silica-bearing minerals within the basalt or surrounding rocks, percolate through vesicles (gas bubbles) or fractures in the basalt. As these fluids cool and/or their chemical conditions change, quartz, chalcedony, or opal precipitates within these voids. This process is common in amygdaloidal basalts, where the vesicles are filled with secondary minerals. The silica source can also be external, introduced by hydrothermal systems.

Usage

Specimen collection, lapidary material (especially when forming agates or jaspers within the basalt), and as a geological indicator for secondary alteration processes and fluid flow within volcanic rocks. Basalt itself is used as aggregate, road base, and in construction.

Age Distribution

Predominantly Cenozoic to Mesozoic, but can occur in basalts of any age where conditions for secondary mineralization are met.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicate (Quartz); Igneous Rock (Basalt)

Group

Tectosilicate (Quartz); Mafic Extrusive Igneous Rock (Basalt)

Crystal System

Trigonal (Quartz); Polycrystalline (Basalt)

Chemical Formula

SiO2 (Quartz); Complex silicate formula (Basalt, e.g., (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 for pyroxene, NaAlSi3O8-CaAl2Si2O8 for plagioclase)

Composition

Silicon dioxide (Quartz); Predominantly plagioclase feldspar, pyroxene, and olivine (Basalt)

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