Quartz and Basalt

Description

Quartz is one of the most abundant minerals in the Earth's crust, a tectosilicate mineral composed of silicon and oxygen atoms in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Basalt is a fine-grained, dark-colored extrusive igneous rock composed predominantly of plagioclase feldspar (typically labradorite or bytownite), pyroxene (augite), and often olivine. It is typically dark grey to black and weathers to brown or rust-red due to the oxidation of its mafic minerals. Its fine-grained texture is a result of rapid cooling of lava.

How to Identify

Color

Quartz: Colorless, white, milky, purple (amethyst), yellow (citrine), pink (rose quartz), brown (smoky quartz), black. Basalt: Dark grey to black, weathering to reddish-brown.

Luster

Quartz: Vitreous (glassy) to greasy. Basalt: Dull to sub-vitreous, sometimes earthy.

Texture

Quartz: Crystalline, granular, massive. Basalt: Aphanitic (fine-grained) to porphyritic (with larger crystals in a fine-grained matrix), vesicular (containing gas bubbles), or amygdaloidal (vesicles filled with secondary minerals).

Crystal Form

Quartz: Hexagonal prisms terminating in hexagonal pyramids; also massive, granular, cryptocrystalline. Basalt: Typically anhedral to subhedral grains of plagioclase, pyroxene, and olivine, often too small to be seen without magnification.

Cleavage

Quartz: None (conchoidal fracture). Basalt: No distinct cleavage planes due to fine-grained nature; minerals within basalt (e.g., pyroxene, plagioclase) have cleavage, but the rock itself fractures irregularly.

Geological Environment

Quartz: Found in igneous (granites, pegmatites), metamorphic (gneisses, schists, quartzites), and sedimentary rocks (sandstones, cherts, veins). Basalt: Associated with volcanic activity, including mid-ocean ridges, oceanic islands (hot spots), continental rifts, and subduction zones (island arcs, continental arcs).

Key Facts

• Hardness: Quartz: 7 on Mohs scale. Basalt: 6-7 (due to constituent minerals like plagioclase and pyroxene).
• Specific Gravity: Quartz: 2.65 g/cm³. Basalt: 2.8-3.0 g/cm³.
• Crystal System: Quartz: Trigonal. Basalt: Not applicable as a rock, but its constituent minerals are typically triclinic (plagioclase) and monoclinic (pyroxene).
• Color: Quartz: Colorless, white, milky, purple, yellow, pink, brown, black. Basalt: Dark grey to black.
• Luster: Quartz: Vitreous to greasy. Basalt: Dull to sub-vitreous.
• Transparency: Quartz: Transparent to translucent to opaque. Basalt: Opaque.
• Fracture: Quartz: Conchoidal. Basalt: Irregular to sub-conchoidal.
• Cleavage: Quartz: None. Basalt: No distinct cleavage as a rock.
• Composition: Quartz: Silicon dioxide (SiO2). Basalt: Primarily plagioclase feldspar, pyroxene, and often olivine, with minor magnetite, ilmenite, and sometimes quartz (rarely, in more evolved basalts).

Quick Check

• Color: Quartz: Variable (colorless, white, purple, etc.); Basalt: Dark grey to black.
• Luster: Quartz: Vitreous; Basalt: Dull to sub-vitreous.
• Streak: Quartz: White; Basalt: Grey to greenish-black.

Physical Characteristics

• Crystal Habit: Quartz: Prismatic, massive, granular, cryptocrystalline. Basalt: Aphanitic (fine-grained), porphyritic, vesicular, amygdaloidal.
• Cleavage Type: Quartz: Absent. Basalt: Not applicable as a rock.
• Fracture Type: Quartz: Conchoidal. Basalt: Irregular to sub-conchoidal.
• Tenacity: Quartz: Brittle. Basalt: Brittle.
• Luster Type: Quartz: Vitreous to greasy. Basalt: Dull to sub-vitreous.

Formation

Quartz: Forms in a wide range of geological environments, including igneous (granites, pegmatites), metamorphic (gneisses, schists, quartzites), and sedimentary rocks (sandstones, cherts). It crystallizes from silica-rich fluids or melts, or precipitates from aqueous solutions. Basalt: Forms from the rapid cooling of magnesium- and iron-rich lava exposed at or very near the Earth's surface. This occurs in various settings such as mid-ocean ridges, oceanic hot spots, continental rifts, and subduction zones (as island arcs or continental arc back-arc basins).

Usage

Quartz: Widely used in electronics (oscillators, filters), optics (lenses, prisms), abrasives, glass manufacturing, ceramics, and as a gemstone. It is also a major component of construction materials (sand, gravel). Basalt: Primarily used as crushed stone for construction aggregate (road base, concrete aggregate, railroad ballast), dimension stone, and in the production of basalt fiber for composites and insulation. It is also used in landscaping and as a raw material for rock wool.

Age Distribution

Quartz: Ubiquitous throughout Earth's history, found in rocks of all ages. Basalt: Formed throughout Earth's history, from the Archean to present day, particularly abundant in oceanic crust (Mesozoic to Cenozoic) and large igneous provinces.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Quartz: Tectosilicate. Basalt: Igneous Rock (Extrusive).

Group

Quartz: Quartz Group. Basalt: Mafic Volcanic Rock.

Crystal System

Quartz: Trigonal. Basalt: Not applicable as a rock.

Chemical Formula

Quartz: SiO2. Basalt: Variable, but typically (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 (pyroxene) + (Ca,Na)(Al,Si)4O8 (plagioclase) + (Mg,Fe)2SiO4 (olivine).

Composition

Quartz: 46.7% Silicon, 53.3% Oxygen by weight. Basalt: Typically 45-52 wt% SiO2, 10-15 wt% Al2O3, 5-12 wt% FeO, 5-12 wt% MgO, 6-12 wt% CaO, 2-5 wt% Na2O, 0.5-2 wt% K2O, 0.5-2 wt% TiO2.

Explore Quartz and Basalt