Included Quartz

Description

Quartz with inclusions refers to any variety of quartz (silicon dioxide, SiO2) that contains visible foreign mineral phases, fluid-filled cavities, or gas bubbles trapped within its crystal structure. These inclusions can be microscopic or macroscopic, and their type, size, and distribution significantly influence the appearance and value of the quartz. Common mineral inclusions include rutile (titanium dioxide), tourmaline (borosilicate mineral group), chlorite (phyllosilicate mineral group), hematite (iron oxide), goethite (iron oxyhydroxide), actinolite (amphibole mineral), and various other silicates, oxides, and sulfides. Fluid inclusions, consisting of water, brine, CO2, or other volatile compounds, are also very common and can sometimes be seen as tiny bubbles or liquid-filled cavities. The presence of inclusions can create unique visual effects, such as 'needles' (rutile, tourmaline), 'moss' (chlorite, actinolite), 'feathers', or 'phantoms' (earlier growth stages outlined by mineral layers).

How to Identify

Color

Variable, depending on the host quartz and the inclusions. Host quartz can be colorless, white, smoky, purple (amethyst), yellow (citrine), or pink (rose quartz). Inclusions add their own colors: golden (rutile), black (tourmaline, hematite), green (chlorite, actinolite), red/orange (hematite, goethite).

Luster

Vitreous (glassy) on crystal faces and fresh fractures.

Texture

Crystalline, often with visible internal structures due to inclusions. Can be massive or euhedral to anhedral crystals.

Crystal Form

Typically hexagonal prisms terminated by rhombohedra. Inclusions are internal and do not alter the external crystal habit of the quartz, though they may outline growth zones (phantoms).

Cleavage

None, but exhibits conchoidal fracture.

Geological Environment

Hydrothermal veins, pegmatites, igneous rocks (granites, rhyolites), metamorphic rocks (gneisses, schists), and sedimentary environments (as detrital grains). The specific type of inclusion often points to a particular formation environment (e.g., rutile in high-temperature hydrothermal veins, chlorite in lower-temperature metamorphic or hydrothermal settings).

Key Facts

• Hardness: 7 on the Mohs scale.
• Specific Gravity: 2.65 g/cm³ (for pure quartz; inclusions may slightly alter this).
• Crystal System: Trigonal.
• Color: Highly variable, depending on the host quartz and the type/color of inclusions.
• Luster: Vitreous.
• Transparency: Transparent to translucent, depending on the density and type of inclusions.
• Fracture: Conchoidal.
• Cleavage: None.
• Composition: Silicon dioxide (SiO2) with various trapped mineral phases or fluids.

Quick Check

• Color: Variable, often colorless to white host with colored internal inclusions.
• Luster: Vitreous (glassy).
• Streak: White.

Physical Characteristics

• Crystal Habit: Prismatic, often with hexagonal cross-section, terminated by rhombohedral faces. Inclusions are internal.
• Cleavage Type: Absent.
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous.

Formation

Quartz with inclusions forms when foreign mineral particles or fluids are trapped within the growing quartz crystal lattice. This can occur during hydrothermal crystallization, magmatic crystallization, or metamorphic recrystallization. The specific type of inclusion depends on the geological environment and the available minerals during quartz growth. For example, rutilated quartz forms when acicular rutile crystals grow simultaneously with or are engulfed by quartz in high-temperature, often hydrothermal, environments. Chlorite inclusions often indicate lower temperature hydrothermal or metamorphic conditions. Fluid inclusions (liquid and gas) are common in many quartz varieties and provide insights into the formation conditions.

Usage

Primarily used as a gemstone or ornamental stone, particularly varieties with aesthetically pleasing inclusions. It is also collected by mineral enthusiasts. Specific types, like rutilated quartz, are highly valued in jewelry. The scientific study of inclusions (fluid inclusions, mineral inclusions) in quartz is crucial for understanding ore genesis, fluid evolution in geological systems, and the thermal history of rocks.

Age Distribution

Found in rocks of all ages, from Precambrian to Cenozoic, wherever quartz forms.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Silicate (Tectosilicate)

Group

Quartz Group

Crystal System

Trigonal (alpha-quartz below 573°C)

Chemical Formula

SiO2 (host mineral)

Composition

Silicon dioxide with various mineral and/or fluid inclusions. Common inclusions include Rutile (TiO2), Tourmaline (complex borosilicate), Chlorite (complex phyllosilicate), Hematite (Fe2O3), Goethite (FeO(OH)), Actinolite (Ca2(Mg,Fe)5Si8O22(OH)2), Pyrite (FeS2), and various fluid inclusions (H2O, CO2, CH4, salts).

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