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Quartz with Mica refers to a metamorphic rock, typically a schist, where quartz (SiO2) and mica (phyllosilicate minerals like muscovite or biotite) are the dominant mineral constituents. The rock exhibits a characteristic foliation (schistosity) due to the parallel alignment of mica flakes, giving it a shiny, often silvery or golden luster. Quartz typically forms granular layers or lenses within the mica-rich matrix.
How to Identify
- Color
- Variable, often silvery, grey, green, brown, or black, depending on the type and abundance of mica (muscovite: silvery/white; biotite: black/dark brown; chlorite: green). Quartz is typically colorless to white.
- Luster
- Schistose luster, often pearly or silky due to mica, with vitreous (glassy) luster from quartz grains.
- Texture
- Foliated (schistose), medium to coarse-grained. Characterized by parallel alignment of platy mica minerals, giving it a layered or wavy appearance. Quartz grains are typically anhedral and intergrown.
- Crystal Form
- Mica forms platy, tabular crystals, often visible to the naked eye. Quartz forms anhedral, interlocking grains, sometimes as elongated lenses or bands.
- Cleavage
- Excellent schistosity (rock cleavage) due to mica alignment, allowing the rock to split along planes. Individual mica minerals have perfect basal cleavage. Quartz has no cleavage.
- Geological Environment
- Common in regional metamorphic terrains, particularly in mountain belts and ancient cratons, associated with convergent plate boundaries. Found in metamorphic facies ranging from greenschist to amphibolite.
Key Facts
- Hardness: Variable, 2-2.5 for mica, 7 for quartz. The rock's overall hardness is moderate, but it can be scratched by a steel knife due to the mica.
- Specific Gravity: 2.7-3.0 g/cm³ (depends on mica type and accessory minerals).
- Crystal System: Quartz: Trigonal; Mica: Monoclinic.
- Color: Silvery, grey, green, brown, black (mica) with colorless to white (quartz).
- Luster: Pearly to silky (mica), vitreous (quartz).
- Transparency: Opaque to translucent (mica), transparent to translucent (quartz).
- Fracture: Irregular to subconchoidal for quartz, but the rock typically exhibits schistose parting.
- Cleavage: Perfect basal cleavage in mica, no cleavage in quartz. The rock has excellent schistosity.
- Composition: Primarily SiO2 (quartz) and various phyllosilicates (e.g., KAl2(AlSi3O10)(OH)2 for muscovite; K(Mg,Fe)3(AlSi3O10)(OH)2 for biotite).
Quick Check
- Color: Silvery, grey, green, brown, or black with white/colorless quartz.
- Luster: Pearly, silky, or vitreous.
- Streak: White (for muscovite) or pale brown (for biotite), but the rock itself does not produce a consistent streak due to its composite nature.
Physical Characteristics
- Crystal Habit: Mica: Platy, tabular, flaky; Quartz: Anhedral, granular, massive.
- Cleavage Type: Perfect basal cleavage in individual mica crystals; excellent schistosity in the rock.
- Fracture Type: Irregular to splintery along foliation; conchoidal to subconchoidal in quartz-rich areas.
- Tenacity: Flexible and elastic (mica); brittle (quartz). The rock is generally brittle but can be split along foliation.
- Luster Type: Pearly to silky (mica); vitreous (quartz).
Formation
Formed through regional metamorphism of argillaceous (clay-rich) sedimentary rocks, such as shales or mudstones, or felsic igneous rocks like granites, under conditions of moderate to high pressure and temperature. The presence of mica indicates a significant clay component in the protolith and specific metamorphic conditions (greenschist to amphibolite facies).
Usage
Historically used as building stone, flagstone, and for decorative purposes. Due to its schistosity, it can be split into thin slabs. Crushed schist is used as aggregate in construction. Mica, when separated, has industrial uses in electronics, paints, and cosmetics.
Age Distribution
Precambrian to Cenozoic, common in orogenic belts worldwide.
Where to Find
Appalachian Mountains, USA
Extensive occurrences of mica schists, including quartz-mica schists, are found throughout the Appalachian orogen, particularly in New England, the Piedmont, and the Blue Ridge provinces.
Scottish Highlands, UK
Well-known for its metamorphic rocks, including abundant mica schists, formed during the Caledonian Orogeny.
Himalayan Orogen, Asia
Large areas of quartz-mica schists are exposed in the Lesser and Greater Himalayan sequences, resulting from the collision of the Indian and Eurasian plates.
Fennoscandian Shield, Northern Europe
Ancient Precambrian shield areas contain significant exposures of various metamorphic rocks, including mica schists.
Finding Tips
Look for Foliation
The most distinctive feature is the parallel alignment of mica flakes, creating a visible layering or schistosity. This often gives the rock a 'sparkly' appearance.
Check for Luster
The abundance of mica gives the rock a characteristic pearly, silky, or metallic sheen, especially on freshly broken surfaces.
Assess Grain Size
Mica flakes are typically visible to the naked eye, distinguishing it from finer-grained phyllite or slate.
Examine for Quartz
Look for translucent to opaque, glassy grains or lenses of quartz intergrown with the mica.
Similar Rocks
Gneiss
Gneiss
Also known as: Banded Gneiss
Phyllite
Phyllite
Also known as: Fine-grained Schist
Slate
Slate
Also known as: Roofing Slate
Quartzite
Quartzite
Also known as: Metaquartzite
Scientific Classification
- Mineral Class
- Quartz: Tectosilicate; Mica: Phyllosilicate.
- Group
- Quartz Group; Mica Group.
- Crystal System
- Quartz: Trigonal; Mica: Monoclinic.
- Chemical Formula
- SiO2 (Quartz); KAl2(AlSi3O10)(OH)2 (Muscovite); K(Mg,Fe)3(AlSi3O10)(OH)2 (Biotite).
- Composition
- Silicon dioxide (quartz) and complex hydrous potassium aluminum silicates (mica).
Explore Quartz-Mica Schist
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