Slate with Quartz Veins

Metamorphic Rock (Foliated)

Slate (metamorphic rock) with Quartz (SiO2) veins

Also known as: Veined Slate

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Description

Slate with quartz veins is a fine-grained, foliated metamorphic rock characterized by its excellent slaty cleavage, allowing it to be split into thin, durable sheets. The distinguishing feature is the presence of distinct veins or stringers of quartz (SiO2) that cut across the slaty cleavage or bedding planes. These veins can vary in thickness from hairline fractures to several centimeters wide and may be straight, undulating, or anastomosing. The color of the slate itself can range from dark gray, black, green, purple, or red, while the quartz veins are typically white, milky, or translucent.

How to Identify

Color: Slate matrix: typically dark gray, black, green, purple, or red. Quartz veins: white, milky, or translucent.
Luster: Slate matrix: dull to sub-vitreous. Quartz veins: vitreous (glassy).
Texture: Slate matrix: very fine-grained, aphanitic, with a distinct slaty cleavage. Quartz veins: crystalline, granular, or massive within the veins.
Crystal Form: Slate matrix: individual mineral grains (micas, chlorite, quartz) are microscopic and not visible to the naked eye. Quartz veins: anhedral to subhedral crystals, often interlocking, forming a massive vein.
Cleavage: Slate matrix: excellent slaty cleavage, splitting into thin, flat sheets. Quartz veins: no cleavage, conchoidal fracture.
Geological Environment: Low-grade regional metamorphic terrains, often associated with ancient mountain belts or convergent plate boundaries where fine-grained sedimentary rocks have been subjected to directed pressure and elevated temperatures. The quartz veins indicate subsequent hydrothermal fluid flow through fractures.

Key Facts

Hardness: Slate: 2.5-4 (Mohs). Quartz veins: 7 (Mohs).
Specific Gravity: Slate: 2.7-2.8. Quartz: 2.65.
Crystal System: Slate: Monoclinic (for constituent micas/chlorite), but rock is massive. Quartz: Trigonal.
Color: Slate: Variable (gray, black, green, purple, red). Quartz veins: White, milky, translucent.
Luster: Slate: Dull to sub-vitreous. Quartz: Vitreous.
Transparency: Slate: Opaque. Quartz: Transparent to translucent.
Fracture: Slate: Splintery to uneven along cleavage, conchoidal perpendicular to cleavage. Quartz: Conchoidal.
Cleavage: Slate: Excellent slaty cleavage. Quartz: None.
Composition: Slate: Predominantly muscovite, chlorite, quartz, illite, with minor feldspar, pyrite, and carbonaceous material. Quartz veins: SiO2 (silicon dioxide).

Quick Check

Color: Dark gray, black, green, purple, or red slate with white/milky veins.
Luster: Dull to sub-vitreous for slate, vitreous for quartz veins.
Streak: White (for quartz) and typically uncolored or very light for slate (though not easily obtained due to hardness).

Physical Characteristics

Crystal Habit: Slate: Microscopic platy minerals aligned. Quartz veins: Massive, granular, or euhedral crystals in vugs.
Cleavage Type: Slate: Slaty cleavage (perfect, planar). Quartz: None.
Fracture Type: Slate: Splintery to uneven (along cleavage), conchoidal (perpendicular to cleavage). Quartz: Conchoidal.
Tenacity: Slate: Brittle. Quartz: Brittle.
Luster Type: Slate: Dull to sub-vitreous. Quartz: Vitreous.

Formation

Slate forms from the low-grade regional metamorphism of fine-grained sedimentary rocks such as shale or mudstone. The presence of quartz veins indicates later hydrothermal activity. During metamorphism, directed pressure causes the platy minerals (like micas and chlorite) to align perpendicular to the stress, creating the characteristic slaty cleavage. Quartz veins typically form when silica-rich fluids, often heated by magmatic intrusions or deep burial, circulate through fractures and fissures within the already formed slate. As these fluids cool or pressure changes, quartz precipitates out, filling these open spaces.

Usage

Historically and currently used for roofing tiles, flooring, paving, blackboards, billiard tables, and decorative landscaping. The quartz veins, while sometimes adding aesthetic appeal, can also be a point of weakness if they are numerous or oriented unfavorably, potentially affecting the workability and durability of the slate for certain applications.

Age Distribution

Varies widely depending on the protolith and metamorphic event, ranging from Precambrian to Cenozoic.

Where to Find

Wales, United Kingdom

Famous for its high-quality roofing slate, often containing quartz veins. Examples include the quarries of Snowdonia.

Vermont, United States

The Slate Valley region of Vermont and New York is a significant source of slate, with various colors and occurrences of quartz veining.

Brazil

Minas Gerais state is a major producer of slate, some of which exhibits quartz veins.

Spain

Galicia region is known for its slate quarries, where quartz veins can be observed.

China

Various regions produce slate, and occurrences of quartz veins are common.

Finding Tips

Look for Outcrops

Search in areas known for regional metamorphism, particularly in road cuts, stream beds, and quarry exposures where slate is present. The quartz veins will stand out against the darker slate matrix.

Examine Cleavage Planes

Slate's characteristic slaty cleavage makes it easy to identify. Look for the quartz veins cutting across or running parallel to these cleavage planes.

Check for Hardness

The quartz veins will be significantly harder (Mohs 7) than the surrounding slate (Mohs 2.5-4), which can be tested with a steel knife or another quartz crystal.

Observe Color Contrast

The white or milky color of quartz veins typically provides a clear contrast against the darker hues of the slate.

Similar Rocks

Phyllite

Also known as: Glossy Slate

Schist

Also known as: Crystalline Schist

Shale

Also known as: Mudstone

Scientific Classification

Mineral Class: Slate: Metamorphic rock (composed of various silicate minerals). Quartz: Tectosilicate.
Group: Slate: Foliated metamorphic rock. Quartz: Quartz group.
Crystal System: Slate: Not applicable to the rock as a whole, constituent minerals are monoclinic/trigonal. Quartz: Trigonal.
Chemical Formula: Slate: Variable, complex silicate mixture. Quartz: SiO2.
Composition: Slate: Primarily phyllosilicates (micas, chlorite) and quartz. Quartz veins: Silicon dioxide.

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