Weathered Metamorphic Rock with Quartz Veins

Description

This describes a metamorphic rock, most commonly schist or gneiss, that has been subjected to surface weathering processes and contains distinct veins of quartz. The metamorphic rock itself will exhibit characteristics of its protolith and metamorphic grade, such as foliation (schistosity or gneissic banding). The quartz veins are typically lighter in color, often white or milky, and stand out against the darker or more varied colors of the host rock. Weathering effects can include discoloration, softening, fracturing, and the development of a friable or crumbly texture on the surface of the rock, while the quartz veins, being more resistant to chemical weathering, may protrude or appear less altered.

How to Identify

Color

Variable for the host metamorphic rock (e.g., grey, black, green, brown, reddish-brown due to iron oxidation from weathering). Quartz veins are typically white, milky, or translucent. Weathering often imparts duller, earthy tones to the host rock.

Luster

Host rock: Dull to earthy on weathered surfaces; fresh surfaces may show pearly (micas in schist) or vitreous (feldspar, quartz in gneiss) luster. Quartz veins: Vitreous to greasy.

Texture

Host rock: Foliated (schistose or gneissic) with visible mineral alignment. Weathered surfaces may be rough, pitted, or friable. Quartz veins: Granular, massive, or crystalline, often cutting across the foliation of the host rock.

Crystal Form

Host rock: Platy minerals (micas) in schist, granular interlocking crystals (quartz, feldspar) in gneiss. Quartz veins: Anhedral to subhedral quartz crystals, often massive or forming interlocking grains within the vein.

Cleavage

Host rock: Schist may exhibit excellent cleavage (schistosity) due to parallel alignment of micas. Gneiss has poor to no cleavage but exhibits gneissic banding. Quartz veins: No true cleavage, but conchoidal fracture.

Geological Environment

Common in ancient mountain belts, cratonic shields, and areas that have undergone regional metamorphism. Often found in outcrops, road cuts, and stream beds where erosion has exposed the bedrock. Weathering is most pronounced in humid and temperate climates.

Key Facts

• Hardness: Host rock: Variable (e.g., micas 2-3, feldspar 6, quartz 7). Quartz veins: 7 (Mohs scale).
• Specific Gravity: Host rock: 2.6-3.0 g/cm³ (variable depending on mineralogy). Quartz veins: 2.65 g/cm³.
• Crystal System: Host rock: Polycrystalline, individual minerals may be monoclinic, triclinic, or orthorhombic. Quartz veins: Trigonal (for quartz).
• Color: Host rock: Grey, black, green, brown, reddish-brown (weathered). Quartz veins: White, milky, translucent.
• Luster: Host rock: Dull, earthy (weathered); fresh surfaces may be pearly or vitreous. Quartz veins: Vitreous to greasy.
• Transparency: Host rock: Opaque to translucent. Quartz veins: Translucent to transparent.
• Fracture: Host rock: Irregular, splintery. Quartz veins: Conchoidal.
• Cleavage: Host rock: Schistosity (perfect in one direction for micas), gneissic banding (no true cleavage). Quartz veins: None.
• Composition: Host rock: Silicates (micas, feldspars, quartz, amphiboles, garnets, etc.). Quartz veins: Silicon dioxide (SiO2).

Quick Check

• Color: Variable host rock (often dull, earthy tones due to weathering) with white/milky quartz veins.
• Luster: Dull to earthy on weathered host rock; vitreous to greasy on quartz veins.
• Streak: White for quartz; variable for host rock, often white or light-colored for weathered material.

Physical Characteristics

• Crystal Habit: Host rock: Foliated (schistose or gneissic), granular. Quartz veins: Massive, anhedral to subhedral crystals.
• Cleavage Type: Host rock: Perfect basal (micas in schist), none (quartz, feldspar). Quartz veins: None.
• Fracture Type: Host rock: Irregular, splintery. Quartz veins: Conchoidal.
• Tenacity: Host rock: Brittle to friable (weathered). Quartz veins: Brittle.
• Luster Type: Host rock: Dull, earthy (weathered); fresh surfaces may be pearly, vitreous. Quartz veins: Vitreous, greasy.

Formation

These rocks form from pre-existing igneous, sedimentary, or other metamorphic rocks that have undergone intense heat and pressure (metamorphism). The original minerals recrystallize and new minerals may form, often developing a foliated texture (schistosity or gneissic banding). Quartz veins typically form later, as silica-rich hydrothermal fluids migrate through fractures and fissures in the metamorphic rock, depositing quartz (SiO2) as the fluid cools or pressure changes. Weathering then acts on the exposed rock, altering its surface and potentially breaking it down.

Usage

Historically, some quartz veins in metamorphic terrains have been mined for gold or other precious metals, as quartz acts as a host mineral for these deposits. The metamorphic rock itself, if sufficiently durable and aesthetically pleasing, can be used as dimension stone or crushed aggregate. Weathered varieties are generally less suitable for structural uses but can contribute to soil formation.

Age Distribution

Precambrian to Cenozoic, depending on the specific metamorphic event and protolith age. Quartz veins can form at various stages during or after metamorphism.

Where to Find

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Scientific Classification

Mineral Class

Silicates (for host rock minerals) and Oxide (for quartz)

Group

Metamorphic Rock (Schist or Gneiss) with Quartz Veins

Crystal System

Polycrystalline (host rock); Trigonal (quartz)

Chemical Formula

Variable for host rock; SiO2 for quartz

Composition

Complex silicate minerals (e.g., micas, feldspars, amphiboles, garnets) for the host rock, with veins of silicon dioxide (quartz).

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