Metamorphic Rock

Description

Metamorphic rocks are a fundamental class of rocks that have undergone significant transformation due to changes in temperature, pressure, and chemical environment. These changes occur deep within the Earth's crust, often associated with tectonic plate collisions, burial, or contact with hot magma. The resulting rocks exhibit a wide range of textures, mineral assemblages, and structures that reflect their metamorphic history. Key characteristics often include foliation (a planar arrangement of mineral grains), banding, and the presence of metamorphic minerals like garnet, staurolite, kyanite, and sillimanite.

How to Identify

Color

Highly variable, depending on the protolith and metamorphic grade. Can range from white (marble, quartzite) to black (slate, amphibolite), green (serpentinite), red, brown, or multi-colored (gneiss, schist).

Luster

Variable, from dull to vitreous, pearly, or silky, depending on the constituent minerals and texture. For example, mica-rich schists can have a silky or pearly luster.

Texture

Often characterized by foliation (e.g., slaty cleavage, schistosity, gneissic banding) or non-foliated textures (e.g., granoblastic in marble and quartzite). Grain size can range from very fine (slate) to coarse (gneiss). Porphyroblastic textures (large metamorphic crystals within a finer matrix) are common.

Crystal Form

Minerals within metamorphic rocks often exhibit anhedral to subhedral forms due to growth under pressure. Porphyroblasts can show euhedral forms. Common metamorphic minerals include garnet (dodecahedral), staurolite (prismatic), kyanite (bladed).

Cleavage

Can exhibit rock cleavage (e.g., slaty cleavage in slate, schistosity in schist) or mineral cleavage within individual grains (e.g., mica, amphibole). Non-foliated rocks like marble and quartzite do not exhibit rock cleavage but may show mineral cleavage.

Geological Environment

Formed in various tectonic settings: convergent plate boundaries (regional metamorphism), near igneous intrusions (contact metamorphism), fault zones (dynamic metamorphism), and deeply buried sedimentary basins (burial metamorphism).

Key Facts

• Hardness: Highly variable, depending on the constituent minerals (e.g., marble 3-4, quartzite 7, schist 2-7).
• Specific Gravity: Highly variable, typically ranging from 2.5 to 3.5 g/cm³, depending on mineral composition (e.g., marble ~2.7, amphibolite ~3.0-3.4).
• Crystal System: Individual minerals within metamorphic rocks exhibit their characteristic crystal systems (e.g., quartz - trigonal, garnet - isometric, mica - monoclinic). The rock itself does not have a single crystal system.
• Color: Extremely diverse, reflecting the protolith and metamorphic mineral assemblage.
• Luster: Variable, from dull to vitreous, pearly, or silky.
• Transparency: Opaque to translucent, depending on the mineral grains and overall rock texture.
• Fracture: Variable, can be conchoidal (quartzite), splintery, irregular, or platy (schist).
• Cleavage: Can exhibit rock cleavage (slaty, schistose) or mineral cleavage within grains. Non-foliated rocks lack rock cleavage.
• Composition: Extremely diverse, reflecting the protolith and metamorphic conditions. Can be rich in silicates (quartz, feldspar, mica, amphibole, garnet), carbonates (calcite, dolomite), or other mineral groups.

Quick Check

• Color: Highly variable (white, black, green, red, multi-colored)
• Luster: Variable (dull, vitreous, pearly, silky)
• Streak: Variable, depending on constituent minerals (e.g., white for marble, gray for slate, no streak for quartzite)

Physical Characteristics

• Crystal Habit: Minerals within metamorphic rocks can exhibit various habits, including granular, bladed, platy, prismatic, or porphyroblastic (large, well-formed crystals).
• Cleavage Type: Rock cleavage (slaty, schistose, gneissic) or mineral cleavage (perfect, good, poor) depending on the rock type and constituent minerals.
• Fracture Type: Conchoidal (quartzite), irregular, splintery, or platy.
• Tenacity: Variable, from brittle (quartzite, marble) to flexible (mica-rich schist) or tough.
• Luster Type: Variable, including dull, earthy, vitreous, pearly, silky, or greasy.

Formation

Metamorphic rocks are formed from pre-existing igneous, sedimentary, or other metamorphic rocks that have been subjected to intense heat, pressure, and/or chemically active fluids. This process, known as metamorphism, causes physical and/or chemical changes to the original rock (protolith) without melting it. The changes can involve recrystallization of existing minerals, growth of new minerals, and development of new textures such as foliation.

Usage

Metamorphic rocks have diverse uses. Marble is widely used in sculpture, architecture, and as a decorative stone. Slate is used for roofing tiles, flooring, and blackboards. Gneiss and schist are used as building materials and decorative aggregates. Quartzite is used in construction, as an abrasive, and in some industrial applications. Serpentine is used as an ornamental stone and historically as a source of asbestos (chrysotile).

Age Distribution

Ranges from Precambrian to Cenozoic, depending on the specific rock and metamorphic event.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Not a single mineral, but a rock class composed of various minerals.

Group

Metamorphic Rock (a rock classification, not a mineral group)

Crystal System

Not applicable to the rock as a whole; refers to individual mineral components.

Chemical Formula

No single chemical formula; represents a diverse range of chemical compositions.

Composition

Composed of various minerals, typically silicates (quartz, feldspar, micas, amphiboles, pyroxenes, garnets, kyanite, sillimanite, staurolite, chlorite, talc, serpentine), carbonates (calcite, dolomite), oxides, and sulfides, depending on the protolith and metamorphic conditions.

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