How to identify Metamorphic Rock

To correctly identify Metamorphic Rock (Metamorphic Rock), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   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.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

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