How to identify Mixed Sedimentary and Metamorphic Rocks

To correctly identify Mixed Sedimentary and Metamorphic Rocks (Various sedimentary and metamorphic rock types (e.g., sandstone, quartzite, chert, slate, granite gneiss)), 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 constituent rock types. Sandstone can be white, grey, yellow, red; quartzite typically white, grey, pink; chert can be grey, black, white, brown; slate is typically dark grey to black; granite gneiss is often banded with light (quartz, feldspar) and dark (biotite, hornblende) minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable. Sandstone and quartzite are dull to vitreous (if quartz grains are prominent). Chert is dull to waxy. Slate is dull to sub-vitreous. Granite gneiss is typically vitreous to dull, depending on mineral composition and weathering.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Variable. Sandstone is clastic, granular. Quartzite is granular, interlocking, often sugary. Chert is cryptocrystalline, smooth to conchoidal fracture. Slate is very fine-grained, foliated (slaty cleavage). Granite gneiss is medium to coarse-grained, foliated (gneissic banding).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual minerals within the rocks will exhibit their characteristic crystal forms, but the overall rock typically does not show macroscopic crystal forms unless it's a coarse-grained metamorphic rock like gneiss with porphyroblasts.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Variable. Sandstone typically has no rock cleavage. Quartzite has no rock cleavage but may fracture across grains. Chert has no cleavage. Slate exhibits excellent slaty cleavage. Granite gneiss exhibits gneissic foliation (banding) but not true mineral cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in orogenic belts (mountain ranges), continental collision zones, subduction zones, and regions that have undergone significant crustal deformation and burial. Often found in ancient cratons and shield areas where deep crustal processes have exposed these rocks.

Key Facts

Hardness

Variable (e.g., Quartzite 7, Slate 2.5-4, Chert 6.5-7, Sandstone 6-7 for quartz grains)

Specific Gravity

Variable (e.g., Quartzite 2.65, Slate 2.7-2.8, Chert 2.5-2.8, Granite Gneiss 2.6-2.9)

Crystal System

Not applicable to a rock type; individual minerals within the rock have specific crystal systems.

Color

Extremely varied, reflecting the diverse mineralogy and protoliths.

Luster

Variable, from dull to vitreous, depending on the dominant minerals and texture.

Transparency

Opaque to translucent, depending on the specific rock type and mineral grains.

Fracture

Variable (e.g., conchoidal in chert, irregular in sandstone, splintery in some schists).

Cleavage

Variable (e.g., slaty cleavage in slate, gneissic foliation in gneiss, no cleavage in quartzite/sandstone).

Composition

Highly variable, consisting of minerals characteristic of both sedimentary (e.g., quartz, feldspar, clay minerals, calcite) and metamorphic rocks (e.g., quartz, feldspar, mica, garnet, staurolite, kyanite, sillimanite).

Physical characteristics

• Crystal Habit: Not applicable to a rock type; refers to individual mineral habits.
• Cleavage Type: Variable, depending on the specific rock type (e.g., slaty, gneissic, or none).
• Fracture Type: Variable, from conchoidal to irregular or splintery.
• Tenacity: Variable, from brittle (quartzite, chert) to somewhat tough (slate, gneiss).
• Luster Type: Variable, from dull to vitreous.

Explore Mixed Sedimentary and Metamorphic Rocks