Gneiss

Main Elements:

Si, Al, K, Na, Fe, Mg, O

Chemical Composition:

Variable, similar to granite but recrystallized: quartz, feldspar, mica, and dark minerals

Density:

2.6-2.9 g/cm³

Compressive Strength:

100-300 MPa

Melting Point:

Variable

Formation:

Forms from granite or other rocks under high-grade metamorphism (intense heat and pressure), causing minerals to separate into distinct bands.

Composition:

Composed of alternating bands of light minerals (quartz, feldspar) and dark minerals (biotite, hornblende, garnet).

Types:

• Biotite Gneiss
• Hornblende Gneiss
• Garnet Gneiss
• Orthogneiss
• Paragneiss

Etymology:

Derived from an old German mining term, possibly related to "gneisto" meaning "to spark" or "to shine", referring to the mica's sparkle.

Uses:

• Construction
• Decorative stone
• Countertops
• Monuments

Distribution:

Found worldwide in ancient mountain ranges and high-grade metamorphic terrains

Historical Use:

Used in construction and monuments. Some of the oldest rocks on Earth are gneisses, over 4 billion years old!

Symbolism:

Represents transformation, strength, and Earth's ancient history

What causes the banding in gneiss?

The banding in gneiss forms when minerals separate into distinct layers under intense heat and pressure during high-grade metamorphism.

How does gneiss differ from granite?

Gneiss has distinct banding of light and dark minerals, while granite has a more uniform, speckled appearance. Gneiss is metamorphic, granite is igneous.