Gneiss
Mineral NameGneiss
Chemical FormulaNot applicable (rock)
Mohs HardnessVariable (typically 6-7 for constituent minerals)
Streak ColorWhite to light gray (variable)
CleavageNot applicable (rock; individual minerals may show cleavage)
FractureUneven to conchoidal
Specific Gravity2.6
Colorswhite, gray, black, pink, brown
TransparencyOpaque
Type LocalityWidespread; specific example: Skagit Gneiss, Washington, USA
Mineral GroupMetamorphic rock
Usesbuilding material, decorative stone, aggregate, dimension stone
Associated Mineralsquartz, feldspar, mica, amphibole, garnet
Raritycommon
Description
Gneiss is a coarse-grained, banded metamorphic rock characterized by alternating light (felsic) and dark (mafic) mineral layers, often exhibiting a wavy or folded texture. The image displays prominent banding with lighter quartz-feldspar rich layers and darker amphibole/biotite rich layers, showing folding and boudinage structures.
Geological Context
Gneiss forms under high-grade regional metamorphic conditions, typically from the metamorphism of pre-existing igneous (orthogneiss) or sedimentary (paragneiss) rocks, involving high temperatures and pressures that cause mineral segregation and recrystallization.
