Schist
Image: James St. John (CC BY 2.0)
Mineral NameSchist
Chemical FormulaVariable, as schist is a rock composed of multiple minerals (e.g., hornblende: (Ca,Na)2-3(Mg,Fe,Al)5Si6-7Al2-1O22(OH,F)2)
Mohs HardnessVariable, depending on constituent minerals (e.g., hornblende 5-6, quartz 7, micas 2-3)
Streak ColorVariable, not typically used for rocks
CleavageExcellent schistosity (foliation) due to parallel alignment of platy minerals; individual minerals have their own cleavage
FractureUneven, splintery along foliation planes
Specific GravityVariable, typically 2.6-3.3 depending on constituent minerals
Colorssilvery-grey, black
TransparencyOpaque as a rock, individual mineral grains may be translucent
Type LocalityRoadcut on the northern side of Route 9, west of the town of Marlboro, southern Vermont, USA
Mineral GroupMetamorphic rock
Usesbuilding material, decorative stone, aggregate
Associated Mineralshornblende, chlorite, quartz
Raritycommon
Description
This schist displays a silvery-grey, finely foliated matrix with numerous radiating, needle-like to prismatic black crystals identified as hornblende amphibole.
Geological Context
This Ordovician-aged schist formed within a highly sheared serpentinite belt in Vermont, associated with chlorite schist, amphibolite, and quartz veins, indicating significant metamorphic processes.
