Hornfels
Image: James St. John (CC BY 2.0)
Mineral NameHornfels
Chemical FormulaVariable, depends on protolith (e.g., shale, sandstone, basalt)
Mohs Hardness6
Streak ColorWhite to gray (variable)
CleavageNone to poor (rock, not a single mineral)
FractureConchoidal to uneven
Specific Gravity2.8
Colorsdark gray, black
TransparencyOpaque
Type LocalityRoute 17 roadcut southeast of Princess Lake & north of Wawa, Ontario, Canada
Mineral GroupMetamorphic rock
Usesbuilding material, aggregate, collector (for specific mineral assemblages)
Associated Mineralsquartz, feldspar, mica, cordierite, andalusite, pyroxene, amphibole, garnet
Raritycommon
Description
A dark gray to black, fine-grained rock with a somewhat blocky appearance, located between a lamprophyre dike and country rock.
Geological Context
Hornfels is a contact metamorphic rock formed by the thermal alteration of country rock by an igneous intrusion, typically characterized by a fine-grained, non-foliated texture.
