Hornfels
Image: <a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a> (CC BY 2.0)
Mineral NameHornfels
Chemical FormulaVariable (depends on protolith, e.g., shale, basalt, sandstone)
Mohs Hardness6
Streak ColorVariable, often white to light gray
CleavageNone (as a rock; constituent minerals may have cleavage)
FractureConchoidal to uneven, sometimes splintery
Specific Gravity2.6
Colorsdark gray, black, brown, green
TransparencyOpaque
Type LocalityRoute 17 roadcut southeast of Princess Lake & north of Wawa, Ontario, Canada
Mineral GroupMetamorphic Rock
Usesbuilding material, aggregate, road construction
Associated Mineralsquartz, feldspar, micas, cordierite, andalusite, pyroxene, amphibole
Raritycommon
Description
Hornfels is a fine-grained, dark gray to brownish contact metamorphic rock, appearing as a distinct layer in the image, characterized by its tough, massive texture.
Geological Context
Hornfels forms through contact metamorphism, where pre-existing country rock is baked and recrystallized by the heat of an igneous intrusion, such as the adjacent lamprophyre dike, typically resulting in a non-foliated texture.
