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)
Streak ColorVariable (often gray to white)
CleavageAbsent or very poor
Fractureuneven
Colorsdark gray, black, brown, greenish-black
Transparencyopaque
Type LocalityRoute 17 roadcut southeast of Princess Lake & north of Wawa, Ontario, Canada
Mineral GroupMetamorphic Rock
Usesaggregate, building material
Associated Mineralsquartz, feldspar, mica, cordierite, andalusite, pyroxene, amphibole
Raritycommon
Description
Hornfels is a fine-grained, dark gray to brownish-black rock, appearing blocky and fractured in the image, showing a dense, 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 lamprophyre dike seen here. This process occurs without significant directed pressure, resulting in a non-foliated texture.
