Hornfels (
German, meaning "hornstone") is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat of
intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. Most hornfels are fine-grained, and while the original rocks (such as
sandstone,
shale and
slate,
limestone and
diabase) may have been more or less fissile owing to the presence of bedding or
cleavage planes, this structure is effaced or rendered inoperative in the hornfels. Though they may show banding, due to bedding, etc., they break across this as readily as along it; in fact, they tend to separate into cubical fragments rather than into thin plates. The most common hornfels (the
biotite hornfelses ) are dark-brown to black with a somewhat velvety
luster owing to the abundance of small
crystals of shining black
mica. The
lime hornfels are often white, yellow, pale-green, brown and other colors. Green and darkgreen are the prevalent tints of the hornfels produced by the alteration of
igneous rocks. Although for the most part the constituent grains are too small to be determined by the unaided eye, there are often larger crystals of
garnet or
andalusite scattered through the fine matrix, and these may become very prominent on the
weathered faces of the rock.
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A hard, dark-colored, dense metamorphic rock that forms from the intrusion of magma into shale or basalt.
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