A sample of banded hornfels, formed by contact metamorphism of sandstones and shales by a granite intrusion
Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable.[1] These properties are caused by fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation.[2][3][4] The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.[5][6]
Most hornfels are fine-grained, and while the original rocks (such as sandstone, shale, slate and limestone) 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 many hornfels show vestiges of the original bedding,[2] they break across this as readily as along it; in fact, they tend to separate into cubical fragments rather than into thin plates.[1] Sheet minerals may be abundant but are aligned at random.[7]
Hornfels most commonly form in the aureole of granitic intrusions in the upper or middle crust. Hornfels formed from contact metamorphism by volcanic activity very close to the surface can produce unusual and distinctive minerals.[2][3] Changes in composition caused by fluids give off by the magmatic body (metasomatism) sometimes takes place.[8] The hornfels facies is the metamorphic facies which occupies the lowest pressure portion of the metamorphic pressure-temperature space.[9]
The most common hornfels (the biotite hornfels) are dark-brown to black with a somewhat velvety luster owing to the abundance of small crystals of shining black mica. Also, most common hornfels have a black streak. The lime hornfels are often white, yellow, pale-green, brown and other colors. Green and dark-green 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 (porphyroblasts) of cordierite, garnet or andalusite scattered through the fine matrix, and these may become very prominent on the weathered faces of the rock.[1][10]
^ abcOne or more of the preceding sentences incorporates text from a publication now in the public domain: Flett, John Smith (1911). "Hornfels". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 13 (11th ed.). Cambridge University Press. pp. 710–711.
^ abcYardley, Bruce W.D. (1989). An introduction to metamorphic petrology. Harlow, Essex, England: Longman Scientific & Technical. pp. 12, 26. ISBN 0582300967.
^ abBlatt, Harvey; Tracy, Robert J. (1996). Petrology : igneous, sedimentary, and metamorphic (2nd ed.). New York: W.H. Freeman. pp. 367, 512. ISBN 0716724383.
^Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 422, 428. ISBN 9780521880060.
^Lawrence, D. J. D et al 2004 Durham Geodiversity Audit, Durham: Durham County Council p20
^Nesse, William D. (2000). Introduction to mineralogy. New York: Oxford University Press. p. 195. ISBN 9780195106916.
^Harry, W. T. (December 1952). "Basic hornfels at a gabbro contact near Carlingford, Eire". Geological Magazine. 89 (6): 411–416. Bibcode:1952GeoM...89..411H. doi:10.1017/S0016756800068114.
planes, this structure is effaced or rendered inoperative in the hornfels. Though many hornfels show vestiges of the original bedding, they break across this...
schistosity. Granofels, which show no obvious foliation or schistosity. A hornfels is a granofels that is known to result from contact metamorphism. A slate...
quartz phengite + kyanite + talc + quartz ± jadeite The albite-epidote-hornfels facies is a facies at low pressure and relatively low temperatures. It...
aureoles at shallow depth are albite-epidote hornfels, hornblende hornfels, pyroxene hornfels, and sillimanite hornfels, in increasing order of temperature of...
hornfels as a result. The recrystallization and phase change of hornfels reflects the composition of the protolith. After the formation of hornfels,...
across two centuries around the town of Keswick, northern England, using hornfels, a stone from the nearby Skiddaw mountain, which is said to have a superior...
Hornfels is an unusual and relatively rare stone used in making flaked stone tools, and which is found in Aboriginal archaeological sites in Victoria,...
Metamorphic rock – A mafic metamorphic rock dominated by green amphiboles Hornfels – series of contact metamorphic rocks that have been baked and indurated...
rocks from the Cambrian in the Mount Morrison region. These dark-colored hornfels, slates, marbles, and schists are found in the western foothills (notably...
blueschist facies and eclogite facies at higher pressures and the contact hornfels facies at lower pressures. As the rock undergoes higher temperatures and...
depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels. Granite often occurs as relatively small, less than 100 km2 stock masses...
encompasses rocks which are known under more specific names, such as marble or hornfels. Goldsmith, Richard (1959). "Granofels, a New Metamorphic Rock Name". The...
amongst the metamorphic rocks of the eastern Highlands of Scotland. It is a hornfels developed from calcareous mudstone. Calcflinta is also found, for example...
feldspar, almandine, cordierite, biotite and quartz in schist, gneiss, hornfels and also rarely in pegmatites. Dumortierite and mullite are similar mineral...
meters taller. The summit of Jabal Maqlā consists mainly of dark-colored hornfels derived from metamorphosed volcanic rocks that originally were silicic...
Whetstone, a sharpening stone used for knives and other cutting tools Hornfels, a type of stone sometimes called whetstone Whetstone (benchmark), a benchmark...
rocks known as hornfels. Goldschmidt made a systematic study of the hornfels. He showed that, of the minerals to be found in the hornfels, only certain...
or regional metamorphism of pelitic rocks. It is especially common in hornfels produced by contact metamorphism of pelitic rocks. Two common metamorphic...
locality at Mont Saint–Hilaire is the only reported occurrence. It occurs in hornfels and siliceous marble xenoliths within and adjacent to a nepheline syenite...
igneous environment, metasomatism produces skarns, greisen, and may affect hornfels in the contact metamorphic aureole adjacent to an intrusive rock mass....
volcanic material remained within the crust. However, the diabase and related hornfels which form the backbone of the mountain are a type of highly erosion resistant...
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