Hornfels
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
The most common hornfels (the
Structure
The structure of the hornfels is very characteristic. Very rarely do any of the minerals show crystalline form, but the small grains fit closely together like the fragments of a mosaic; they are usually of nearly equal dimensions. This has been called pflaster or pavement structure from the resemblance to rough pavement work. Each mineral may also enclose particles of the others; in the quartz, for example, small crystals of graphite, biotite, iron oxides, sillimanite or feldspar may appear in great numbers. Often the whole of the grains are rendered semi-opaque in this way. The minutest crystals may show traces of crystalline outlines; undoubtedly they are of new formation and have originated in situ. This leads us to believe that the whole rock has been recrystallized at a high temperature and in the solid state so that there was little freedom for the mineral molecules to build up well-individualized crystals. The regeneration of the rock has been sufficient to efface most of the original structures and to replace the former minerals more-or-less completely by new ones. But crystallization has been hampered by the solid condition of the mass and the new minerals are formless and have been unable to reject impurities, but have grown around them.[1]
Compositions
Pelitic
In the rocks of this group cordierite also occurs, not rarely, and may have the outlines of imperfect hexagonal prisms that are divided up into six sectors when seen in polarized light. In biotite hornfels, a faint striping may indicate the original bedding of the unaltered rock and corresponds to small changes in the nature of the
Carbonate
A second great group of hornfels are the
Mafic
From diabases, basalts, andesites and other igneous rocks a third type of hornfels is produced. They consist essentially of feldspar with hornblende (generally of brown color) and pale pyroxene. Sphene, biotite and iron oxides are the other common constituents, but these rocks show much variety of composition and structure. Where the original mass was decomposed and contained calcite, zeolites, chlorite and other secondary minerals either in veins or in cavities, there are usually rounded areas or irregular streaks containing a suite of new minerals, which may resemble those of the calcium-silicate hornfelses above described. The original porphyritic, fluidal, vesicular or fragmental structures of the igneous rock are clearly visible in the less advanced stages of hornfelsing, but become less evident as the alteration progresses.[1]
In some districts hornfelsed rocks occur that have acquired a schistose structure through shearing, and these form transitions to schists and
Facies
Metamorphic facies in pressure-temperature space. The various hornfels facies occupy the low pressure region of the space. |
The hornfels facies occupies the portion of the metamorphic pressure-temperature space of lowest pressure and low to high temperature. It is subdivided into a low-temperature regime of albite-epidote hornfels, a medium-temperature regime of hornblende hornfels, a high-temperature regime of pyroxene hornfels, and an ultra-high-temperature sanidinite regime. The latter is sometimes regarded as a separate facies. Maximum pressures are around 2 kbar and temperatures are around 300-500 C for the albite-epidote hornfels facies, 500-650 C for the hornblende hornfels facies, 650-800 C for the pyroxene hornfels facies, and above 800 C for the sanidinite facies.[9][4]
The actual minerals present in each facies depends on the composition of the
For an ultramafic protolith, the albite-epidote facies is characterized by serpentine, talc, tremolite, and chlorite, giving way to forsterite, orthopyroxene, hornblende, chlorite, and characteristic minor aluminum spinel and magnetite in the hornblende facies, which in turn gives way to forsterite, orthopyroxene, augite, plagioclase, and aluminum spinel in the pyroxene hornfels facies. The sanidinite facies for this composition differs from the pyroxene hornfels facies only in the disappearance of aluminum spinel.[9]
For a pelitic protolith, the sequence is quartz, plagioclase, muscovite, chlorite, and cordierite in the albite-epidote facies; quartz, plagioclase, muscovite, biotite, cordierite, and andalusite in the hornblende hornfels facies; and quartz, plagioclase, orthoclase, andalusite, sillimanite, cordierite, and orthopyroxene in the pyroxene hornfels facies. The sanidinite facies features quartz, plagioclase, sillimanite, cordierite, orthopyroxene, sapphirine, and aluminum spinel.[12][9]
For a calcareous protolith, the sequence is
Acoustic properties
Hornfels have the ability to resonate when struck. Michael Tellinger had described these stones in South Africa also known as "ring-stones" due to their ability to ring like a bell.[13] The Musical Stones of Skiddaw are an example of a lithophone made from hornfels.[14]
See also
- List of rock types – List of rock types recognized by geologists
References
- ^ . Vol. 13 (11th ed.). Cambridge University Press. pp. 710–711.
- ^ ISBN 0582300967.
- ^ ISBN 0716724383.
- ^ ISBN 9780521880060.
- ^ "Holwick Scar & Low Force : Pamphlet" (PDF). Explorenorthpennines.org.uk. Retrieved 2015-03-17.
- ^ Lawrence, D. J. D et al 2004 Durham Geodiversity Audit, Durham: Durham County Council p20
- ISBN 9780195106916.
- .
- ^ a b c d e f Blatt and Tracy, p.378-380, 512
- ^ Yardley 1989, p.161
- ^ Dickey, John S.; Obata, Masaaki (1974). "Graphitic hornfels dikes in the Ronda high-temperature peridotite massif". American Mineralogist. 59 (11–12): 1183–1189. Retrieved 23 August 2020.
- .
- ^ "2014 Ancient Hidden Technology of the Annunaki (Fallen Angels)". see from 43 min. Archived from the original on 2016-08-09. Retrieved 2016-06-02.
- ^ "The Musical Stones of Skiddaw - Allerdale Borough Council". Allerdale.gov.uk. Archived from the original on 2010-06-19. Retrieved 2015-03-17.
- . Vol. 13 (11th ed.). Cambridge University Press. pp. 710–711. This article incorporates text from a publication now in the
External links
Media related to Hornfels at Wikimedia Commons