What is Hornfels?
Hornfels is from a German word means hornstone because of its much hardness and texture both resembled to animal horn is a metamorphic rock. These properties are due to fine grained non aligned crystals. Hornfels are also called as Whetstone in England. These rocks are mostly fine grained while the original rocks such as sandstone, shale, slate and limestone may be more or less fissile because to the presence of bedding or cleavage planes, these are inoperative in hornfels. These rocks may show banding due to bedding of the original rock but is differ in breaking than that of the original rock as they break into thin plates, hornfels breaks into cubicles.
How hornfels form?
Hornfels is a group designated for a series of contact metamorphism that have been baked and by the heat of magma chamber or from the intrusive igneous masses and are made into massive, hard, splintery, and in some cases exceedingly tough and durable. As of the contact metamorphism, pressure is not a factor in the formation of hornfels, it lacks the foliation as seen in many metamorphic rocks formed under high pressure and temperature. Pre-existing bedding and structure of the parent rock is generally destroyed in hornfels.
Composition of Hornfels
Slates, shales and clays yield biotite hornfels in which the most conspicuous mineral is biotite mica, the small scales of which are transparent under the microscope and have a dark reddish-brown colour and strong dichroism. There is also quartz, and often a considerable amount of feldspar, while graphite, tourmaline and iron oxides frequently occur in lesser quantity. In these biotite hornfels the minerals, which consist of aluminium silicates, are commonly found; they are usually andalusite and sillimanite, but kyanite appears also in hornfels, especially in those that have a schistose character. The andalusite may be pink and is then often pleochroic in thin sections, or it may be white with the cross-shaped dark enclosures of the matrix that are characteristic of chiastolite. Sillimanite usually forms exceedingly minute needles embedded in quartz.
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 polarised 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 sediment deposited. More commonly there is a distinct spotting, visible on the surfaces of the hand specimens. The spots are round or elliptical, and may be paler or darker than the rest of the rock. In some cases they are rich in graphite or carbonaceous matter; in others they are full of brown mica; some spots consist of rather coarser grains of quartz than occur in the matrix. The frequency with which this feature reappears in the less altered slates and hornfels is rather remarkable, especially as it seems certain that the spots are not always of the same nature or origin. Tourmaline hornfels are found sometimes near the margins of tourmaline granites; they are black with small needles of schorl that under the microscope are dark brown and richly pleochroic. As the tourmaline contains boron, there must have been some permeation of vapours from the granite into the sediments. Rocks of this group are often seen in the Cornish tin-mining districts, especially near the ludes.
A second great group of hornfels are the calc–silicate hornfels that arise from the thermal alteration of impure limestone. The purer beds recrystallise as marbles, but where there has been originally an admixture of sand or clay lime-bearing silicates are formed, such as diopside, epidote, garnet, sphene, vesuvianite and scapolite; with these phlogopite, various feldspars, pyrites, quartz and actinolite often occur. These rocks are fine-grained, and though often banded, are tough and much harder than the original limestones. They are excessively variable in their mineralogical composition, and very often alternate in thin seams with biotite hornfels and indurated quartzites. When perfused with boric and fluoric vapors from the granite they may contain much axinite, fluorite and datolite, but the altiminous silicates are absent from these rocks.
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 colour) 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.
In some districts hornfelsed rocks occur that have acquired a schistose structure through shearing, and these form transitions to schists and gneisses that contain the same minerals as the hornfels, but have a schistose instead of a hornfels structure. Among these may be mentioned cordierite and sillimanite gneisses, andalusite and kyanite mica-schists, and those schistose calcite-silicate rocks that are known as cipolins. That these are sediments that have undergone thermal alteration is generally admitted, but the exact conditions under which they were formed are not always clear. The essential features of hornfelsing are ascribed to the action of heat, pressure and permeating vapors, regenerating a rock mass without the production of fusion (at least on a large scale). It has been argued, however, that often there is extensive chemical change owing to the introduction of matter from the granite into the rocks surrounding it. The formation of new feldspar in the hornfelses is pointed out as evidence of this. While this felspathization may have occurred in a few localities, it seems conspicuously absent from others. Most authorities at the present time regard the changes as being purely of a physical and not of a chemical nature.
Types and colour of hornfels
The most common hornfels are the biotite hornfels which are dark brown to black with somewhat velvety luster owing to the abundance of small crystals of shinning black mica. The limestone hornfels are often white, yellow, pale green, brown and other colours. Although for the most part the constituent grains are too small to be determined by the unaided eye, there are often larger crystals of cordierite, garnet or andalusite scattered through the fine matrix, and these may become very prominent on the weathered faces of the rock.
