Everything about Hornfels totally explained
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.
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're usually of nearly equal dimensions and from the resemblance to rough pavement work. This has been called pfiaster or pavement structure. 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're 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.
Compositions of Hornfels
Slates, shales and
clays yield biotite hornfels in which the most conspicuous mineral is black mica, the small scales of which are transparent under the microscope and have a dark reddish brown color 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 aluminiun silicates, are commonly found; they're usually andalusite an
sillimanite, but
kyanite appears also in hornfels, especially in those which 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 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
sediment deposited. More commonly there's 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're rich in graphite or carbonaceous matter; in others they're 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're 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 vapors 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
calcite-
silicate-hornfels that arise from the thermal alteration of impure
limestone. The purer beds
recrystallize 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,
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 (andalusite, &c.) 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 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 a reas 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 isn't 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's 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.
Further Information
Get more info on 'Hornfels'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://hornfels.totallyexplained.com">Hornfels Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
