Pelitic Clays, sedimentary slates and shales 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 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 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 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 vapors from the granite into the sediments. Rocks of this group are often seen in the Cornish
tin-mining districts, especially near the lodes.
Carbonate A second great group of hornfels are the
calc–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 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.
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. 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. ==Facies==