HORNFELS (a

See also:

• GERMAN
• GERMAN, DUTCH AND SCANDINAVIAN

German word meaning hornstone)  , the group designation for a series of rocks which have been baked and indurated by the heat of intrusive granitic masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable . Most hornfelses 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, &c., they break across this as readily as along it; in fact they tend to separate into cubical fragments rather than into thin plates . The commonest hornfelses (the " biotite hornfelses ") are dark-brown to black with a somewhat velvety lustre owing to the abundance of small crystals of shining black mica . The " lime hornfelses " are often white, yellow, pale-green, brown and other colours . Green and dark-green are the prevalent tints of the hornfelses 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 . The structure of the hornfelses 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 and from the resemblance to rough pavement work this has been called' pilaster structure 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 felspar 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 . Slates, shales and clays yield biotite hornfelses in which the most conspicuous mineral is black mica, in small scales which under the microscope are transparent and have a dark reddish-brown colour and strong dichroism . There is also quartz, and often a considerable amount of felspar, while graphite, tourmaline and iron oxides frequently occur in lesser quantity .