What Hornfels Is and How It Forms

The Different Types of Hornfels

All hornfels is fine-grained and hard, but its toughness, color, and durability greatly depend on the composition of the original rock. Hornfels may be classified according to its source.

Pelitic hornfels: The most common hornfels comes from the heating of clay, shale, and slate (sedimentary and metamorphic rocks). The primary mineral in pelitic hornfels is biotite mica, with quartz, feldspar, and assorted aluminum silicates. Under magnification, the mica appears as dichroic red-brown scales. Some specimens contain cordierite, which forms hexagonal prisms when viewed under polarized light.

Carbonate hornfels: Carbonate hornfels are calcium silicate rocks made from heating impure limestone, a sedimentary rock. Higher purity limestone crystallizes to form marble. Limestone containing sand or clay forms a variety of minerals. Carbonate hornfels is often banded, sometimes with pelitic (biotite) hornfels. Carbonate hornfels is stronger and tougher than limestone.

Mafic hornfels: Mafic hornfels result from heating of igneous rocks, such as basalt, andesite, and diabase. These rocks exhibit varied compositions, but consist mainly of feldspar, hornblende, and pyroxene. Mafic hornfels is typically green in color.

Where to Find Hornfels

Hornfels occurs worldwide. In Europe, the largest reserves are in the United Kingdom. In North America, hornfels occurs in primarily in Canada. South American countries with large reserves include Bolivia, Brazil, Ecuador, and Colombia. Asian reserves are found in China, Russia, India, North Korea, South Korea, and Thailand. In Africa, hornfels is found in Tanzania, Cameroon, East Africa, and Western Africa. The rock is found in Australia and New Zealand, as well.

Architectural and Musical Uses

The primary use of hornfels is in architecture. The hard, interesting-looking stone may be used to make interior flooring and decorations as well as exterior facing, paving, curbing, and decorations. The rock is used in the construction industry to make road aggregate. Historically, hornels has been used to construct monuments, cemetery markers, whetstones, artworks, and artifacts.

One noteworthy use of hornfels is to construct lithophones or stone bells. In South Africa, the rock may be called "ring stones." The "Musical Stones of Skiddaw" refers to a series of lithophones made using hornfels mined from Skiddaw mountain, near the town of Keswick in England. In 1840, stonemason and musician Joseph Richardson built an eight-octave lithophone, which he played on tour. The lithophone is played like a xylophone.

How to Identify Hornfels

It can be hard to identify hornfels unless you view it under magnification and know the geological history of its source to verify the presence of a magma body. Here are some tips:

• Strike the rock with a hammer. Hornfels makes a ringing sound.
• The bulk of the rock should have a fine, velvety appearance. While larger crystals may be present, most of the rock should be free of obvious structure. Under magnification, crystals may appear granular, plate-like, or oblong and display random orientation.
• Note how the rock breaks. Hornfels does not display foliage. In other words, it doesn't break along well-defined lines. Hornfels is more likely to break into rough cubes than into sheets.
• When polished, hornfels feels smooth.
• While hardness is variable (around 5, which is the Mohs hardness of glass), you can't scratch hornfels with a fingernail or penny, but you can scratch it with a steel file.
• Black or brown is the most common color, but others are common. Banding is possible.

Hornfels Key Points

• Hornfels is a type of metamorphic rock that gets its name from its resemblance to animal horn.
• Hornfels forms when magma heats other rock, which may be igneous, metamorphic, or sedimentary.
• The most common colors of hornfels are black and dark brown. It may be banded or occur in other colors. The colors depend on the composition of the original rock.
• Key properties of the rock include velvety texture and appearance, conchoidal fracture, and fine grain. It may be very hard and tough. 
• It is a contact metamorphic rock, formed when magma bakes its source material.

Source

• Flett, John S. (1911). "Hornfels". In Chisholm, Hugh. Encyclopædia Britannica. 13 (11th ed.). Cambridge University Press. pp. 710–711.