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Hornfels Facts

Hornfels Facts
Hornfels are a classified group of contact metamorphic rocks that have been baked under high temperatures by the heat of igneous intrusions and as a result, have become massive, splintery, extremely hard, and in some cases exceedingly tough and durable. The majority of hornfels are dark and fine-grained. The most common hornfels are biotite hornfels that are dark-brown to black with a velvety luster that comes from the abundance of small crystals of mafic mica. There are also lime hornfels that are commonly white, yellow, brown, pale-green and other colors. The green and dark-green color tint of the hornfels is established by the alteration of igneous rocks. Most of the grains of the hornfels are too small to be determined by the naked eye. There are larger crystals scattered throughout the fine-grained matrix of the hornfels that become extremely prominent where the surface is weathered.
Interesting Hornfels Facts:
The structure of the hornfels is characterized by the small-grained mosaic make-up.
Hornfels are used in a number of applications like in the field of construction and landscaping. It is used as a decorative rock in gardens. In the olden times, it is used as a tool like scrapers and knives.
Hornfels is used as a road base and in concrete and is most often dark blue or almost a black color.
The interior use of hornfels is found in homes and businesses in the decorative aggregates, flooring, countertops, and bathrooms.
The exterior use of hornfels is viewed in building construction, paving stones, and a variety of gardening decorations.
In prehistoric times, hornfels was used to make simple tools such as knives, scrapers, and arrowheads.
Hornfels are defined by the physical properties such as hardness, strength, grain size, fracture, porosity, and streak. It is these physical properties that determine usage.
Because pressure is not a main factor in the formation of hornfels, and the texture is granular, platy or elongated crystals, there is a lack of foliation as often seen in many metamorphic rocks formed under high pressure.
During the formation of hornfels, the pre-existing rock is destroyed.
Hornfels are typically found only by microscopic observation and not witnessed by eye alone. However, under a microscope the structure becomes very distinctive revealing the small-grained mosaic design.
There is a second group of hornfels are called the calc-silicate hornfels which originate from the thermal alteration of impure limestone. These rocks are fine-grained, and even though they are often banded, they are tough and much harder than the original limestone.
Hornfels have the ability to resonate when struck. The stones in South Africa are called "ring-stones" due to their ability to ring like a bell after being struck with an object.


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